Heat production and retained energy in lactating cows held under hot summer conditions with evaporative cooling and fed two rations differing in roughage content and in vitro digestibility

Abstract

The objective of this study was to measure the effect of feeding two total mixed rations (TMRs), differing in their roughage content and in vitro dry matter (DM) digestibility, on the physiological response and energy balance of lactating cows. The partitioning of metabolizable energy intake (MEI) between heat production (HP) and retained energy (RE) of cows held under hot weather conditions and external evaporative cooling was measured. In all, 42 lactating cows were divided into two similar sub-groups, each of 21 animals, and were fed either a control (CON) ration containing 18% roughage neutral detergent fiber (NDF) or an experimental (EXP) TMR containing 12% roughage NDF and used soy hulls as partial wheat silage replacer. The in vitro DM digestibility of the CON and EXP TMR was 75.3% and 78.6%, respectively (P < 0.05). All cows were cooled by evaporative cooling for 2 adaptation weeks plus 6 experimental weeks under hot weather conditions. The EXP diet reduced rectal temperature and respiratory rate of the cows while increasing their DM intake (DMI) from 23.1 to 24.7 kg/cow per day, milk yield from 41.9 to 44.2 kg and yield of energy-corrected milk from 38.7 to 39.7 kg, as compared with the CON group. Cows fed the EXP TMR had increased RE in milk and body tissue, as compared with the CON group, but the diets had no effect on the measured HP that was maintained constant (130.4 v. 130.8 MJ/cow per day) in the two groups. The measured MEI (MEI = RE + HP) and the efficiency of MEI utilization for RE production were also similar in the two dietary groups.

Full text

Animal
(2008), 2:6, pp 843–848 &The Animal Consortium 2008
doi: 10.1017/S1751731108001900
animal
Heat production and retained energy in lactating cows held
under hot summer conditions with evaporative cooling and
fed two rations differing in roughage content and
in vitro
digestibility
J. Miron
1
-
, G. Adin
2
, R. Solomon
2
, M. Nikbachat
1
, A. Zenou
1
, A. Shamay
1
, A. Brosh
1
and
S. Y. Mabjeesh
3
1
Agricultural Research Organization, PO Box 6, Bet-Dagan, 50250, Israel;
2
Extension Service, Ministry of Agriculture, Israel;
3
Faculty of Agriculture, The Hebrew
University of Jerusalem, Israel
(Received 30 October 2007; Accepted 4 February 2008)
The objective of this study was to measure the effect of feeding two total mixed rations (TMRs), differing in their roughage
content and
in vitro
dry matter (DM) digestibility, on the physiological response and energy balance of lactating cows. The
partitioning of metabolizable energy intake (MEI) between heat production (HP) and retained energy (RE) of cows held under
hot weather conditions and external evaporative cooling was measured. In all, 42 lactating cows were divided into two similar
sub-groups, each of 21 animals, and were fed either a control (CON) ration containing 18% roughage neutral detergent fiber
(NDF) or an experimental (EXP) TMR containing 12% roughage NDF and used soy hulls as partial wheat silage replacer. The
in vitro
DM digestibility of the CON and EXP TMR was 75.3% and 78.6%, respectively (
P
,0.05). All cows were cooled by
evaporative cooling for 2 adaptation weeks plus 6 experimental weeks under hot weather conditions. The EXP diet reduced
rectal temperature and respiratory rate of the cows while increasing their DM intake (DMI) from 23.1 to 24.7 kg/cow per day,
milk yield from 41.9 to 44.2 kg and yield of energy-corrected milk from 38.7 to 39.7 kg, as compared with the CON group.
Cows fed the EXP TMR had increased RE in milk and body tissue, as compared with the CON group, but the diets had no
effect on the measured HP that was maintained constant (130.4
v
. 130.8 MJ/cow per day) in the two groups. The measured
MEI (MEI 5RE 1HP) and the efficiency of MEI utilization for RE production were also similar in the two dietary groups.
Keywords: dairy cows, energy expenditure, evaporative cooling under heat load, retained energy, soy hulls as roughage replacement
Introduction
It is well accepted in the literature that the main factors
that might affect the partitioning of energy in lactating cow
are level of dry matter intake (DMI), environmental condi-
tions that might affect both DMI, energy expenditure as
heat production (HP), the production level of milk and body
tissue gain and individual variability between cows with
respect to the efficiency of energy utilization for main-
tenance and production (Berman, 2005; Brosh, 2007).
Under heat-stress conditions, lactatingcowstendtoreduce
DMI and milk production (West
et al
., 2003). Voluntary DMI
can decrease by as much as 6% when ambient temperatures
rise above 308C (Eastridge
et al
., 1998). Therefore, under hot
weather conditions during the summer, most of the dairy
farms in Israel cool the lactating cows by ‘evaporative
cooling’, namely an arrangement whereby first the cows are
made wet using sprinklers and then exposed to fans to
promote evaporation (Flamenbaum
et al
., 1986; Berman,
2005). At present, in Israel only minor changes are made to
the total mixed ration (TMR) composition in order to adapt
it to the hot weather conditions.
In this respect, we have demonstrated (Miron
et al
., 2003;
Halachmi
et al
., 2004) that partial replacement of the
roughage component of the TMR with readily digestible
neutral detergent fiber (NDF)-rich by-products may increase
the milk yield of lactating cows by 10% under hot weather
conditions. Unfortunately, those studies were operated under
inadequate cooling of the cows. There is limited information
in the literature concerning the effect of combining intensive
-
E-mail: jmiron@volcani.agri.gov.il
843

evaporative cooling with such a feeding regime, which uses
highly digestible by-products such as roughage replacer, on
the voluntary DMI, energy status and performance of lactat-
ing cows experiencing heat load conditions.
A major part of the metabolizable energy intake (MEI) by
cows is dissipated as HP and another part is retained in milk
and body tissue (retained energy, RE). Recently, the research
group of Brosh and Aharoni developed a new method that
can directly measure the HP and RE in milking cows held
under commercial dairy barn conditions (Aharoni
et al
., 2005;
Brosh, 2007). The HP is quantitatively assayed by measuring
the volume of O
2
consumption (VO
2
). In mammals, most of
the measured VO
2
is transferred to the tissues through the
heart; therefore, multiplying of daily measured heart rate (HR)
by a short measurement of VO
2
per heart beat was used for
estimation of cows’ HP (Brosh, 2007).
This new method provides a powerful tool to understand
the effect of changing the
in vitro
digestibility and roughage
content of the TMR, on the partitioning of the MEI between
HP and RE.
The objective of this study was to measure the effect of
feeding two rations, differing in their roughage content and
in vitro
DM digestibility, on the physiological response, DMI,
performance and partitioning of the MEI between HP and
RE of lactating cows held under hot weather conditions and
external evaporative cooling.
Material and methods
Cows, diets, environmental conditions and sampling
procedures
In all, 42 lactating multiparous Holstein cows (average of
2.9 60.1 lactations) were fed for a pre-experimental period
of 3 weeks the control TMR described below (Table 1). After
the standardization period, the cows were divided into
two feeding groups, each of 21 cows, similar in average
(means 6s.e.) days in milk (125 65.3), daily milk yield
(39.0 60.75 kg) and DMI (23 61 kg) that were fed for 2
adaptation weeks plus additional 6 weeks of experiment,
two different TMR. The control TMR fed to the CON group
contained 18% roughage NDF, whereas the EXP TMR fed to
the EXP cows contained just 12% roughage NDF, in which
one-third of the dietary wheat silage was replaced with soy
hulls (Table 1). All the cows were housed at the Agricultural
Research Organization (ARO) dairy farm, in one shaded
corral with free access to water, and were cooled in the
milking parlor holding yard. Evaporative cooling was repe-
ated seven times per day, at 0415, 0800, 1115, 1400, 1700,
1915 and 2200 h. Each cooling period lasted 45 min and
comprised nine repeated cycles: 1 min of wetting the cows
with water using sprinklers, followed by 4 min exposure to
fans promoting evaporation. Thus, the total shower cooling
time of the cows lasted about 5.25 h/day, which allows
enough time for eating and rumination as seen in the high
intake and milk production levels of the cows. The cool-
ing system makes use of 10 high-velocity fans (air speed of
3 m/s). The two TMRs were offered once daily at 1030 h
with
ad libitum
intake, allowing for 5% to 10% orts, and
the cows were milked three times daily at 0500, 1200 and
2000 h. Cows were fed individually via a computerized
monitoring system, designed to electronically identify indi-
vidual cows and to control and automatically record daily
individual feed intake (Miron
et al
., 2003).
TMRs were sampled daily and pooled on a weekly basis
to produce 10 samples for each dietary treatment. DMI was
determined by oven drying at 1058C for 24 h a portion of
the weekly TMR samples. The rest of the weekly TMR
samples were oven dried at 608C for 48 h, ground through a
1-mm sieve and used for
in vitro
digestibility evaluation and
chemical analyses.
The milk yield of each cow was recorded daily by automatic
meters (Afimilk SAE, Kibbutz Afikim, Israel). Milk samples
were collected during three sequential milkings on a weekly
basis throughout the study. Each set of milk samples for each
cow was stored at 48C in the presence of a preservative
Bronopol (2-bromo-2-nitropropane-1,3-diol) tablet, until ana-
lyzed for content of fat, true protein, lactose and urea by
infrared analysis (Israeli Cattle Breeders Association Laboratory,
Table 1
Ingredients and chemical composition of the two total mixed
rations
%ofDM
EXP CON s.e.
P
value
Ingredients
Wheat silage 18.7 29.7
Soybean hulls 11.8 0.0
Soybean meal (solvent extract) 8.4 9.2
Vetch hay 4.3 4.3
Ground corn grain 18.3 18.3
Ground barley grain 17.4 17.4
Whole cottonseeds 4.5 4.5
Corn gluten feed 7.9 7.9
Distillers dry grains 5.3 5.3
NaHCO
3
0.70 0.70
NaCl 0.4 0.4
CaCO
3
1.3 1.3
Protected fat 0.90 0.90
Trace mineral 1vitamin mixture
1
0.10 0.10
Water (l/cow per day) 3.36 0.0
Chemical composition
DM (%) 64.6 63.2 0.5 0.22
OM (% of DM) 92.1 91.7 0.2 0.53
CP (% of DM) 16.0 16.0 0.1 0.70
Ether extract (% of DM) 4.6 4.6 0.1 0.70
NDF (% of DM) 36.0 30.8 0.02 0.01
Roughage NDF (% of TMR DM) 12.0 18.0 0.02 0.01
In vitro
DM digestibility (%) 78.6 75.3 0.02 0.03
In vitro
NDF digestibility (%) 68.8 59.2 0.02 0.01
EXP 5experimental total mixed ration (TMR) containing 12% NDF of
roughage origin; CON 5control TMR containing 18% NDF of forage origin;
DM 5dry matter; OM 5organic matter.
1
Trace minerals 1vitamins mix contained (g/kg DM): Zn, 24; Fe, 24; Cu,
12.8; Mn, 24; I, 1.44; Co, 0.32; Se, 0.32; vitamin A, 16000 000 IU; vitamin
D
3
, 3 200 000 IU; vitamin E, 48 000 IU.
Miron, Adin, Solomon, Nikbachat, Zenou, Shamay, Brosh and Mabjeesh
844

Caesaria, Israel, using Milkoscan 4000, Foss Electric, Hillerod,
Denmark). Body weight (BW) data were recorded by an
automated walkover weigher each time the cow entered the
milking parlor. The animal performance study was carried out
according to the guidelines and under the supervision of the
ARO Animal Care Committee.
Chemical analyses and
in vitro
digestibility measurements
Replicate samples of the weekly composites of each TMR
and of the orts of the individual cows were assayed in
triplicate for DM content (drying at 1058C for 24 h) and
residual ash (4 h at 6008C). Dry TMR samples were ground
through a 1-mm sieve, and analyzed for crude protein (CP)
content according to the Kjeldahl method (AOAC, 2001)
and for the content of NDF (without sodium sulfite and with
heat-stable amylase, Van Soest
et al
., 1991). The Ankom
apparatus (Ankom
220
; Fairport, NY, USA) was used for
extracting and filtering the NDF. Net energy (NE
L
) content of
each TMR was calculated based on the NE
L
content of the
individual feeds (NRC, 2001).
In vitro
digestibility of DM and NDF in the weekly com-
posites of TMR was analyzed in triplicate for each sample.
The procedure involved incubating 0.5 g dry plant material
with rumen fluid for 48 h and then with 0.1 N HCl and 0.2%
pepsin for another 48 h, according to the two-stage fer-
mentation technique of Tilley and Terry (1963). Rumen fluid
was obtained before morning feeding via rumen fistula
from three dry cows fed a mixture of the CON and EXP
diets. Residual NDF in the
in vitro
tubes was determined
according to Van Soest
et al
. (1991).
Calculations and statistical methods
Yield of energy-corrected milk (ECM) was calculated according
to the NRC (2001) equation, ECM (kg/day) 5milk yield (kg/
day) 3[(0.3887 3% milk fat) 1(0.2356 3% milk protein 2
urea) 1(0.1653 3% milk lactose)]/3.1338 MJ/kg.
Comparison between the two feeding groups with
respect to physiology, individual intake, performance and
energy status parameters was carried out according to the
repeated measurement Proc mixed model of SAS (SAS,
1996), and presented in Tables 2 and 3 as the daily mean
for the 6-week experimental period. The
F
-test was used to
differentiate between means (SAS, 1996).
Differences between the two TMRs (six weekly compo-
sites of each TMR) with respect to composition and
in vitro
digestibility were tested for significance using an analysis of
variance (SAS, 1996).
Physiological measurements
Rectal temperature was measured with a digital electronic
thermometer (Toshiba Glass Co. Ltd, Shizuoka, Japan) and
respiration rate (RR) was visually measured in each cow
Table 3
Milk performance and energy partitioning in cows fed the two dietary treatments
Treatments
Variable EXP CON s.e.
P
value
DMI (kg/day) 24.7 23.1 0.28 0.04
Milk yield (kg/day) 44.2 41.9 0.15 0.04
Energy-corrected milk yield (kg/day) 39.7 38.7 0.16 0.05
Efficiency of milk production (kg milk/kg DMI) 1.79 1.81 0.01 0.35
RE
1
in milk (MJ/cow per day) 124.4 121.3 0.15 0.05
RE
1
in BCS change (MJ/cow per day) 20.11 1.526 0.10 0.04
Total RE
1
(MJ/cow per day) 126.4 122.8 0.59 0.02
Heat production
2
(MJ/cow per day) 130.4 130.8 3.35 0.36
Measured MEI
3
(MJ/cow per day) 256.8 253.6 3.87 0.50
Efficiency of energy utilization for production
(Total RE/measured MEI) 0.492 0.484 0.012 0.50
EXP 5experiment TMR containing 12% roughage NDF; CON 5control TMR containing 18% roughage NDF; DMI 5dry matter intake; RE 5retained energy;
BCS 5body condition score; MEI 5metabolizable energy intake.
1
RE in milk and/or BCS change according to Aharoni
et al
. (2006).
2
Total heat production (HP) 5heart rate (HR) 3O
2
P320.47/1000 360 324.
3
Measured MEI 5total RE 1HP.
Table 2
Respiratory rate, rectal temperature, heart rate, pedometer
activity, body weight and body condition score (BCS) changes, of
cows fed the two total mixed rations
Treatments
Variable EXP CON s.e.
P
value
Respiratory rate/min (morning)
1
52.5 59.8 1.88 0.01
Respiratory rate/min (afternoon)
1
84.3 91.7 2.15 0.04
Rectal temperature 8C (morning)
1
37.9 38.2 0.05 0.05
Rectal temperature 8C (afternoon)
1
38.7 39.0 0.06 0.05
Average heart rate (beats/min) 78.8 81.9 1.70 0.34
Average pedometer activity
(steps/h)
115.0 118.8 4.18 0.30
Average body weight (kg/cow) 596 562 6.2 0.01
BCS change/day (units 1 to 5) 0.002 0.0016 0.0004 0.45
EXP 5experiment total mixed ration (TMR) containing 12% roughage NDF;
CON 5control TMR containing 18% roughage NDF.
1
Morning measurements at 0700 h (immediately after cooling) and
afternoon measurement at 1700 h (2 h post cooling).
Energy partitioning in milking cows fed two model rations
845

3 days per week of the experiment at 700 and 1700 h.
A trained veterinarian, the same one throughout the study,
measured body condition scores (BCS) on a weekly basis in
a scale of 1 to 5 according to NRC (2001).
The HR and HP of the individual cows were measured as
described previously by Aharoni
et al
. (2005) and Brosh
(2007). These parameters were measured in each cow
twice, at 3-week intervals, during the experimental period.
The HR was recorded with a Polar instrument (Polar
Electro Oy, Kempele, Finland), a model T51H HR transmitter,
and a watch model S610 data logger and receiver (details in
Aharoni
et al
., 2006). The devices were attached to the
thorax behind the forelegs by means of a specifically
designed elastic belt (Pegasus, Eli-ad, Israel). HR was mea-
sured continuously for 4 days in each measurement; the data
logger was programmed to record HR at 1-min intervals.
VO
2
was measured for in each cow with a facemask
open-circuit respiratory system simultaneously with the
HR for 10 to 15 min for each cow from 0900 to 1100 h
immediately after 4 days of HR measurement. The HP (kJkg
BW
20.75
/day) was calculated by multiplying the means of
HR measured throughout 3 to 4 days by the VO
2
per
heartbeat O
2
pulse (O
2
P), and by 20.47 kJ/l VO
2
(Nicol and
Young, 1990). For the non-cooled cows, O
2
P during the hot
hours was corrected for the effect of temperature humidity
index (THI) on O
2
P, as suggested by Aharoni
et al
. (2003).
The HP and the energy retained in the milk and BW gain
were measured for each cow in each treatment. The
equation used to calculate HP was (Brosh, 2007)
Daily HP ðMJ=cow per dayÞ¼specific HP kg BW0:75 ;
where
Specific HP ðkJ=kgBW0:75=dayÞ¼HR O2P20:47=1000
6024;
HR 5heart rate (beats/min), O
2
P5lofO
2
/(beat 3kg
BW
0.75
), 20.47 5kJ/l O
2
consumption.
Environmental climate data in the barn including ambient
temperature, humidity, wind speed and directions were
monitored continuously during the experimental period
using a weather monitoring station (RWMS-8; Rotem
Computerized Controllers, Petach-Tikva, Israel).
The THI was calculated (Aharoni
et al
., 2003) with the
equation
THI ¼td ð0:55 0:55 RHÞðtd 58Þ;
where
td
is the dry bulb temperature (8F), RH is the relative
humidity expressed as a decimal.
Results and discussion
The distribution in the barn of ambient air temperature,
relative humidity and THI data over an average day and
night of the experimental period is illustrated in Figure 1.
Environmental conditions in the barn were similar for the
two dietary groups and stable throughout the 6 weeks of
the experiment. Ambient temperature was high (268Cto
278C) during most of the day hours and early evening, with
daily averages of 25.78C.
RH was the highest in the early morning hours and its daily
average was 66.6%. Thus, THI was confined within a narrow
range (74 to 76) during the experimental period, which was
typical of heat load conditions. Therefore, we have used an
external evaporative cooling method to improve the voluntary
intake and performance of all the cows.
The
in vitro
digestibility of the EXP TMR was significantly
higher than the CON TMR (78.6
v
. 75.3, Table 1) and reflected
the large difference in NDF digestibility of the two TMR
(68.8%
v
. 59.2%). This difference affected the RR and rectal
temperature of the cows and their voluntary DMI (Table 2).
23.5
24.0
24.5
25.0
25.5
26.0
26.5
27.0
0:00 2:00 4:00 6:00 8:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00
Time
T°C
60.0
62.0
64.0
66.0
68.0
70.0
72.0
74.0
76.0
78.0
RH/THI
Figure 1 Average diurnal pattern of ambient air temperature (
T
8C6s.e.) &; relative humidity (RH 6s.e.) m; and temperature humidity index (THI 6s.e.)
Jin the barn.
Miron, Adin, Solomon, Nikbachat, Zenou, Shamay, Brosh and Mabjeesh
846

Cows fed the EXP TMR responded by a decrease of their RR
by 13% in the morning hours and by 9% in the afternoon
hours, as compared with the CON group. A positive sig-
nificant effect of the feeding regime on reducing rectal
temperature of the cows by 0.38C was manifested in both
the morning and afternoon measurements (Table 2). How-
ever, the effect of the feeding regime on HR and daily steps
(activity) was not significant. Unfortunately, there is limited
information in the literature regarding the relationship
between body temperatures and feeding of high-energy
diets to lactating cows under heat load conditions.
Total daily DMI of cows fed EXP TMR relative to cows fed
CON TMR was higher by 6.9% (24.7
v
. 23.1 kg/day per cow,
P
,0.04, Table 3). This higher DMI of the EXP cows might
be attributable partially to their better heat balance status
reflected in lower rectal temperatures and RR (Table 2), and
to improved digestibility and increased rate of passage for
the soy hull-containing diet. Similar daily DMI differences
in mid-lactating cows were reported in previous studies
employing soy hulls as corn silage replacement under heat
load conditions without evaporative cooling of the cows
(Miron
et al
., 2003; Halachmi
et al
., 2004). The higher DMI
observed for cows fed the EXP TMR and the higher DM and
NDF
in vitro
digestibility of this TMR as compared with the
CON diet (Table 1) were reflected in concomitant increase
of 5.5% in milk production and only by 2.4% in ECM yield
of the cows (
P
,0.05, Table 3).
These moderated although significant differences in
digestibility, DMI and ECM yield between the two dietary
treatments resulted in a 2.9% increase (
P
,0.05) in RE for
milk 1body tissue production of the cows fed the EXP
TMR, as compared with the CON group. This increase in RE
was not followed by any significant increase in the HP of
the cooled cows, which was maintained at a constant level
of 130.4 to 130.8 MJ/cow per day. Similar constant levels
of total HP in the range of 122.6 to 135.6 MJ/cow per day
were also measured in previous studies with lactating
non-cooled Holstein cows, held under various feeding
regimes, climate conditions and over different stages of
lactation (Aharoni
et al
., 2005 and 2006; Miron
et al.
, 2007,
unpublished). The finding of this study and of the previous
ones that shower cooling and various feeding regimes
hardly affect the HP of mid-lactation Holstein cows deserves
further investigation. Total HP is actually the sum of HPp
(HP for milk and body tissue production) plus HPm (HP for
maintenance including pregnancy and embryo needs). The
constant level of HPp 1HPm 5130 MJ/cow per day, found
in this study, in accordance with previous data (Aharoni
et al
., 2005 and 2006; Miron
et al.
, 2007 unpublished),
suggests that any increase in HPp of the cows is enabled by a
concomitant decrease in their HPm. Thus, under severe heat
load conditions without external cooling, HPm of the lactating
cows might increase (see Berman, 2005) and therefore HPp
should be concomitantly decreased to maintain the total HP
level. However, under external evaporative cooling, the HPm
might decrease as demonstrated previously (Berman, 2005),
allowing for concomitant increase in HPp. Based on this study
and on the previous ones (Aharoni
et al
., 2005 and 2006;
Miron
et al.
, 2007 unpublished), we suggest that an HP level
of around 130 MJ/cow per day is the upper limiting range of
heat disposal capability that a high-producing mid-lactation
cow (yielding 35 to 45 kg milk/day) can handle under Israeli
heat load conditions, while maintaining its body rectal
temperature at a level below 408C. Consequently the cows
respond to external heat load conditions by reducing the level
of production and HPp in order to maintain their internal
HP level.
The possible mechanism involved in the control of
voluntary DMI by the lactating cow under heat load versus
external cooling conditions is still unknown. Forbes (1986)
suggested that temperature receptors occur in many parts
of the cow body, including in the skin and anterior hypo-
thalamus as well as in the rumen wall and the abdomen,
with the afferent fibers in the splanchnic nerves, and in this
way heating or cooling may affect the short-term control of
voluntary DMI. Based on the present study findings we
suggest that the cow’s heat loss capability limits its total
HP, which indirectly affects its voluntary intake and milk
production level.
The constant level of total HP found in this study suggests
that since calculated MEI 5RE 1HP, there should be a linear
correlation between RE and calculated MEI. Indeed, in this
study the
r
2
value of the correlation between MEI and RE was
0.93. This finding suggests that changes in the composition
and/or digestibility of the TMR, which affect DMI and may
increase MEI, affect directly the level of production expressed
by RE and has a minor effect, if any, on the HP.
Milk production efficiency (kg milk /kg DMI, Table 3) and
efficiency of energy utilization formilkandtissueproduction
(total RE/measured MEI, Table 3) were hardly influenced by
the dietary treatments, despite the higher
in vitro
digestibility
of the EXP TMR (Table 1). This finding implies that the
increased DM and NDF potential digestibility (
in vitro
)ofEXP
TMR is expressed mostly in increasing daily DMI, which in
turn elevates milk and ECM production, and is not manifested
by improved feed utilization efficiency along the digestive
tract. This premise is supported by the high coefficient of
correlation for the linear regression in this study between DMI
andECMyield(
r
2
50.94). This high correlation supports the
important influence of DMI on milk production, as described
also in previous reports (Van Soest, 1994).
One possible explanation for the low effect of
in vitro
TMR
digestibility on ECM yield (Table 3) is that the higher DM and
NDF voluntary intake by cows fed the EXP TMR increased, in
turn, the passage rate of particles and NDF from the rumen,
causing a reduced rate and extent of NDF digestion along the
gastrointestinal tract (Van Soest,1994;NRC,2001).Similar
findings have been documented in previous studies that
compared various diets differing in their potential
in vitro
DM
digestibility, where a great effect on voluntary DMI was
observed, but with little influence on milk production effi-
ciency (Solomon
et al
., 2005; Miron
et al
., 2004a and 2004b).
Data of this study, based on direct measurements of total
HP and total RE, enable comparison between the predicted
Energy partitioning in milking cows fed two model rations
847

NE
L
value of the diets, which was 6.67 and 6.58 MJ/kg
DMI in the EXP and CON diets, respectively (based on NRC,
2001 data)and the direct measurements of RE in the two
treatments (Table 3). This comparison shows the under-
estimation of 20% to 24% in the total RE measured in this
study as compared with the predicted NE
L
values (NRC,
2001). This gap between estimated and measured NE
L
values probably originated from the higher HP values
measured in this study (130 MJ/cow per day operated under
hot summer conditions) as compared with the lower HP
estimations ( ,90 MJ/cow per day) used by NRC (2001).
Additional explanation for this gap is the lower quality of
Israeli forages as compared with National Research Council
standard (NRC), which contributes to the higher HP in the
digestive tract that might further increase the HP level. This
gap between predicted and measured NE
L
values suggests
that the methodology used for the estimation of the
nutritive value of forages in Israel (NE
L
estimation according
to NRC (2001) Tables) should be re-evaluated and adapted
to the quality of Israeli forages and to the actual HP values
measured under Israeli conditions.
Conclusions
This study demonstrates that feeding lactating cows with
soy hulls as partial roughage replacer increased DMI by
6.9% and milk yield by 5.5% (
P
,0.05). The feeding
regime hardly affected the efficiency of energy utilization
for ECM and BCS production, suggesting that the EXP
treatment affected mostly the intake level rather than the
digestibility along the gastrointestinal tract. This was
reflected by an increase in the total RE for milk and body
tissue reserves, while the HP was maintained fairly constant
at a level of 130 MJ/cow per day. Based on the present
study findings, supported by previous studies, we can
hypothesize that the cow’s total HP capability (upper level
of about 130 MJ/cow per day) limits its voluntary intake,
level of production and RE.
Acknowledgments
We wish to express our appreciation to the Volcani Center
dairy farm team for their efforts in cooling and feedings the
cows, and to Mr Zwi Sarid and his team from ‘Yavne Feeding
Center’ for preparing and delivering the two TMR. This study
was supported by a research grant # 362-071-06 of the Israeli
Milk Council.
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