Wet Foods for Poultry

Abstract

Historically feeding of wet mashes to poultry has not been recommended for use in large-scale commercial poultry production, on the basis that it does not offer any nutritional advantage and is difficult to apply. However, food soaked and re-dried is utilised more efficiently and recent work has shown repeatedly that mixing conventional foods with sufficient water to make a porridgy consistency (typically1.5-2.0 kg of water per kg of air-dry food) either increases the proportional retention of nutrients by broilers or increases voluntary food intake. Foods of lower nutrient density (e.g. with a high content of cereal) can therefore be used to get the desired growth rate. The improvement in digestion with wet feeding is not likely to be due to activation of endogenous enzymes in the feed but probably involves more rapid penetration of digestive juices into food particles, more rapid and complete digestion and thus the opportunity for higher food intakes. The benefits are seen with both male and female broilers and are greater the earlier in life the wet feeding is started. Growing ducks and laying hens are also more efficient with wet feeding. Practical advantages of giving food in the wet form include the ability to use high levels of cereals without pelleting, the opportunity to fine-tune the composition of the diet on a daily basis (e.g. amino acids, medicines) and the great reduction in dust in intensive houses. Disadvantages include the danger of wet litter and dirty feathers, risk of spread of disease and the high cost of feeding equipment. So far wet feeding is not in commercial use although it might be of particular benefit in hot environments.

Full text

Wet Foods for Poultry
J. Michael Forbes
Centre for Animal Sciences, School of Biology, University of Leeds, Leeds LS2 9JT, UK
ABSTRACT
Historically feeding of wet mashes to poultry has not been recommended for use in large-scale commercial poultry production,
on the basis that it does not offer any nutritional advantage and is difficult to apply. However, food soaked and re-dried is utilised
more efficiently and recent work has shown repeatedly that mixing conventional foods with sufficient water to make a porridgy
consistency ( typically 1.5 ^ 2.0 kg of water per kg of air-dry food) either increases the proportional retention of nutrients by broi -
lers or increases voluntary food intake. Foods of lower nutrient density (e.g. with a high content of cereal) can therefore be
used to get the desired growth rate. The improvement in digestion with wet feeding is not likely to be due to activation of
endogenous enzymes in the feed but probably involves more rapid penetration of digestive juices into food particles, more rapid and
complete digestion and thus the opportunity for higher food intakes. The benefits are seen with both male and female broilers and
are greater the earlier in life the wet feeding is started. Growing ducks and laying hens are also more efficient with wet feeding.
Practical advantages of giving food in the wet form include the ability to use high levels of cereals without pelleting, the opportunity
to fine-tune the composition of the diet on a daily basis (e.g. amino acids, medicines) and the great reduction in dust in intensive
houses. Disadvantages include the danger of wet litter and dirty feathers, risk of spread of disease and the high cost of feeding
equipment. So far wet feeding is not in commercial use although it might be of particular benefit in hot environments.
Keywords: broiler chickens; wet feeding; digestive ef®ciency
1. INTRODUCTION
Many foods are high in their content of water but,
either through natural or arti®cial drying, most are
eaten in a relatively dry form. It is common for such
dried foods to be reconstituted with water for human
consumption, usually by wet cooking, but this is not
common for animal foods. In this review the effect of
adding water to dry food is investigated, with parti-
cular reference to food intake and growth of poultry
and to the possible mechanisms of the effects. The
subject is of interest and importance in view of the fact
that many bene®ts of wet feeding have been uncov-
ered, although counterbalanced in some cases by
potential disadvantages, and this may have practical
applications in some parts of the world.
1.1. Background
Of the diets that produce obesity in rats many are foods
with a high moisture content (Sclafani and Xenakis,
1984). Ramirez (1987b) found that rats fed a diet
which had been lique®ed by the addition of water ate
more calories and grew faster than rats fed the same
food in dry form. Wet diets tended to produce more
persistent overeating than did dry high-fat or high-
sucrose diets and adding water to a high-starch diet
stimulated energy intake and weight gain for a longer
period than a high-sucrose diet (Ramirez, 1991). Rats
eventually come to prefer the wet diet when given a
choice between wet and dry foods (Ramirez, 1987a),
suggesting that experience of the bene®ts of the wet
diet render it more palatable than the dry diet. Addition
of water to the diet for rats not only increased energy
intake and weight gain, but also improved the ef®-
ciency of utilisation of protein. Keane et al. (1962)
reported that the level of water in a puri®ed diet may
in¯uence the growth rate and feed ef®ciency of rats fed
sub-optimal (90 gykg) protein diets.
For pigs, the nutritional bene®ts are probably of less
importance than the ease of automatic delivery of wet
food through pipelines but together they have resulted
in wet feeding being widely used in the pig industry.
Water content is high (typically 80%) to ensure easy
Avian and Poultry Biology Reviews 14 (4), 2003, 000 ± 000
*To whom correspondence should be addressed: j.m.forbes@leeds.ac.uk

¯ow and to avoid blockages of pumps and pipelines,
and pigs readily eat such liquid food (Geary et al.,
1996). With ruminants, wetting of concentrate supple-
ments has been seen to increase rate of eating by dairy
cows (Clough, 1972) and wetting of poor quality
forage has improved growth rate and carcass composi-
tion in beef cattle (Holzer et al., 1976). Fresh herbage
and silage are, of course, high in moisture and fed to
ruminants very widely.
In comparison with these species, relatively few
studies have been made of wet feeding for poultry.
This review covers the effects of water addition to food
either immediately before, or not more than 48 hours
before, being offered. Whilst most of the research
reported has been carried out with broiler chickens
and laying hens, a little has also been performed with
ducks.
1.2. History
Wet mash feeding has been practised for many decades
in back-yard poultry keeping using, for example, waste
food scraps, potatoes and their peelings and many
other available materials mixed up to give a sloppy
mash. Also, wet feeds are useful under very hot
conditions where they might partly alleviate the
effects of heat stress on feed intake and performance.
However, teaching was against wet feeding as there
was no scienti®c evidence of improved performance or
ef®ciency under the conditions of modern temperate-
region intensive broiler production. Brown (1915), in
his textbook on poultry husbandry, stated (p. 195) that
``As a general rule ::: the grains are fed in a dry state,
either given whole or milled. Meals can, however, be
fed either in the form of a crumbly, moist mixture ± a
wet mash or soft food ± or in their original condition,
forming a dry mash. The former is the one generally
used in [the UK]. There does not appear to be any
difference whatsoever in the digestibility of the meals
::: when fed wet or dry.'' Also writing about UK
poultry husbandry, Robinson (1948) said (p. 178): ``It
is dif®cult to understand why wet mash is so constantly
recommended for natural rearing, when dry mash is
equally satisfactory and requires so much less labour.''
In the USA, Jull (1938) discussed ``Dry and Wet
Mashes'' and wrote (p. 316) that ``Dry-mash feeding
involves less labour than wet-mash feeding and except
under certain circumstances is preferred. On the other
hand, wetting the mashes, especially if skim milk is
used, tends to increase their palatability. In the feeding
of chicks and layers the mash should be moistened so
that it is still crumbly.''
Early research into complete wet synthetic diets for
chickens reported a major osmotic pressure distur-
bance resulting in a critical loss of tissue water
indicated by dehydration and mortality (Kop¯er and
Wilkinson, 1963). In a series of short-term (2 ± 7 days)
studies with chickens 1 to 2 weeks of age it was found
that weight gain was signi®cantly less with wet food
than with the dry diet (Waibel et al., 1966). On both
practical and experimental grounds, therefore, wet
feeding of poultry was contra-indicated.
Pittard (1969) implied that it is well known that wet
feeding of poultry improves feed intake and ef®ciency.
Although his patent is for a piece of apparatus it does
include rather obscure statements about advantages of
wet feeding (p. 1): ``One method of enticing the
broilers to consume more feed than they normally
would is by uniformly mixing the dry poultry feed
with water.'' Then, again without giving any evidence,
it says (p. 2): ``A further important object of the
present invention is to provide an automatic feeding
device which entices poultry to eat solid feed so as to
produce a desirable conversion ratio between pounds
of feed fed and pounds of poultry produced.''
1.3. Wetting and re-drying
Fry et al. (1958) reported that the nutritive value of
barley was improved by the addition of 100% water
and soaking for 8 hours, followed by drying for 15
hours. Pre-soaking and redrying hard wheat resulted in
signi®cantly improved growth by chicks when
compared to untreated hard wheat (Adams and
Naber, 1969).
Different ratios of water to food were used by
Lepkovsky and Furuta (1960) and the food was then
dried and fed to the chickens. Their results indicated
that growth was increased whether the wet food was
partially or completely dried. This showed that it was
not the water per se that improved the feed but it was
due to some change which had taken place in the food
that increased nutritional value. Accordingly, chickens
were fed dry food and during the feeding period they
were given water directly into their crops. It was found
that intubation of 20 ± 40 ml of water into the crop of
chickens eating a dry food did not give any better
growth than chickens given dry food and ad libitum
water. When the intubated amount of water was
2 J. Michael Forbes

increased to 80 ml, both food intake and growth were
decreased.
2. WET FEEDING OF BROILER
CHICKENS
2.1. Bene®ts of wet feeding demonstrated
Despite the historical advice, and encouraged by the
results of wetting and re-drying, Yalda and Forbes
(1995) re-examined wet feeding for growing chickens,
on the fanciful hypothesis that providing food with
slightly less water incorporated than the birds required,
and withholding any separate source of drinking water,
might encourage birds to eat more food in order to
obtain the water they needed. Surprisingly large
increases in food intake and body weight gain were
induced in male broilers by wet feeding (700 g
waterykg)
1
, irrespective of whether drinking water
was available. Female broiler chicks responded to
wet feeding in a similar manner to males and the
apparent retention of dry matter (DM) was increased
from 0.65 for the dry food to 0.73 for the wet. In
contrast to the broilers, cockerels of an egg-laying
strain did not increase their intake of food dry matter
when it was fed in the wet form (700 g waterykg) but
there was a signi®cant increase in body weight gain,
i.e. there was a signi®cantly improved conversion
ef®ciency.
If the primary effect of wet feeding was to increase
food intake then it would be expected that there would
be a large increase in fat deposition. However, the
composition of the carcass, in terms of its fat : protein
ratio was not changed signi®cantly. The results
suggest, therefore, that wet feeding stimulates growth
directly. The mechanisms for this are unknown but
presumably are associated with the improved nutrient
retention (Yalda and Forbes, 1995). Yalda and Forbes
(1996b) reported that wet feeding improves protein
retention by a similar margin as for dry matter
retention.
Digesta moves through the gut very quickly in the
broiler chicken, with an average retention time in 3-
week old birds of about 3 h (Almirall and Garcia,
1994). It is therefore imperative for digestion to start
quickly once food has been eaten if the maximum
proportion is to be absorbed. Pre-wetting food
increases its solublisation and allows a more rapid
penetration of digestive juices (Yasar and Forbes,
2001), rendering the food more digestible. Therefore,
a reduction in food intake, such as restriction of intake,
which results in food particles spending longer in the
digestive tract, should result in digestion proceeding
further towards its potential. However, this was not the
case, as those birds given wet food at the restricted
level had DM metabolisability intermediate between
those given dry and wet foods ad libitum (Yalda and
Forbes, 1996b). This paradox is addressed below.
2.2. Optimum water content
The bene®ts of wet feeding outlined above are
surprising as established wisdom is that wet mash
feeding confers no bene®t over dry feeding (see
Introduction). However, the concept of wet feeding
has been different in the past to that suggested from the
results of our work, which is to use much more water,
per kilogram of food, than previously used. The
addition of too little water results in no bene®cial
effect; too much and birds are reluctant to put their
beaks through the layer of water on top in order to
reach food beneath. The appropriate consistency is
described as `porridgy' and this is achieved by
different amounts of added water for foods with
different ingredients and in different physical forms.
Robinson (1948), writing about ``Consistency of
Wet Mash'' (p. 325), stated that ``Although it is
customary to speak of wet mash, the mash should be
fed in a crumbly-moist condition. Only a small
quantity of water should be added to the dry meal ±
just suf®cient to make it hold together when pressed
between the hands. It should break up when thrown to
the ground.'' However, he was presumably talking
here about ¯oor-feeding as he goes on to say ``This
is the correct consistency for all wet mashes except
those used for trough-feeding and cramming, which
should be mixed to the consistency of thick cream.''
This latter consistency is similar to the `porridgy'
consistency recommended by Yalda and Forbes
(1995) that was achieved in wet foods ranging from
640 to 723 g waterykg.
Addition of up to 0.75 parts by weight of water per
part of air-dry food to give a water content of about
450 gykg did not affect growth or intake in a consistent
manner. Thus the effect of water addition begins
somewhere between 0.75 and 1.5 parts of water per
Wet Foods for Poultry 3
ÐÐÐÐÐÐ
1
Note that in this review, water content of the diet
is quoted where it is known, otherwise the weight
of water added to a kg of air-dry food is stated.

part of air dry food (see below, section on in vitro
solubilisation). The upper limit to water addition is that
which results in a layer of free water on top of the food
discouraging feeding. For typical foods this occurs
somewhere between 2.25 and 3.0 but is lower (1.6 ±
2.0) for some other commercial poultry foods, espe-
cially where fat has been added (A.Y. Yalda and J.M.
Forbes, unpublished results).
The effects of adding different levels of water to the
diet or restricting intake of diets to which water was
added were included in a study with hard and soft
types of wheat (Scott and Silversides, 2003). In
addition to the dry-fed control, water was added at
0.6 or 1.2 gyg air dry food. The higher inclusion of
water signi®cantly increased body weight gain
compared to all other treatments while the lower
inclusion gave gains intermediate between the high-
water and the dry-fed control. The effects, both on gain
and intake were much greater with diets based on the
Hard Red Spring (HRS) wheat cultivar than diets
based on the Durum wheat cultivar. The authors
observed that the 0.6 diet was ``crumbly compared to
porridge-like for WET1.2 diets'' and approximately
50% more water would be needed with pelleted wheat
to obtain the same consistency of diets that contained
unpelleted wheat. The lower level of water inclusion
was not limiting to water consumption because all
birds had access to drinking water. It seems more
likely that extra time was required for the feed with
the lower addition of water to become hydrated in the
gut and facilitate digestion and passage through the
gut.
2.3. Food composition
The advantages of increased retention of DM and
protein with wet feeding can either be exploited by
feeding conventional diets and expecting to see
improved feed conversion ef®ciency, or by reducing
the cost of the diet by reducing its content of digestible
nutrients. Dilution of a standard broiler grower food
with 0.4 corn¯our (almost pure carbohydrate) signi®-
cantly reduced growth when given in the dry form. By
contrast, when given in the wet form (650 g waterykg)
food intake and growth increased signi®cantly,
compared to the dry fed controls (Yalda and Forbes,
1996b). Wetting signi®cantly increased the apparent
retention of dry matter and protein while dilution with
corn¯our signi®cantly reduced protein retention.
Wetting the corn¯our diet increased carcass weight
from 1249 to 1771 g and feed conversion improved
from 0.45 to 0.55 kgykg. The potential therefore exists
for reducing the protein content of feeds to be fed in
wet systems and this warrants further investigation.
2.3.1 Cereal-based foods
Wet feeding improved utilisation of nutrients,
including protein and suggests that, compared to dry
feeding, higher proportions of cereals could be used.
Food intakes and body weights of broiler chickens
given cereal based-foods have been markedly
improved by addition of 1.3 kg water per 1.0 kg food
(Yasar and Forbes, 1999). Chicks were fed wheat-,
barley- or oat-based food (61 ± 63% cereal in the
starter food, 66 ± 70% in the grower food) in the dry
or wet form from 7 to 42 d. Wetting food signi®cantly
increased food intake, total water intake and body
weight gain and ef®ciency of food utilisation was
similar in all treatments (Table 1). Dry matter retention
of food tended to increase in birds given wet food
compared to the dry fed birds but only up to 21 d.
Providing wet feed also affected gut morphology.
Compared with birds on dry rations there was a
4 J. Michael Forbes
Table 1 Effects of wetting feeds, based on wheat, barley or oats respectively, on food intake, weight gain, dry matter (DM) retention,
crypt cell proliferation rate (CCPR) and digesta viscosity (Yasar and Forbes, 1999, experiment 2)
Treatment Dry Wet
Wheat Barley Oats Wheat Barley Oats
Food intake 28 ±42d, g/d 121.3
ab
134.5
ac
108.6
b
143.6
c
184.9
d
173.5
d
Weight gain 28 ± 42d, g 390.5
ab
375.8
ab
329.2
a
444.3
bc
538.4
c
563.8
d
Feed ef®ciency 28 ± 42d, g/g 0.46 0.39 0.43 0.44 0.42 0.46
DM retention 28 ± 42d, g/kg 806.6
a
735.7
ab
685.7
bc
754.2
b
759.3
ab
660.1
c
Water intake, g/d 103.5
a
131.3
b
126.3
b
28.4
c
52.6
d
27.8
c
CCPR, dividing cells/crypt/2 h, middle of small intestine 44.41
a
39.54
b
34.98
b
29.30
c
23.10
d
23.30
d
Viscosity, middle of small intestine, cPs 7.45
a
4.84
b
3.74
b
3.33
b
2.76
b
2.36
b
Means in any one row with the same superscript letter do not differ signi®cantly (P  0.05).

signi®cantly thicker gland layer in proventriculus and
gizzard and also an increased villus height in the small
intestine, caecae and colon. Furthermore, there was a
signi®cantly (P50:05) reduced crypt cell proliferation
rate (CCPR) throughout the digestive tract of birds on
the wet feed (Yasar and Forbes, 1999). This was
associated with a signi®cant decrease in the mean
viscosity of gut contents and concentration of volatile
fatty acids (VFA) in the caecae. Reduction in the rate
of division of gut epithelial cells is likely to have been
due to the reduced viscosity of digesta in wet-fed birds,
which presumably caused less physical damage to the
villi. Lower CCPR reduces the nutrient cost of main-
taining the integrity of the gut thereby releasing more
nutrients for body growth and supporting more ef®-
cient conversion. The increased villus height with wet
feeding might be via high VFA production in the gut
but there was no information on microbial populations
of the gut with wet and dry feeding.
2.3.2 Wheat variety
Wetting of a food (1.2 gyg air dry food) that included
80% wheat and a commercial wheat enzyme and
developed as a test diet for Apparent Metabolisable
Energy (AME) determination, increased feed intake
and weight gain to 17 d by almost 20% (Scott, 2002).
Feed conversion ef®ciency (FCE) was reduced as
compared to the dry-fed birds but there was no effect
on AME. Of the two types of wheat used, the intake of
Durum-based food was increased much less by wet
feeding than that of HRS-based diets, an effect also
observed by Scott and Silversides (2003). A second
experiment by Scott (2002) included several varieties
of the two types of wheat in which wet feeding
increased weight gain, feed intake and feed conversion
and reduced digesta viscosity (Table 2). The AMEs of
wet foods were lower than the equivalent dry food and
it was suggested that this was due to a shorter
residence time in the digestive tract as a result of the
higher food intake.
Once again feed intake of HRS-based food was
increased to a greater extent with wet feeding than of
Durum-based food, and was associated with a greater
reduction in digesta viscosity with HRS diet. The
reduction in AME due to wetting was greater with
HRS. Feed intake and weight gain were signi®cantly
increased with enzyme supplementation of dry diets
but not wet diets. The decrease in digesta viscosity
with wet feeding was smaller than that caused by
enzyme supplementation of wet or dry diets. There
was improvement in AME of diets with enzyme
supplementation of dry diets, whereas supplementation
had no effect on AME of wet diets.
Scott (2002) was of the opinion that these results
``strongly support the hypothesis that water hydration
rate andyor capacity is important in determining feed
intake''. Type of wheat in¯uences the ability to absorb
water and might be a major reason for the differences
in results between different experiments. Thus, wetting
Durum wheat-based food gave similar increase in food
intake and weight gain, whereas with HRS there was a
proportionately greater increase in intake. The latter
has ``very high water absorption potential'' (Canadian
Wheat Board, 2001). Particle size after grinding may
be important with respect to water absorption rate and
Durum wheat, considered to be the hardest wheat,
would be expected to produce larger particles with
grinding.
Although wet feed and enzymes both lowered
digesta viscosity, wet feeding had a much greater
positive effect on feed intake. This challenges the
Wet Foods for Poultry 5
Table 2 Effect of wet versus dry feeding of wheat-based foods (means for Durum and HRS wheat) on para-
meters of broiler chickens from 0 to 17 days (Experiment 1) and 0 to 21 days (Experiment 2) (Scott, 2002)
Experiment 1 Experiment 2
Dry Wet Dry Wet
Weight gain, g 449
a
523
b
591
a
682
b
Food intake, g/b/d 34.2
a
41.5
b
35.7
a
47.2
b
FCR, g/g 1.3
a
1.35
b
1.47
a
1.67
b
Digesta viscosity, cPs 9.9
c
6.0
d
AME of diet, kcal/kg 3030 2960 2860
a
2510
b
ME intake, kcal/b/d 103.5
a
122.1
b
101.7
a
114.9
b
Water intake, ml/bird/d 71.6 77 70.2
a
94.3
a
feed/water, g feed: ml water 0.49
a
0.55
b
0.52 0.51
Means in any one row and experiment with the same or no superscript letter do not differ signi®cantly
(
a,b
, P50.01;
c,d
, P50.05, ANOVA).

theory that broiler chickens are capable of consuming
adequate amounts of wheat-based diets to attain the
required nutrient intake to satisfy their genetic poten-
tial for growth; growth rate of broilers on all wheat-
based diets was increased by wet feeding, suggesting
that feeding wheat-based diets in a dry form is limiting
to feed intake (Scott and Silversides, 2003).
2.4. Enzymes
Enzymes are commonly added to broiler diets to
enhance the digestion of carbohydrates and protein.
One of their effects is to reduce the viscosity of digesta
(Bedford and Schulze, 1998), another may be to reduce
CCPR (Silva and Smithard, 2002). As these are also
effects of wet feeding is it important to know whether
or not their effects are additive. In a factorial experi-
ment with wet and dry food, with or without added b-
glucanase, there were signi®cant positive effects on
growth and ef®ciency of broilers due to both enzyme
and wetting, with no signi®cant interactions, i.e. the
effects were additive (Yalda and Forbes, 1996b). FCE
was improved from 0.52 to 0.57 by wetting while
addition of enzyme to the wet food further improved
FCE to 0.59. Water and enzyme addition to food both
increased metabolisability, food intake and growth and
both reduced the viscosity of digesta. However, with
foods high in wheat (Scott, 2002; Scott and
Silversides, 2003) the enzyme did not always increase
food intake despite reducing digesta viscosity. Feed
intake was increased so much more with wet feeding
than with enzyme addition that it can be concluded that
the effect of the former was not solely because of
activation of endogenous wheat enzymes.
Both water-soaking and enzyme treatment reduce
the viscosity of viscous pentosans (Pawlik et al.,
1990). Wetting and enzyme supplementation (xyla-
nase, b -glucanase and protease activity, at the
commercially-recommended level of inclusion) of
wheat (type unspeci®ed) grain-based foods both
caused great changes in the gut, with larger villi and
reduced CCPR in the epithelium suggesting reduced
endogenous loses of material (Yasar and Forbes,
2000). However, this did not lead to improvements
in FCE although food intake (Figure 1a) and body
weight gain were increased by wetting. In this case
the enzyme had no signi®cant effect on growth or food
intake which suggests that the mode of action of wet
feeding was not primarily through its effects on
viscosity and CCPR. Figure 1b shows that either
enzyme inclusion, or wetting, or both, reduced the
mean viscosity of digesta in samples taken from six
sites along the digestive tract to similar extents,
compared to dry food without enzyme. Similarly,
either wet feeding or enzyme addition reduced CCPR
but the combination had no further additive effect.
(Silva and Smithard, 2002) demonstrated that CCPR
was signi®cantly reduced by addition of a xylanase
enzyme to a rye-based diet, but only at levels much
higher than those used commercially.
The soluble fraction of mixed linked b-glucans
decreased after 22 hours of soaking whole barley, as
did viscosity but steaming to inhibit endogenous
enzyme activity did not affect this decrease (Svihus
et al., 1996). Therefore, it may be that microbial
growth in wet feed is responsible for the enzyme
reduction of viscosity. Soaking at 0

C, again designed
to inhibit the activity of endogenous enzymes, did not
reduce b-glucan as much as soaking at 22

C, but not
the viscosity of an acid extract (Svihus et al., 1997b).
b-glucan content and viscosity reduced even further in
the presence of exogenous enzyme (Table 2).
2.5. Level of feeding
It would be expected that the longer the time spent in
the gut by digesta the greater the nutrients absorbed
and the smaller the bene®cial effect of wetting the
food. This is borne out by the results of two experi-
ments (Yalda and Forbes, 1996b) in which food was
given at restricted, equal DM amounts in the wet or dry
form. The effect of wetting on metabolisability was
less with feed restriction than with ad libitum feeding.
Lower intake produces a slower rate of passage, with
more time for digestion and an anticipated reduced
effect of wetting. However, when food intake is
restricted there are periods when birds are without
food so that when fresh food is given there is an
intensive feeding period (Savory, 1988). Such large
intakes in short period of time is likely to be followed
by increased rate of passage. Yalda and Forbes (Yalda
and Forbes, 1996b) demonstrated, with 28 ± 49-day
old female broilers given commercial grower pellets,
that 95% of the normal 24 hour intake was eaten
during 8 hours of each experimental day. Thus, its
rate of passage through the digestive tract would be
more rapid than normal, giving less opportunity for
digestion to proceed towards completion. This would
give a lower metabolisability than for ad libitum wet-
fed birds whose intake was more evenly distributed
6 J. Michael Forbes

Wet Foods for Poultry 7
Fig. 1a Food intake (g Dry Matter/day) of broilers fed on wheat-based food given in dry (- ----) or wet ( ) form with (j) or without
(h) enzyme. SEM 1.72 (Yasar and Forbes, 2000)
Fig. 1b Digesta viscosity of the gut content (mean of 6 segments) in broiler chickens fed on wheat-based food given in dry (- ----)orwet
(
) form with (j) or without (h) enzyme. SEM 0.18 (Yasar and Forbes, 2000)

throughout the 23 hour light period. Metabolisability
was 630, 670, 720 gykg for dry, wet restricted and wet
ad libitum, respectively, but these differences were not
signi®cant.
Mixing ground food with water just before force-
feeding in the true metabolisable energy (TME) test
(Lessire, 1990) had no effect on nitrogen balance or
TME value. However, the amount of food given is
small in relation to the test birds' requirements and
would pass slowly through the digestive tract; an effect
on the proportion of food retained in the body would
not, therefore, be expected.
Scott and Silversides (2003) restricted wet feeding to
a level 1.2 times the DM intake of the dry-fed control
birds to see whether reducing intake of wet food
would, by slowing rate of passage of digesta, increase
metabolisability. AME and FCE were close to those
for ad libitum feeding (Table 3), even though the
intake of air dry food was signi®cantly less than with
ad libitum feeding of the ``WET1.2'' diet. Again, it is
likely that there was a faster rate of passage due to wet
feeding per se and there is a need to measure rate of
passage of digesta in experiments of this type.
Much greater improvements in growth and food
intake have been observed with enzyme inclusion in
the diet of broilers than in slower-growing layer
chicks. Furthermore, broilers increase their perfor-
mance much less in response to enzyme inclusion
when fed at restricted levels rather than ad libitum
(Scott et al., 2001). This supports the proposal that the
utilisation of nutrients in full-fed broilers is limited by
the short time spent by digesta in the gut.
2.6. Soaking time
Knowledge of the optimum soaking time for wet
feeding of poultry is critical for the design of a
commercial feeding system. It had seemed possible
that the improvement in retention with wet feeding was
due to activation of natural enzymes in the food. In this
case it would be necessary to soak the food for several
hours in order to obtain the maximum effect by
allowing some pre-digestion, either by activation of
endogenous enzymes or by microbial growth. Soaking
for excessive periods allows mould growth, which is
clearly undesirable. Another proposed mode of action
of wet feeding, that of more rapid penetration of
digestive juices throughout food particles, implies
that the water and food only need to be mixed a
short time before feeding.
Pre-soaking food with 1.6 g wateryg air-dry food for
0, 12 and 24 hours for male broilers increased body
weight gains signi®cantly compared with dry food, the
best results coming from the zero soaking time with
which DM retention was increased signi®cantly from
677 to 714 gykg (Yalda and Forbes, 1996b). Growth
and carcass weight were also only signi®cantly
improved by the shortest soaking time which suggests
that overlong soaking is detrimental to the effects of
wet feeding, perhaps due to fungal growth. In this case
food was mixed and offered once per day so that the
average soaking time was 12 hours longer than the
stated pre-soaking times. Shorter mean soaking times
were achieved by mixing and offering wet food at 8-
hourly intervals for 4 days. Under these conditions DM
retention was signi®cantly increased from 634ykg for
dry food to 659 for that freshly mixed with water and
664 for that soaked for one hour between mixing and
offering (Yalda and Forbes, 1996b).
In another experiment, broilers given food soaked
for a shorter time gained weight signi®cantly faster
than those on food soaked overnight, which in turn
increased body weight gain compared to dry feeding.
The foods were given in equal amounts of DM to all
8 J. Michael Forbes
Table 3 Effect of wet feeding and enzyme addition on b -glucans and viscosity of barley and on weight gain, food intake,
feed conversion ef®ciency, illeal digestibility and digestible energy of broiler chickens (Svihus et al., 1997b)
Untreated Enzyme Wetting Enzyme plus wetting
Total b-glucans, g/kg DM 49.3
a
NA 4.50
b
4.44
b
Acid extract viscosity, cps 2.67
a
NA 2.32
a
1.60
b
Weight gain, 14 ± 28d, g 689
a
831
b
758
ab
833
b
Food intake, 14 ± 28d, g DM 1220 1259 1204 1283
FCE, g gain/g DM intake 0.56
a
0.66
b
0.63
b
0.65
b
Ileal DM digestibility, g/g 0.53
a
0.61
ab
0.53
a
0.64
b
Digestible energy, MJ/kg DM 15.68
a
16.88
b
15.53
a
16.57
b
Means in any one row and experiment with the same or no superscript letter do not differ signi®cantly (P  0.05, ANOVA.).
NAÐnot applicable

treatments so the FCEs were also signi®cantly
different, in line with the improvements in the propor-
tion of DM absorbed with the two wet treatments
(Yalda and Forbes, 1996b).
When cereals were soaked for 8 hours and then dried
before feeding, growth was signi®cantly improved and
more effectively with rye and barley than with wheat
(Fry et al., 1958). If the cereals were autoclaved
(which would inactivate any endogenous enzymes)
prior to soaking the results were essentially the same.
Therefore, it is not likely to be an enzymatic effect, but
rather a physical effect, probably on the hydration of
carbohydrates.
There is thus no advantage for nutrient retention, and
therefore for growth and ef®ciency, of pre-soaking
when food is to be given in the wet form to broilers;
no provision needs to be made, therefore, for long-term
storage of wet food once it has been mixed.
2.6.1 Germination
The short periods of soaking referred to above are
insuf®cient for germination to start. However, germina-
tion of cereals or legumes consistently improves nutri-
tive value (Finney, 1983) but would demand special
facilities and a great deal of storage space if applied in
large-scale commercial production. Of the many
changes taking place during germination the one of
most relevance in the context of wet feeding is the
release of enzymes such as phytase, amylases, proteases
and phosphorlyases (Cornell and Hoveling, 1998).
Barley grain soaked for 24 hours and then allowed to
germinate for 48 hours improved broiler performance
to a similar extent as did enzyme addition. This was
attributed to lower digesta viscosity in both treatments
caused by endogenous b-glucanase released during
germination (Svihus et al., 1997a). Scott and
Campbell (1998) have found pre-germination of
barley and wheat to improve broiler performance by
increasing voluntary food intake growth, with little
effect on feed ef®ciency. Digesta viscosity was
lowered to levels similar to those with ungerminated
grains treated with enzyme and no further improve-
ment was seen when pre-germinated grains were also
treated with enzyme.
2.7. Viscosity
Addition of guar gum to food for broilers greatly
increased the viscosity of digesta. Wetting such food
resulted in a signi®cant reduction in the viscosity of
digesta and improved body weight gain in broiler
chickens (Yasar and Forbes, 1997). It is likely that
this reduction of digesta viscosity will reduce CCPR in
the gut epithelia because the feeding of non starch
polysaccharides (NSP) signi®cantly increases CCPR in
the gut of rats (Lynn et al., 1994) and wet feeding
greatly reduces CCPR in broilers (Yasar et al., 1996).
2.8. Hydration and in vitro solubility
Food carbohydrates bind water chemically, to a vari-
able extent, and their 3D structure provides interstices
in which water becomes entrapped (Whistler and
BeMiller, 1997). The extent of water hydration of
wheat ¯our varies according to the type of wheat,
growing conditions, agronomic practices, grinding and
processing (Grant, 1998). Scott (2002) proposed that
these differences are responsible, in part at least, for
the different effects of wet feeding on food intake and
FCE between experiments. Moreover it was suggested
that measurements of diet solubility or hydration rate
should be used in the assessment of the feed value of
different batches of cereals.
The effect of added water on the solubilisation of
dietary components was studied by incorporating
water into a commercial broiler feed to give water
contents ranging from 240 to 700 g waterykg food
(Yasar and Forbes, 2001). To these was added pepsin
and HCl to mimic the gastric environment, and the
samples were incubated for between 10 and 240
minutes at 42

C before being ®ltered to measure the
proportion of DM solubilised. Solubility was similar
for foods containing 240 and 340 gykg water as for dry
feed (80 g waterykg) but was signi®cantly increased in
samples with 540, 580, 630 and 700 g waterykg
(Figure 2). Maximum solubilisation occurred within
20 minutes in the food with 700 g waterykg, while that
of the other foods increased throughout the 6 hour
incubation period.
Further support for the more rapid solubilisation of
food when pre-wetted comes from the use of an inert
marker incorporated in the food and measured in
digesta from several sites in the digestive tract after
killing (Yasar, 1998). Given the very rapid transit of
food, particularly in broilers, this early solubilisation
gives more time for absorption to take place. This
allows the actual digestibility of the food to approach
more closely the potential digestibility that would be
achieved if the food stayed longer in the tract.
Wet Foods for Poultry 9

2.9. Wet by-products
Historically, some ingredients such as milk, whey and
butter milk were fed to poultry in a liquid form or
mixed with ingredients such as wheat shorts or bran
and fed to laying hens. Attempts to use alternative
foodstuffs in the form of Methane Digester Ef¯uent
(Caldwell et al., 1986) or Alfalfa Juice Protein
Concentrate (Tsiagbe et al., 1987) were not very
promising. Birds compensated for the moisture levels
by increasing total intake and therefore maintaining
dry matter intake with very little improvement, if any,
in productive performance.
Recent studies of wetting food with water or whey F.
Shariatmadari and J.M. Forbes (unpublished observa-
tions) have con®rmed the bene®cial effects of adding
water. However, whey is deleterious if it is given in a
way in which the birds cannot avoid an excess intake
of lactose. Thus, offering whey as the drink accom-
panying dry food greatly depressed DM intake and
growth. By contrast, when incorporated in wet food, in
which the intake of whey was lower, there was no
deleterious effect. The older the birds, the better they
coped with whey. It was concluded that whey can be
used in diets for broiler chickens by incorporating it in
the food as long as drinking water is offered ad
libitum. Whey may be offered as a drink if the food
is mixed with 1.8 times its weight of water but it is
better to dilute the whey with an equal volume of water
whether it is added to food or given as drink. Good
results can also be obtained when undiluted whey is
offered alternately with water, either in half-day or
full-day periods. A lactase enzyme could be used but
the galactose produced is also toxic if absorbed in too
large amounts. There is also a potential problem with
the amount of sodium in whey.
2.10. Age of bird
Despite the obvious advantages wet feeding has been
shown to offer intensive poultry producers, its applica-
tion has been effectively prohibited by the consider-
able effort and expense involved in developing and
implementing systems for automatic mixing and
delivery of a wet ration to hopper fed birds.
However, the current commercial practice of ¯oor
feeding chicks for the ®rst ten days of life gives rise
to the question: Would chicks fed a high moisture
content mash during this initial period show the
previously reported performance advantages? If so,
to what extent would these persist following the
change to a conventional dry diet?
Previous work has shown particular bene®ts from
wet feeding with poor quality feeds so, in order to
improve the chances of showing clear results, the
experiments reported here use a relatively low speci-
®cation feed (Slade and Forbes, 1997). A commercial
starter crumb was offered from 1 ± 10 days after
10 J. Michael Forbes
Fig. 2 Solubilisation of a compound food mixed with different volumes of a pepsin/HCl solution for different lengths of time (Yasar and
Forbes, 2001). Water content of food: d, 80, m, 530, r, 630, j, 700 g ¯uid per kg.

hatching in the dry or wet form, followed by dry food
up to 21 days. The wet-fed birds gained signi®cantly
more ef®ciently during the ®rst 10 days and still had
signi®cantly heavier and carcass weights at 21 days.
There was a signi®cant reduction in apparent meta-
bolisability as broiler chickens aged from 28 to 42 days
after hatching (Yalda and Forbes, 1996b). Given the
generally negative relationship between the viscosity
of digesta and digestive ef®ciency (Bedford, 1993) it
might be supposed that the decline in digestibility
would be accompanied by an increased in viscosity.
However, it is observed that gut viscosity decreases as
broiler chickens aged (Petersen et al., 1999), especially
so when the food is dry and does not contain added
enzyme (Yasar and Forbes, 2000). We are effectively
left with no clear explanation of the decline in digest-
ibility with age.
Weight gain of broiler chicks from 3 to 19 days of
age was signi®cantly greater when the food was wet
(2.0 kg waterykg diet), without a signi®cant effect on
DM intake, giving a 13% improvement in FCE (A.Y.
Yalda and J.M. Forbes, unpublished results). When
wet feeding was started at 7, 11 or 15 days, weight
gains were intermediate (Table 4). Feed intake
(expressed as air-dry feed) was not signi®cantly
affected by treatment but tended to be higher with
the wet feeds, particularly at the end of the experiment.
Carcass weight was signi®cantly increased by feeding
wet diets, that for chicks on wet feed from 3 days being
20% greater than dry-fed controls, but there was no
effect of wetting on the weight of abdominal fat. These
results show that growth is improved when the feed is
given in the wet form from 3 days of age, to a similar
extent to that previously shown with older broilers.
The bene®cial effects of wet feeding from 28 to 42
days persisted for two weeks after return to dry
feeding, i.e. to 56 days (Yalda and Forbes, 1995).
The increased crop and intestine weight tended to
persist and presumably allowed the improvement in
food utilisation to continue even after dry feeding was
resumed. This is important from a commercial point of
view as periods of dry feeding, for example when
equipment breaks down, will not be detrimental.
2.11. Water source ± bowls versus drinkers
In many of the studies of effects of wet feeding using
individually-caged birds in our laboratory have
provided drinking water from nipple drinkers rather
than from an open water source. There was a suspicion
that birds might experience some dif®culty in drinking
from the nipples. Restricted access to water reduces
food intake and growth (Kellerup et al., 1965) and low
water intake from nipples might account for some of
the bene®cial effect of wetting the food observed in the
previous experiments. Clearly there is a general bene®t
from wet feeding, independent of type of drinker, as it
is seen in the absence of drinking water (Yalda and
Forbes, 1995) as long as the food includes suf®cient
water.
In a factorial experiment with dry or wet (1.8 kg
water per kg of air-dry feed) food and water provided
from nipples or bowls (G.D. Amos and J.M. Forbes,
unpublished results), it was observed that dry-fed
broilers with bowls were eating and drinking signi®-
cantly greater quantities than dry-fed birds with
nipples (P50:05). With wet-fed birds there was no
difference in food intake between the two modes of
access to water. It was concluded that the nipple
dispensers used in this investigation were indeed
restricting water intake, and that this restriction is
causing a secondary decrease in DM feed intake.
Wet feeds remain a possible solution to the problem
of ensuring that water requirements of broilers are met,
Wet Foods for Poultry 11
Table 4 Effect of two levels of addition of water to broiler foods based on two types of wheat (HRS and Durum), offered ad libitum
or restricted to 1.2 times the dry matter intake of dry-fed birds (Scott and Silversides, 2003)
Treatment
1
DRY WET1.2 WET0.6 RWET1.2
wt gain, 1 to 21 days, g Overall mean 656
d
751
a
702
b
681
c
intake, g/bird/day Overall mean 39.8 48.6 45.3 43.2
Durum 38.6
e
43.2
c
42.3
c
40.7
cde
HRS 41.4
cd
54.8
a
46.7
b
46.0
b
Mix of durum and HRS 39.5
de
47.7
b
46.9
b
42.7
c
Feed Conversion Ratio 0 to 21d Overall mean 1.42 1.49 1.51 1.48
AME, d 20, kcal/kg diet Overall mean 3000 2730 2800 2700
1
DRY, air dry; WET1.2, DRY with 1.2 g water g
ÿ 1
feed; WET0.6, DRY with 0.6 g water g
ÿ 1
feed; and RWET1.2, WET1.2 diet
treatment restrictedÐfed to 120% of the previous day's DRY feed intake (on an air-dried basis) for each diet type.
Means in any one row with the same or no superscript letter do not differ signi®cantly (P  0.05, ANOVA).

but the use of pressure-regulated nipple dispensing
units, so long as they are providing a consistently
desirable nipple ¯ow rate, may offer a similarly
effective and far more economic alternative.
Although free water intake is usually signi®cantly
reduced in birds given wet food, total water intakes
from the water bottle plus that from food were
signi®cantly higher in wet-fed birds than dry-fed
birds (Yasar and Forbes, 1999). However, DM intake
was higher with wet feeding and the ratio of total water
to dry food intake was similar in both feeding regi-
mens. Total water intake was higher for barley-based
food than for those including wheat or oats, probably
the former is usually higher in NSPs which bind much
water. By contrast, other studies (A.Y. Yalda and J.M.
Forbes, unpublished results) have shown signi®cantly
higher ratios of total water intake to food DM intake.
2.12. Mould inhibitor
Although it is usually several days before wet food
shows obvious signs of surface mould growth at room
temperature, it has been suggested that the addition of
a mould inhibitor to wet food for poultry might be
bene®cial. Scott (2002) included a propionic acid
treatment in order to establish if a mould inhibitor
was necessary. The wet diets were prepared daily so it
is not surprising that there were no signi®cant main
effects or interactions with effects of other treatments
(type of wheat, addition of wheat enzyme). Inclusion
of a mould inhibitor may become necessary if wet diets
were not to be changed frequently or if food troughs
are not cleaned regularly.
2.13. Preference for wet food
Animals given a choice of two foods with different
nutritional properties and different in at least one
sensory property, learn to eat that mixture of the two
that best matches their nutrition requirements (Forbes,
1995). Thus, ``palatability'' of a food depends not
simply on the sensory properties of the food, but also
on the animal and its physiological state, and previous
interactions between the animal and food(s). In obser-
ving birds' preferences for wet food, when given in
choice with dry food, it is necessary to monitor the
situation for many days, not just for a few minutes. In
the absence of drinking water it is obvious that birds
will prefer wet food to dry. This was con®rmed by
observations that individually-caged growing cock-
erels of an egg-laying strain with previous experience
of only dry food ate more dry food than wet when
drinking water was available but more wet food than
dry when water was not offered separately (A.Y. Yalda
and J.M. Forbes, unpublished observations).
A more detailed study involved groups of day-old
cockerels given dry or wet food for a week, with
drinking water available throughout. In the following
week, when given a choice of foods, they initially
ate more of the food to which they were accustomed
but gradually converged so that after 5 days they
were eating approximately equal amounts of dry
matter from the two foods. Other groups given wet
and dry foods on alternate days in the ®rst week
chose equal amounts of the two foods in the second
week (A.Y. Yalda and J.M. Forbes, unpublished
observations).
A third study involved individually-caged 3 ± 6
week-old female broiler chicks offered ground wheat
and a pelleted grower food, each in the dry or wet form
(J.M. Forbes, A.Y. Yalda, and M. Covasa, unpublished
results). Intake of wheat declined from about 20% to
10% of the total except that those given a choice of dry
pellets and wet wheat ate signi®cantly more wheat and
less pellets than the birds on the other three treatments
in the second and third week, the ratio decreasing in
these latter treatments as the experiment progressed.
There was no effect of treatment on weight gain,
ef®ciency of food utilisation or weight of the abdom-
inal fat pad. Wide individual variation was noted in
diet selection between birds on any one treatment. It is
concluded that giving food in the wet form does not
lead to strong preferences or aversions but can lead to
increased intake of cereals.
2.14. Fermentation
Wet feeding of pigs is often done is such a way as to
encourage the growth of lactobacilli whose production
of lactic acid reduces the pH of food to around 3.5.
Fermented diets improve growth performance of pigs,
compared with non-fermented diets. Gastric pH is
reduced and there are fewer coliform bacteria in the
gastrointestinal tract with some indications that
fermented diets may positively affect pancreatic secre-
tion, villus architecture, digestibility and absorption of
dietary nutrients (Scholten et al., 1999).
Wet feeding of poultry, at least as practised by us,
does not encourage fermentation and, indeed,
prolonged soaking, such as might allow fermentation
12 J. Michael Forbes

to occur, is not bene®cial as far as growth and
ef®ciency are concerned (see above).
2.15. Alleviation of heat stress
Heat stress is a limiting factor for broiler production in
tropical and sub-tropical countries and even in summer
in temperate climates when the birds are approaching
slaughter weight and are very densely stocked.
It was the opinion of Robinson (1948, p. 233) that if
appetite ¯ags in hot weather then ``a small feed of wet
mash at midday will be much relished, while the birds
will often consume greedily grain that has been soaked
in water for twenty-four hours.''
Tadtiyanant et al. (1987) found that feeding hens a
diet containing 50% moisture under environmental
temperatures of 21.1

C or 29.4

C did not improve
laying performance. However, in a later study
(Tadtiyanant et al., 1991) reported that feeding hens
wet food (50% moisture) stimulated dry matter intake
at 33.3

C by 38%. Presumably the temperatures used
in the former experiment were not suf®ciently high to
depress food intake.
Offering broiler chickens a food with the addition of
33 or 50% water increased both food intake and body
weight in the hot season in Nigeria (37

C). However,
food intake and body weight were reduced signi®-
cantly by wet feeding under normal temperatures
(20

C) (Abasiekong, 1989), but it is unclear as to
why this should be.
In two studies in which broilers were heat stressed
(35 to 37

C) for 8 hours per day wet feeding
(1.2 kgykg air dry food, Coskun and Kutlu, 1997;
1.5 kgykg air dry food, Kutlu, 2001) increased DM
intake and abdominal fat signi®cantly, but depressed
DM conversion ef®ciency as the increase in body
weight gain was not signi®cant.
Shariatmadari (2001) has reported that, when
compared with dry feed, wet feeding signi®cantly
improved broiler performance at high environmental
temperatures, irrespective of whether drinking water
was available. At the extreme temperature of 36

C,
however, wet feeding did not completely overcome the
depression in food intake and growth. Shariatmadari
suggested that wet feeding could be a useful technique
for overcoming some of the adverse effects of high
environmental temperatures on feed intake and growth
of broiler chickens.
Ascites and ``sudden death'' have been associated
with rapid growth and it might be expected that when
growth is increased by wet feeding there would be
increased mortality. This has not been the case,
however, (T.A. Scott, personal communication; J.M.
Forbes and colleagues, unpublished results) and it may
be that the release of the heat of hydration outside
rather than inside the body assists in this respect.
The bene®t of wet feeding in situations of heat stress
is therefore variable and further work will be necessary
to establish the optimum ways in which to add water to
food in hot climates.
3. WET FEEDING TO LAYING HENS
Laying hens fed a diet containing methane digester
ef¯uent to give 400 g waterykg had egg production and
food ef®ciency similar to those hens fed an air-dried
control diet (Vandepopuliere and Lyons, 1983).
However, Caldwell et al. (1986) found that egg
production was decreased when hens were fed diets
over 300 g waterykg by the addition of methane
digester ef¯uent. The suppression in performance in
this case was attributed to fungal growth but it is not
clear for how long the wet foods were kept before they
were fed.
An automated feeding system has been developed to
deliver high moisture by-product diets with 50% added
water to laying hens (Thorne et al., 1989) and
improvements in egg production, egg weight and
food ef®ciency have been found.
With both conventional (11.0 MJ MEykg) and low
energy (10.0 MJ MEykg) foods hens given wet food
(1.8 kg waterykg) laid many more eggs than dry-fed
controls (Table 6; El Kaseh and Forbes, 1995). With
the higher-quality food there was not a signi®cant
increase in food intake demonstrating a large increase
in ef®ciency and showing that the bene®t of wet
feeding was not due to lifting a restriction on water
intake from the nipple drinkers. With the lower-quality
food there was a signi®cant increase in food intake and
in ef®ciency. Also in a second experiment, shell
colour, yolk colour and albumen height were signi®-
cantly improved by wet feeding and there was
increased metabolisability of DM (569 versus
600 gykg, NS) and protein (500 versus
565 gykg,P50:05). Egg production was much more
ef®cient, in terms of food utilisation, when the food
was presented in the wet form, compared to conven-
tional dry feeding of the same food, although this was
only signi®cant in the ®rst experiment. This large
improvement in food utilisation was achieved with
Wet Foods for Poultry 13

no effect on egg size and a signi®cant improvement in
egg quality.
Further work is required to characterise the optimum
conditions for wet feeding of laying hens as the birds
used in this work were initially laying at a low level
and were not representative of hens kept under
commercial conditions. It will also be interesting to
see the response of pullets to wet feeding.
4. WET FEEDING TO DUCKS
It has been generally considered that dry pellets are
better than wet mash for growing ducks (Elkin, 1986)
but Roberts (1934) found moist mash to be better than
dry pellets. However, the results were very variable,
some birds were killed by rats and no proper statistical
analysis was performed although it was claimed that
there were ``probably signi®cant differences'' in some
cases.
Two comparisons of dry and wet (1.8 kgykg) feeding
of commercial Pekin-type ducks have been carried out,
one with a commercial pelleted duck grower food
(11.7 MJ MEykg and 190 g proteinykg) given from
21 to 42 days of age, the other with a lower energy
food (10.8 MJ MEykg and 165 g proteinykg) given
from 25 to 46 days (Yalda and Forbes, 1996a). There
was a clear effect of wet food on body weight
throughout both experiments so that by slaughter the
birds on wet food had gained 12% and 9% more than
those on dry food. Ef®ciency of utilisation of food was
signi®cantly improved from 0.35 to 0.38 in experiment
1 and from 0.32 to 0.38 in experiment 2. It was
intended to measure retention of nutrients in these
experiments but the excreta were liquid and could
not be collected completely. Despite this there was
no soiling of feathers in these individually-penned
birds, but the risk must be noted in any consideration
of commercial use of wet feeding.
5. PRACTICAL APPLICATION
Attempts to utilise wet diets with conventional ingre-
dients or under commercial conditions have been few
(Valarezo and Perez, 1970, 1972). The consistent
improvements in food intake andyor FCE of broilers
in the experiments detailed above would, if seen in
commercial practice, provide great bene®ts to the
poultry industry without compromising the welfare of
the birds.
Under semi-commercial conditions six pens each of
110 broiler chicks were offered a standard pelleted
grower food (ME, 13.2 MJykg; crude protein,
209 gykg; crude ®bre, 27 gykg) from 11 to 41 days
post-hatching; food from the same batch was mixed
with 1.6 times its weight of water, left overnight and
then fed to a further six pens (Yalda et al., 1995). The
third feed was the same formulation but left unpelleted
and mixed with 1.2 times its weight of water the day
before being offered to the remaining six pens (this
was the amount of water necessary to achieve the
required porridgy consistency). In both the wet-fed
treatments there were signi®cantly lower intakes of dry
matter compared to the dry control (Table 5).
However, by day 34 both wet-fed groups were signi®-
cantly heavier than those on dry feed but at the end of
the experiment only those on wet mash were signi®-
14 J. Michael Forbes
Table 5 Effect of adding 2.0 kg water/kg food on the performance of male broiler chickens starting at different ages (A. Y. Yalda and
J. M. Forbes, unpublished results)
Age at start of wet feeding Dry 3 days 7 days 11 days 15 days SED
Body weight, g/bird 3 days 77 78 77 78 77 2.8
7 days 167
b
181
a
166
a
169
a
161
a
5.8
11 days 293
b
323
a
306
ab
287
b
294
b
9.7
15 days 457
b
510
a
498
ab
482
b
451
b
12.7
19 days 637
b
736
a
700
ab
677
bc
675
bc
24.2
Food intake, g DM/bird/d 3 ± 7 days 27 28 27 27 26 1.3
8 ± 11 days 43 43 45 42 44 1
12 ± 15 days 64 64 68 64 63 4.2
16 ± 19 days 78 86 83 82 87 5.9
Total food intake, g DM*/bird 3 ± 19 days 848 884 889 862 884 16.9
Carcass weight, g/bird 19 days 422
b
508
a
492
a
489
a
483
a
20.3
Abdominal fat, g/bird 19 days 5 5.7 5.7 5.1 5.5 1.4
Means in any one row with the same or no superscript letter do not differ signi®cantly (P  0.05, ANOVA).

cantly heavier than controls. FCE was signi®cantly
improved by both wet feeding treatments while there
were no signi®cant effects of treatment on carcass
weight, composition or abdominal fat.
In a test of the ef®cacy of wet feeding on a full
commercial scale, two broiler houses were divided into
two equal halves and each pen was stocked with 4,250
broilers of mixed sex (A.Y. Yalda, C. Heaton, E.
Heaton, J.M. Forbes, unpublished results). The birds
in one half of each house were given dry food and in
the other half wet food (1.8 kg waterykg air-dry food)
ad libitum from 8 days until slaughter at 46 days. Very
similar amounts of air-dry food were provided to each
pen but there was considerable wastage in the wet-fed
groups, due to the initial dif®culty of designing a
trough suited for wet food in a commercial house.
Body weight gain was signi®cantly improved by the
wet food from day 38 and by day 46 the wet-fed birds
were on average 128 g heavier than the dry-fed.
Subsequent large-scale trials demonstrated the dif®-
culty of managing birds in wet feeding systems. Wet
litter and dirty feathers were problems and the
improvements in ef®ciency, clearly demonstrated
above, were not maintained in these later experiments.
During the period of our studies exogenous enzymes
were increasingly being used in commercial feeds and
it may be that the improvements in ef®ciency due to
enzymes was reducing the ef®cacy of wet feeding.
However, an enzyme included at the commercially-
recommended rate had little effect on digestion or
ef®ciency in one of our experiments (Yasar and
Forbes, 2000).
The perceived advantages, disadvantages and uncer-
tainties of wet feeding on a commercial scale are
summarised in Table 8 and described in more detail
below.
5.1. Practical advantages of wet feeding
In several experiments summarised above, wet feeding
gave more ef®cient conversion of food to weight gain,
thereby reducing cost of production. Alternatively,
when wetted, foods with poorer speci®cation than
those normally used can give growth rates in broilers
and ducks, and laying hens, similar to conventional
foods given in the dry form. Unfortunately, because of
the variable results obtained in experiments it is
dif®cult to predict the improvement in ef®ciency to
be achieved with wet feeding on a commercial scale.
Wet Foods for Poultry 15
Table 6 Performance of laying hens given dry food or the same with 1.8 parts of water per unit of
food (El Kaseh and Forbes, 1995)
Experiment 1 Experiment 2
Food ME content, MJ/kg 11.0 10.0
Treatment Dry Wet Dry Wet
Food consumption, g air dry matter/bird/day 143.7 148.9 148.4
a
164.2
b
Egg production, % 58.6
a
76.0
b
70.9
a
85.2
b
Egg weight, g 68.4 68.4 64.8 65.7
Egg mass, g/bird/day 40.2 51.7 45.2
a
55.8
b
Egg mass/food consumption, g/g) 0.28 0.35 0.30 0.34
Means in any one row and experiment with the same or no superscript letter do not differ signi®cantly
(P  0.05; paired t-test).
Table 7 Effect of wet feeding dry pellets, wet pellets (1.6 g water / g air dry food) and wet mash
(1.2 g/g) on food intake, growth and carcass of broiler chickens under semi-commercial conditions
from 16 to 42 days after hatching (Yalda et al., 1995)
Treatment Dry pellets Wet pellets Wet mash
Feed intake, g DM/bird/day 136
a
126
b
123
b
Water drunk, ml/bird/day 161
a
25
b
31
b
Total water intake, ml/bird/day 161
a
227
b
180
a
Body weight at slaughter, g 2303
a
2356
ab
2367
b
Carcass weight, g 1677 1660 1654
Carcass protein, g/kgDM 511 527 513
Carcass lipid, g/kgDM 361 360 343
Means in any one row with the same or no superscript letter do not differ signi®cantly (P  0.05, ANOVA).

Although the dust from the bedding in deep litter
houses is not eliminated by wet feeding, there is no
dust from the food. Houses in which wet feeding is
being practised are noticeably less dusty than conven-
tional houses, to the bene®t of both birds and staff.
Pelleting, normally used for broiler feeds, involves
transport of raw materials to a feed mill as well as the
costs of operating the pelleter itself. There is no point
in pelleting a food if it is then to be mixed with water,
which breaks down the structure of the pellets. The
savings are thus considerable, especially if home-
grown cereals are to be used in the formulation of
poultry rations with a saving in transport as well as
processing costs.
Broiler production normally involves three different
foods: starter, grower and ®nisher. Much of the time,
therefore, the ever-changing nutrient requirements of
the birds are not being met exactly. Wet feeding
involves mixing of ingredients close to the site of
feeding, allowing the formulation to be changed far
more frequently and to be tailored to the needs of the
birds on a daily basis. Locally-produced materials can
be blended with proprietary concentrates on a daily
basis and the content of major nutrients adjusted
according to the birds' calculated requirements based
on growth potential, environmental temperature,
disease, etc. In addition, it becomes possible to add
trace materials, such as amino acids and medicines, on
a daily basis without having to order large quantities of
feed in advance. In other words, the diet can be
tailored to the needs of the system daily, or even
more frequently.
5.2. Disadvantages of wet feeding
The appropriate porridgy consistency of the food is not
always easy to achieve or maintain. Different batches
of food are likely to have different water absorption
capacities and a close watch must be kept for dirty
birds when there is a change of food. Too wet, and the
litter becomes dirty. Extreme wetness can result in a
layer of water over the food through which the birds
are reluctant to feed. Too little water and the digestive
bene®ts of wet feeding are not maximised. Moreover,
the texture of semi-wet food allows more wastage as
the birds can pick up and drop large lumps of food
(Scott and Silversides, 2003) and such a consistency
puts more strain on the machinery delivering the food
into the house
The cost of installing mixing tanks and water-
resistant feeders will be considerable. However, if
new equipment is to be installed anyway, then the
additional cost, over dry feeding, will not be so great.
Suitable mixing and feeding equipment needs to be
developed and used on a large scale to identify and
correct any problems that arise. The machinery and
troughs must be non-rusting and able to handle the
greatly increased weight of food material to be deliv-
ered to the birds. The number of points at which birds
can drink can be considerably reduced in view of the
great reduction in intake of free water by wet-fed birds.
Wetting the food increased the water content of
excreta (Yalda and Forbes, 1996b) which might be
expected to increase the risk of wet litter in a
commercial situation, which can be very detrimental
to cleanliness and welfare (Pattison, 1989). On the
other hand, incorporating corn¯our into the diet
increased excreta DM content so that the wet corn¯our
diet gave excreta which were not much wetter than
those produced from the conventional food given in
the dry form. Also, by increasing the proportion of the
food which is absorbed, wet feeding gives a lower
weight of faecal material than dry feeding of the same
food, so a small increase in water concentration in
excreta does not necessarily increase the total amount
of water excreted onto the litter. Faster growth also
means a shorter growing period, again resulting in a
reduced burden of excreta.
The water content of excreta was also increased by
wet feed in of laying hens but, due to the increase in
metabolisability, almost the same total weight of water
in the excreta, compared to dry feeding (El Kaseh and
Forbes, 1995).
16 J. Michael Forbes
Table 8 Summary of advantages, disadvantages and uncertain-
ties of wet feeding of poultry
Advantages
Improved ef®ciency
Lack of dust
Reduced cost of processing
Optimal formulation of diets
Use of home-grown cerealsÐno pelletingÐ
saving of cost
Disadvantages
Cost of machinery for mixing and delivery
Risk of wet litter
Maintaining the optimum water content
Risk of dirty feathers
Danger of mould and other contamination
Uncertainties
Animal welfare
Patenting and licensing restrictions
Alleviation of heat stress
Lack of research

Wet food is prone to growth of moulds and is likely
to transmit pathogens such as Salmonella more freely
than dry food. Mould inhibitors such as propionic acid
can be used and studies on disease transmission will be
required before wet feeding can be adopted commer-
cially.
Unnecessarily wet food can lead to dirty feathers due
to wet litter and to birds rubbing excess food from their
beaks onto other birds. This can be controlled by
adjusting the water content of the food to maintain
the appropriate consistency of the food so that it is
suf®ciently wet so as not to stick to the beak.
5.3. Uncertainties of wet feeding
The natural feeding behaviour of chickens is to search
for dry seeds and insects on the ground. They drink
from open sources of water such as puddles and,
although drinking usually takes place in association
with feeding, it is a separate activity. Wet food is
unnatural and, given free choice between wet and dry
foods, chickens do not prefer wet food (although they
do not ®nd it aversive). The fact that birds still take
some free water even when they are taking in suf®cient
with their food suggests that drinking is an important
activity.
With wet food poultry cannot perform their well
developed pecking and searching functions. On this
basis it could be suggested that it denies them the
opportunity to engage in a natural activity. However,
day-old chicks take very readily to wet food and it
usually bene®ts growing chickens in terms of faster
growth, or greater yield of nutrients per unit food
intake, or both, which are generally considered to be
signs of a good diet.
The increased risk of wet litter is potentially of
detriment to welfare as it makes it more likely that
birds will suffer hock burn and breast blisters. Also,
the propensity of wet-fed birds to wipe their beaks on
the backs of other birds presumably increases discom-
fort as chickens normally keep themselves clean by
preening their feathers. Wet litter and dirty birds can
be controlled by careful monitoring of the consistency
of the food to ensure that it is not allowed to become
too liquid; the correct porridgy consistency does not
facilitate food spillage or sticking to the beak.
When we initially discovered bene®ts of wet feeding
we assumed that the practice would not be able to be
protected by patents or other means. However, because
claims of increased ef®ciency or productivity due to
wet feeding in large, commercial operations had not
previously been made, we were able to ®le a patent
application for the process (Patent application
PCTyGB92y01134 made by The University of Leeds
on 22 June 1992) but we did not proceed to defend this
application.
Apart from the need to develop new methods of
mixing and delivering feed, the main uncertainty with
wet feeding is the lack of research into this area.
Optimal use of wet feeding will depend on improved
understanding of its mode on action, but whether this
system will replace the classical feeding system of
poultry ®nally depends on economic considerations.
6. CONCLUSION
Despite past teaching, mixing poultry food with suf®-
cient water to produce a porridgy consistency (typi-
cally 1.5 parts of water per part of air dry food) can
increase the ef®ciency of utilisation by broiler
chickens, laying hens and growing ducks. This is
often accompanied by increased voluntary intake of
food, likely to be due to the increased rates of digestion
and passage engendered by more rapid penetration of
digestive juices into the pre-wetted particles of food.
Attempts to make use of these ®ndings in commer-
cial practice in the UK have come up against problems
of wet litter and dirty feathers and it may well be that
wet feeding is better suited to warm, dry climates
where wet litter is less likely to cause problems. In
addition, the ef®cacy of wet feeding appeared to
decrease over a period of a few years, possibly
because of the increased use of exogenous enzymes
in commercial broiler foods. T.A. Scott is of the
opinion that more research is required to establish
how voluntary feed intake relates to water hydration
capacity of the feed and advocates considerations of
practical applications of wet feeding of diets to support
rapid gain and maximise nutrient retention (Scott,
2002; Scott and Silversides, 2003).
ACKNOWLEDGEMENTS
The author is pleased to acknowledge ®nancial assis-
tance by Dalgety Agriculture Ltd for our research
summarised in this review and especially Sotiris
Papasolomontos, Simon Green, Eilir Jones and Jim
Sainsbury. In the University of Leeds particular thanks
are due to Akram Yalda, Sulhattin Yasar and Ed Ruck-
Keene. Tom Scott, Agriculture and Agri-Food Canada
Wet Foods for Poultry 17

(currently University of Sydney), provided invaluable
advice on the manuscript.
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Wet Foods for Poultry 19