Wood ash in livestock nutrition: 2. Different uses of wood ash in animal nutrition

Abstract

Wood ash is a mineral source that is readily available in most subsistence farming communities. The potential use of wood ash as a mineral source in animal nutrition is evaluated and reviewed. The predominant mineral in wood ash is calcium (Ca), and thus a potential Ca source in animal diets. However, the wide, source-dependent variations of Ca and other minerals in wood ash are limiting factors in the extensive use of ash in livestock diets. Ca sources with a less variable mineral composition such as feed lime (CaCO3) and limestone are therefore preferable. This restricts the inclusion of wood ash in diets except where a good estimate of its Ca content is available. Specific situations exist under subsistence farming conditions in which wood ash could be used as a Ca supplement, for example when kitchen waste high in cereal products or cassava tuber meal that is low in Ca is fed to livestock. Variation in the concentrations of most trace elements in wood ash is even wider than that of Ca. However, it is suggested that trace elements in wood ash formed when combustion is at temperatures below 500 ºC have a relatively high bioavailability. Therefore, wood ash could be a valuable source of trace elements under subsistence farming conditions. Wood ash is an alkaline product which has been utilized in the alkaline treatment of roughages to improve digestibility and also to reduce the tannin content of feedstuffs. Contrary to other hazardous alkali such as NaOH used in the treatment of feedstuffs, wood ash is a non-corrosive alkali. It is suggested that these treatments would be feasible under resource-limited farming conditions. However, attention will have to be given to the problem that products containing ash would have a high Ca to P ratio which could result in a phosphorus deficiency in livestock. ______________________________________________________________________________________

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Review
Wood ash in livestock nutrition: 2. Different uses of wood ash in animal
nutrition
J.B.J. van Ryssen#
Department of Animal & Wildlife Sciences, University of Pretoria, Pretoria, South Africa
______________________________________________________________________________________
Abstract
Wood ash is a mineral source that is readily available in most subsistence farming communities. The
potential use of wood ash as a mineral source in animal nutrition is evaluated and reviewed. The predominant
mineral in wood ash is calcium (Ca), and thus a potential Ca source in animal diets. However, the wide, source-
dependent variations of Ca and other minerals in wood ash are limiting factors in the extensive use of ash in
livestock diets. Ca sources with a less variable mineral composition such as feed lime (CaCO3) and limestone
are therefore preferable. This restricts the inclusion of wood ash in diets except where a good estimate of its
Ca content is available. Specific situations exist under subsistence farming conditions in which wood ash could
be used as a Ca supplement, for example when kitchen waste high in cereal products or cassava tuber meal
that is low in Ca is fed to livestock. Variation in the concentrations of most trace elements in wood ash is even
wider than that of Ca. However, it is suggested that trace elements in wood ash formed when combustion is at
temperatures below 500 ºC have a relatively high bioavailability. Therefore, wood ash could be a valuable
source of trace elements under subsistence farming conditions. Wood ash is an alkaline product which has
been utilized in the alkaline treatment of roughages to improve digestibility and also to reduce the tannin
content of feedstuffs. Contrary to other hazardous alkali such as NaOH used in the treatment of feedstuffs,
wood ash is a non-corrosive alkali. It is suggested that these treatments would be feasible under resource-
limited farming conditions. However, attention will have to be given to the problem that products containing
ash would have a high Ca to P ratio which could result in a phosphorus deficiency in livestock.
______________________________________________________________________________________
Keywords: Calcium source, Ca : P ratio, detanninification, mineral bioavailability
# Corresponding author: jvanryssen@gmail.com
Abbreviations: Al: aluminium; Ca: calcium; Cu: copper; Fe: iron; K: potassium; Mn: manganese; Mg:
magnesium; Mo: molybdenum, P: phosphorus; Se: selenium; Zn: zinc
Introduction
When plant material including wood is burnt, the inorganic residue left is ash, and ash is usually readily
available in subsistence farming communities. Ash is considered a waste product, though it has the potential
to be used in agriculture. Wood ash is a source of Ca (Van Ryssen & Ndlovu, 2018) which can be used in
resource-limited farming situations.
Application of ash to soil appears to be a practical method of disposing of wood ash, because it helps to
recycle nutrients back onto the soil (Campbell, 1990). In fact, wood ash has been found to be a good fertilizer,
and can be used as a source of plant mineral nutrients when applied to forestry and agricultural soils (Naylor
& Schmidt, 1986; Campbell, 1990; Olanders & Steernari, 1995). Österås (2004) noted that the restoring of
nutrients to soil through fertilizing with wood ash is critical, especially in areas where trees are harvested
commercially. Although ash has low fertilizing properties, it can be used as a substitute for lime and limestone
to neutralize acidic soils through supplementing Ca, K, P, Mg, and replacing microelements that would have
been depleted from the soil during plant growth and harvesting (Naylor & Schmidt, 1986). Liming with wood
ash may reduce the toxic effects of Al and Mn on plants grown in acidic soils. An indirect benefit of using
wood ash as a fertilizer would be the rise in the pH level of acidic soils that would increase the bioavailability
of elements such as P, Mg, Mo, and Se in plants (Reid & Horvath, 1980), which in turn would be beneficial to
animals consuming those plants.

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The objective of this presentation is to review the use or potential use of wood ash in animal nutrition
with special reference to minerals in the ash of subtropical tree species, as reported by Van Ryssen & Ndlovu
(2018).
Wood ash as a dietary ingredient
Cereal grain-based balanced diets require the inclusion of Ca, especially in diets for laying hens (NRC,
1994). Wood ash as a Ca source has been included in such diets by replacing limestone (Van Ryssen et al.,
2014). However, limestone is a relatively cheap source of Ca and is readily available. This limits the need for
alternative sources of Ca. Therefore, the inclusion of wood ash in animal diets as a Ca source would be
restricted to situations where large quantities of ash are readily available, and where it is economically
justifiable to analyse the ash before it is included in the diet.
In subsistence farming systems animal performance is relatively low and wood ash can be used
effectively as a Ca source for poultry (Ochetim, 1988). Van Ryssen et al. (2014) substituted feed lime with
wood ash in a diet for slow-growing broilers and recorded no difference between the two sources of Ca in
growth and leg characteristics of the birds. Similarly, Oso et al. (2011) reported that the performance of broilers
were similar when comparing Ca sources such as oyster shell, snail shell, wood ash, and limestone. However,
they recorded that chickens on a diet containing wood ash developed leg problems such as slipped tendon.
When formulating a complete diet in which ash is used, more ash than limestone must be included in
order to supply the Ca required in a diet. To be sufficient in Ca and P, the diets formulated by Van Ryssen et
al. (2014) contained 19 to 23 g limestone/kg and 26 to 33 g wood ash/kg, and those of Oso et al. (2011) 22.7
g limestone and 37.1 g wood ash/kg. Wood ash would dilute the concentration of other nutrients in the diet
and therefore the concentration of, for instance, available energy. In both studies approximately 3% wood ash
was included in the diets. These findings suggest that wood ash should not be included in diets intended for
high-performing livestock.
In rural and subsistence farming communities, kitchen waste containing cereal grains is often available
and fed to livestock. However, cereal grains and their by-products are deficient in Ca (Bredon et al., 1987). In
such situations, wood ash would be an excellent Ca source. Cassava tuber is another feedstuff that is used
widely in tropical regions worldwide, both in animal and human nutrition (Oke, 1978). One of the nutritional
limitations of the cassava tuber is its very low Ca content (Oke, 1978). Chávez et al. (2005) analysed 600
different cassava genotypes and reported mean concentrations of 0.76 g Ca/kg and 1.65 g P/kg on a DM basis.
Under these feeding situations wood ash would fit in well as a supplemental Ca source.
A practical problem that Van Ryssen et al. (2014) identified in their study was that fine wood ash tended
to separate from other dietary ingredients, except when macadamia oil cake meal that contained relatively high
levels of oil was included in the diet. Another characteristic of wood ash and wood bark ash is the variety of
other mineral elements they contain, apart from Ca (Van Ryssen & Ndlovu, 2018). However, this is similar to
many commercial sources of Ca such as limestone (Naylor & Schmidt, 1986; Van Ryssen, 1993) because the
mineral composition of limestone depends on the composition of the rock mined. Van Ryssen (1993) recorded
a limestone source containing 5000 mg Mn/kg compared to another source containing 45 mg Mn/kg.
Mavromichalis (2015) indicated that dolomitic limestone contains more than 100 g Mg/kg, while it has also
been documented that high levels of fluoride are often present in Ca and P sources (Suttle, 2010).
With the dominant presence of Ca in wood ash, it can be assumed that a large proportion of trace
elements in wood ash would be present as carbonates and bicarbonates, considering the low temperatures of
wood burning in homestead fires, as suggested by Van Ryssen & Ndlovu (2018). Although the relative value
(RV) for carbonate salts is usually lower than reference salts, as quoted by Ammerman et al. (1995), it can be
assumed that high levels of trace elements in wood ash would make a substantial contribution to the supply of
trace elements to animals consuming diets containing wood ash. To what extent their bioavailability would be
comparable to those of trace minerals in limestone is yet to be determined, although Ammerman et al. (1995)
indicated that the bioavailability of Mg and Fe in dolomitic limestone is relatively low, and Suttle (2010)
reported the low bioavailability of Fe sources such as oxides, hydroxides, and carbonates in soil originating
from rock. It is proposed that the bioavailability of trace elements in wood ash would be relatively high
compared to those in a product such as limestone.
A high bioavailability of minerals in wood ash can be significant, especially in cases where the wood
ash contains high concentrations of trace elements (Van Ryssen & Ndlovu, 2018). At a trace element
concentration in ash of 500 mg/kg, as was the case for Fe, Mn, and Zn in some of the samples, 3% wood ash

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in a diet would supply 15 mg of Fe, Mn or Zn to a kg of the diet, which would constitute between 30% and
50% of the requirements for livestock (NRC, 1994; 1996). However, due to the large number of variations in
mineral and trace element concentrations in wood ash, the effects of trace elements in wood ash in livestock
rations are unpredictable. Oso et al. (2011) included wood ash from bakeries in diets for broilers and reported
that the birds developed leg problems, including swelling of joints and slipped tendon. Consequently these
researchers did not recommend the use of wood ash as a Ca source in poultry nutrition. Considering that slipped
tendon is associated with Mn deficiency in poultry (Suttle, 2010), low concentrations of Mn in the wood ash
as used by Oso et al. (2011), viz. 42 mg/kg as compared to a mean concentration of 798 mg/kg in wood ash
from subtropical trees (Van Ryssen & Ndlovu, 2018), may indicate that broilers, as reported by Oso et al.
(2011) in their study, might have suffered from an induced Mn deficiency. These findings emphasize the
challenge presented by the wide variation and unpredictability of the trace element concentration in wood ash.
Although the concentration of trace elements in feed ingredients is usually not a major determining factor
when formulating diets, it can be assumed that wood ash can be a source of trace elements in livestock diets.
Treating animal feeds with ash solutions
The alkalinity of wood ash makes it suitable and effective in treating a deficiency in high-fibre roughages
to improve poor digestibility, and also in the detanninification of tannin-rich feedstuffs.
Alkaline treatment of high-fibre roughages has been investigated extensively. Several reviews relate to
the use of alkaline treatment to improve the feeding value of roughages for ruminants (Jackson, 1978; Wanapat
et al., 1985; Nolte et al., 1987). Products such as NaOH, urea, and anhydrous ammonia have been used
successfully, although they are all corrosive and thus hazardous. Wood ash is a non-corrosive product, and is
economical and readily available. Nolte et al. (1987) pointed out that wood ash can be used to treat wheat
straw and improve its digestibility.
According to Nolte et al. (1987) alkaline solutions prepared using wood ash are referred to as solutions
of ashes in water or the supernatant fluid when ash is dissolved in water. There are different techniques to
prepare these alkaline solutions: Ramirez et al. (1991) used 10%, 20% or 30% of wood ash, mixed it with
water (w/v) and allowed the ash to dissolve (w/v) until the insoluble material settled out. These authors used
a 359 L wood ash solution to soak 60 kg ground maize stover for six hours. The treated stover was dried and
fed to ruminants as part of their diet. Imbeah (1999) prepared a 20% (w/v) mixture of wood ash and water and
added 4% NaCl to improve its palatability. The mixture was sprinkled on chopped hay (I L per 5 kg hay),
mixed, and dried.
Ramirez et al. (1992) reported that the DM digestibility of diets containing maize stover treated with
20% wood ash increased by up to 20% in sheep above that of the control group. Nolte et al. (1987) found that
the treatment of wheat straw with a 30% solution of wood ash for 6 h significantly increased the digestibility
of DM, organic matter (OM), neutral detergent fibre (NDF), and acid detergent fibre (ADF) of the straw fed
to goats. Treating hay with a 20% wood ash solution also increased the ash content of the treated product by
142%, the Ca content by 443%, Cu by 112%, and Zn by 285% (Imbeah, 1999), thus fortifying the hay with
minerals.
In utilizing treated roughages, it should be noted that generally wood ash contains very low
concentrations of P, and that the inclusion of wood ash treated feedstuffs in diets could result in a wide Ca : P
ratio. When treating hay with a 20% wood ash solution, Imbeah (1999) recorded that the treated hay had a
Ca : P ratio of 4 : 1. While Ca, Mg, Cu, and Zn concentrations in the serum of sheep increased over a six-week
experimental period, serum P concentration decreased significantly from 98.2 mg/L in week 2 to 60.3 mg/L in
week 6, suggesting a reduction in bioavailability of P. In vitro digestibility studies by Ramirez et al. (1991;
1992) tested the effect of 10%, 20%, and 30% wood ash solutions on the fibre digestibility in sorghum straw
and maize stover, respectively. In the study by Ramirez et al. (1991) treated straw contained Ca : P ratios of
21 : 1, 71 : 1, and 117 : 1, and when included in a balanced diet in an in vivo digestibility study, ratios of 26 :
1, 54 : 1, and 74 : 1 respectively, were recorded. Ramirez et al. (1992) treated maize stover and recorded a
lower Ca : P ratio, but at the 30% ash treatment level, a Ca : P ratio in a balanced diet was still 15 : 1. Although
these were short-term in vivo and laboratory investigations, the wide Ca : P ratios demonstrate the potential
risk associated with P deficiency if the Ca : P ratio is not corrected when livestock are fed such treated products
in long-term feeding regimens. A practical problem would be that P supplementation might be too expensive
under subsistence farming situations, thus diminishing the advantages of using such wood ash treated products.

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Makkar (2003) reviewed strategies to overcome the detrimental effects of feeding tannin-rich feeds to
livestock, and reported that wood ash has the potential to render inactive or remove tannins in feedstuffs.
Mohammed & Ali (1988) mixed firewood ash with water at a ratio of 1 : 5 and left it for 24 h. The filtrate was
used to treat high-tannin sorghum. The sorghum was soaked in the wood ash extract and then washed several
times with water until the washings were clear. The treated sorghum was then sun-dried, ground, and included
successfully in a diet for broiler chicks. Kyarisiima et al. (2004) mixed 1 kg of hard wood ash with 20 L water
and left it overnight. The supernatant with a pH of 11.8 was removed and filtered through cotton cloth before
2 L of the extract was added to 1 kg of high-tannin sorghum grain, soaked for 12 h, and then dried in the sun.
Treatment of oak leaves with a 10% solution of oak wood ash decreased the content of total phenols,
condensed tannins and the protein precipitation capacity in oak leaves by 66%, 80%, and 75%, respectively
(Makkar, 2003). The pH of the ash solutions was between 10.5 and 11.3. Mohammed & Ali (1988) reported
that the soaking of high-tannin sorghum in a wood ash extract improved the performance of broiler chicks to
be similar to those which received low-tannin sorghum in their diets. Using a wood ash extract, Kyarisiima et
al. (2004) reported a 62% reduction in tannins when high-tannin sorghum grain was soaked in the extract, and
after germination of the seed an 85% reduction in tannin content was recorded. The treated high-tannin
sorghum replaced maize effectively in a broiler diet. However, Tshabalala et al. (2013) recorded that different
methods of detannification including wood ash were ineffective in improving intake and digestibility of
Vachellia nilotica fruits by goats. They recommended that detannification methods should be tested on
different tree species to establish if they are effective or not.
Conclusion
As is the case with most by-products that can be used in animal nutrition, variation in the nutrient
composition of wood ash is high. However, wood ash, and specifically homestead ash, is a resource that is
available in most subsistence farming situations and can be considered a source of Ca in animal nutrition. If to
be used in feeding programmes, it is recommended that at least the Ca content of the product be measured. To
cut cost, it is perhaps worth considering assessing the Ca and mineral content on a regional basis where similar
fire-making products are used. The variation in concentration of other elements, especially that of trace
elements, is very high and it has been suggested that the bioavailability of these trace elements in wood ash
would also be high.
As a non-corrosive alkaline, wood ash has the potential to improve the digestibility of fodder, or to
suppress the effect of tannin in plants on the animal. Since a certain amount of ash might be present in the
treated products if not washed after treatment, it could result in high Ca : P ratios in the final diet. This would
require the supplementation of P to diets containing such treated products. The problem is that under
subsistence farming conditions P supplementation might not be feasible.
It is concluded that in resource-poor / subsistence farming situations:
 Wood (homestead) ash can be used as a source of Ca when Ca-deficient products such as cereal
in kitchen waste and cassava tuber meal are fed to livestock;
 Although the alkaline treatment of fibrous feedstuffs gave excellent results in in vitro and short-
term digestibility studies, the high Ca : P ratio in the treated products would require the
supplementation of P, which might be unaffordable under subsistence farming conditions.
 Although the concentration of trace elements in ash can vary widely, wood ash can supply
highly bioavailable trace elements which subsistence farmers would probably otherwise not be
able to supply to their livestock.
Conflict of interest
The author declares that he has no conflict of interest.

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