Vol. 280, Issue 2, R418-R427, February 2001
Effect of water restriction on feeding and metabolism in dairy
cows
Martine Steiger
Burgos1,
Markus
Senn1,
Franz
Sutter2,
Michael
Kreuzer2, and
Wolfgang
Langhans1
Institute of Animal Sciences, 1 Physiology and Animal
Husbandry and 2 Animal Nutrition, Swiss Federal
Institute of Technology, CH-8092, Zurich,
Switzerland
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ABSTRACT |
We investigated how lactating cows are able to cope with a
sustained water restriction. In experiment 1, body weight
and meal patterns were recorded with ad libitum access to water
(baseline) and during 8 days of 25 and 50% restriction of drinking
water relative to ad libitum intake. In experiment 2,
indirect calorimetry was combined with nitrogen and energy balance and
plasma hormone and metabolite measurements to assess the effects of
50% water restriction on digestion and metabolism. In experiment
1, food intake and body weight declined during the first 3 days of
water restriction depending on the restriction level and stabilized thereafter at a lower level. The daily food intake reduction with 50%
water restriction was entirely due to a reduction of meal size. The
size of the first meal on every day was markedly (>50%) reduced with
25 and 50% water restriction. In experiment 2, urea concentrations in milk and blood as well as plasma sodium and hematocrit were increased by 50% water restriction. Energy balance was
not affected by 50% water restriction, but nitrogen balance became
negative, because, relative to intake, nitrogen excretion via urine and
milk was higher. The lower energy intake during 50% water restriction
was compensated by a lower milk production, a higher digestibility of
organic matter and energy, and, apparently, a more efficient energy
use. Through these changes and a preserved water balance, the cows
reached a new equilibrium at a lower water turnover level, which
enabled them to cope with a sustained drinking water restriction of
50%.
energy balance; nitrogen balance; water balance; dehydration; meal patterns; body heat production; adaptation
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INTRODUCTION |
RESTRICTION OF WATER
INTAKE has often been shown to reduce food intake in humans
(18) and various animal species (e.g., Ref.
1), including ruminants (e.g., Refs. 7,
19, 26). Ruminants differ from monogastric
animals because much more saliva is secreted during eating
(3) and because they have a large fluid reserve in the
rumen, which can buffer osmotic changes in the rumen derived from
digesta. In previous studies from our laboratory, pygmy goats
(24) and lactating cows (34) progressively
reduced food intake during water deprivation and did not compensate for the dehydration-induced weight loss by increasing food intake during
the subsequent rehydration period. These results contrast findings in
rats, which are known to compensate for dehydration-induced body weight
loss by markedly increasing food intake during the subsequent
rehydration (1). This different response suggests that
ruminants are better able to cope with dehydration than rats, i.e., a
similar degree of dehydration presumably provokes a smaller energy
deficit in ruminants than in rats. Two mechanisms may contribute to the
limitation of the dehydration-induced energy deficit in ruminants:
1) the digestibility of forage-based diets may be higher during dehydration (4, 10, 35) and 2) resting
metabolic rate may decrease with dehydration (9). Such
adaptive mechanisms have mainly been shown in nonlactating, mostly
small and desert-adapted ruminants (4, 9, 10, 35). Whether
similar mechanisms or other compensatory changes in digestion and
metabolism are activated by dehydration in lactating dairy cows, which
are more susceptible to water shortages than desert goats, is unknown. The present study addressed this question by trying to identify and
quantify such mechanisms in addition to the food intake suppression during graded levels of water restriction in lactating dairy cows.
In one experiment we characterized the feeding behavior of lactating
cows in response to different levels of water restriction to see
whether the cows are able to reach and maintain a new equilibrium under
these conditions. In a second experiment we determined water, energy,
and nitrogen balances as well as plasma metabolite and hormone
concentrations when cows had ad libitum access to water and when they
were subjected to the higher water restriction level of the first experiment.
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METHODS |
Animals and experimental procedures.
In experiment 1, 17 Brown Swiss cows [268 ± 14 (mean ± SE) days post partum, all pregnant (137 ± 8 days of pregnancy), 644 ± 13 kg body wt] were used. The cows
were housed in a tying stable with ad libitum access to water. After
food and water intake as well as milk yield for 5 days (baseline) were
recorded, the cows were divided into two groups (matched for baseline
food intake, milk yield, and water intake) that were subjected to 25%
(n = 9) and 50% (n = 8) water
restriction relative to individual ad libitum drinking water intake
(mean baseline) for 8 days. These treatments are subsequently referred
to as 25 and 50% water restriction, respectively. Note, however, that
the total water intake (drinking water plus water in food) restriction
level was somewhat less (see RESULTS) because food intake
was not reduced by 25 and 50%. The restriction period was followed by
a 4-day rehydration period with ad libitum access to drinking water.
During the restriction period, the cows had access to drinking water
every day from 0600, at the same time when food was first presented in
the morning, until the allotted amount of drinking water was consumed.
The cows were weighed once during baseline and every day during the restriction and rehydration periods.
In experiment 2, six lactating Brown Swiss cows [191 ± 7 (mean ± SE) days post partum, four of them pregnant (59 ± 5 days of pregnancy), 601 ± 23 kg body wt] had ad libitum
access to drinking water for 5 days (baseline) and were then subjected
to 50% drinking water restriction relative to individual baseline
intake for 9 days (restriction period). During the restriction period,
animals had access to drinking water from 0900, at the same time when food was first presented in the morning, until the allotted amount of
water was consumed. The 5 baseline days and the 5 last days of the
restriction period (days 5-9 of restriction) were used for data collection, assuming steady-state according to the results of
experiment 1. During these days, feces, urine, and milk were collected. For respiratory measurements on days 3 and
4 of the baseline and on days 7 and 8 of the restriction period, two cows each were randomly placed into two
respiration chambers. During all other days, they were housed in a
tying stable with partly slatted floor designed for balance trials. On
the last day of the baseline and restriction periods, the cows were
weighed (at 0800) and a blood sample was taken (at 1500).
Feeding and meal patterns.
During both experiments, the cows had ad libitum access to a mixed diet
of grass silage (57%), corn silage (33%), and hay (10%). Diet
composition was as follows: 51.9% dry matter (DM), 6.1 MJ/kg DM net
energy for lactation, 78.1 g/kg DM digestible protein in the intestine,
124 g/kg DM crude protein, and 257 g/kg DM crude fiber. In
experiment 1, the diet was provided three times a day (at
0600, 1030, and 1530). In experiment 2, the diet was provided at 0900 and 1530. The animals were adapted to the diet for at
least 2 wk before the experiments. Food intake and meal patterns were
continuously recorded online by a computerized feeding system similar
to the one described previously by Senn et al. (34). Meals
were defined as weight changes 
50 g after electronic identification
of a cow at the feeding trough, lasting 1 min (minimum meal
duration), and separated by 8 min (minimum intermeal interval) from
other weight changes, as described previously (34). Meals
defined and recorded this way accounted for 99.9% of total daily food intake.
Drinking and water restriction.
The drinking troughs were controlled by computer-assisted water meters
and water valves (Bürkert, Fluid control systems, Ingelfingen,
Switzerland). This system allowed for continuous recording (±1%) and
automatic restriction of individual 24-h drinking water intake when a
preset maximum volume of water was consumed, i.e., the water valve for
a particular cow's drinking trough was automatically closed until the
next morning when this cow had consumed the amount allotted for the day.
Milk recording.
Milk yield was recorded automatically throughout both experiments
(METATRON, Westfalia Separator, Oelde, Germany).
Respiratory measurements.
During experiment 2, oxygen consumption and the output of
carbon dioxide and methane were measured by open circuit calorimetry (38) in the respiration chambers for 2 × 24 h
in each data-collection period (baseline and restriction). The two
chambers were air conditioned to match the climate in the tying stable
[21.8 ± 0.1°C (mean ± SE) temperature and 59.8 ± 0.4% relative humidity]. Air flow was maintained at 28.6 ± 0.6 m3/h and recorded with inline electronic flowmeters
(SWINGWHIRL DV 630, Flowtec, Reinach, Switzerland). Gaseous composition
of the air flowing into and out of the chambers (internal volume: 20 m3 each) was measured using infrared analyzers (BINOS,
Leybold-Heareus, Zurich, Switzerland) for carbon dioxide and methane,
and a paramagnetic analyzer (OXYMAT 3, Siemens, Dietikon, Switzerland)
for oxygen. The whole system was manually calibrated each day before
the onset of the respiratory measurements. Heat production of each
animal was estimated according to the following equation
(12): heat production (kJ) = 16.179 × O2 (l) + 5.022 × CO2 (l) 
2.168 × CH4 (l) 5.989 × N in urine (g).
Collection and processing of samples in experiment 2.
During the 5-day baseline period and during the 5 data collection days
of the restriction period in experiment 2, samples were
taken either daily (feces, urine, milk) or every second day (food). The
food samples were dried (60°C, 48 h) and milled for later
analysis of total ash, crude fiber, gross energy, carbon, and nitrogen
content as described below. The daily samples of feces were stored at
4°C until the end of the respective collection periods, when they
were mixed to an aliquot for each period and cow. One part of each
aliquot was dried and milled for the analysis of total ash, crude
fiber, and gross energy, the other part was frozen and stored at
20°C until later analysis of carbon and nitrogen content.
Twenty-four hour urine was collected in two containers via a flexible
urinal fixed on VELCRO tape that was glued onto the body with special
adhesive (CYANOLIT, 3M, Rueschlikon, Switzerland). The urine in one
container was mixed with 60 ml sulfuric acid (5 M) to avoid N
losses for further analysis of nitrogen. The urine in the second
container was used for carbon analysis. The daily samples of acidified
urine were stored at 4°C, whereas the nonacidified samples were
frozen until the end of the collection periods. At the end of each
collection period, the daily samples were pooled, frozen, and stored at
20°C until analysis of carbon and nitrogen.
For milk composition analysis, two samples per milking were taken
automatically in the stable and manually in the respiration chambers.
One of these samples was conserved with sodium acid (BROMOPOL, BSM2,
D&F Control, San Ramon, CA) and stored at 4°C for the weekly analysis
of fat, protein, and lactose by the Swiss Brown Cattle Breeders'
Federation (Zug, Switzerland). The other sample was frozen and stored
at 20°C until the end of the collection period and then also
pooled. One part of this pooled sample was then frozen in liquid
nitrogen and lyophilized for the analysis of dry matter and gross
energy. The other part was used for the carbon and nitrogen analysis.
At 1500 on the last day of the baseline and restriction periods, ~35
ml blood was taken from the jugular vein. The blood was collected into
EDTA (for hormone analysis), heparin, and NaF tubes (for analysis of
metabolites, osmolality, and electrolytes). One milliliter of the
EDTA-blood was mixed with 500 kallikrein inactivating units (APROTININ,
Böhringer Mannheim, Rotkreuz, Switzerland) for later analysis of
glucagon. All blood samples were immediately centrifuged (1,600 g, 4°C, 15 min), and the plasma was then frozen at
20°C until analysis.
Laboratory analyses.
The Weende method was used to analyze total ash and crude fiber in food
and feces (32). Carbon and nitrogen in food, feces, urine,
and milk were analyzed using an automatic C/N analyzer (Leco-analyzer,
Type FP2000, Leco Instruments, St. Joseph, MI): the samples were
oxygenized at 950°C, and the carbon was then measured by an infrared
cell; nitrogen was measured by a detector for heat conductivity with
helium. The gross energy content of food, feces, and milk was measured
with an adiabatic bomb calorimeter (IKA-calorimetry system C 700 T,
IKA-Analysentechnik, Heitersheim, Germany). The gross energy content of
urine was calculated by the following equation (20): gross
energy in urine (kJ) = 33.1 × C in urine (g) + 9.2 × N in urine (g). Fat, protein, and lactose in milk were measured by
an infrared-photospectrometer (MILKOSCAN 4000, Foss Electric,
Hillerød, Denmark). Plasma glucose, lactate, free fatty acids (FFA),
-hydroxybutyrate (BHB), triglycerides, glycerol, urea, and protein
were measured according to standard enzymatic procedures
(16) using an automatic analyzer (Cobas-Mira, Roche
Diagnostics, Basel, Switzerland). A freezing point osmometer (MULTI-OSMETTE, Precision Systems, Natick, MA) was used to measure plasma osmolality. Plasma sodium, potassium, and chloride were determined by flame photometry (FLM3, Radiometer Copenhagen,
Instrumenten-Gesellschaft, Zurich, Switzerland). Commercially available
radioimmunoassay kits were used for the determination of plasma
cortisol and glucagon (Diagnostic Products, Los Angeles, CA) and
insulin (Pharmacia, Uppsala, Sweden). EDTA-blood was used to measure
the hematocrit (Hettich Hematocrit centrifuge, 5 min).
Calculation of energy and nutrient balances.
Intake was opposed to excretion for calculation of water, nitrogen,
carbon, and energy balance. Metabolizable energy is defined as gross
energy intake minus fecal, methane, and urinary energy. Retained energy
comprises energy in milk and body energy balance. Body fat balance was
calculated from carbon and nitrogen balances (12).
Statistics.
In experiment 1, a repeated-measures ANOVA was performed to
test for the effects of day and group and for day × group
interactions. The data were analyzed in two steps: first baseline and
restriction periods were compared, then baseline and rehydration. When
the day effect was significant, pair-wise comparisons of selected days
were made with the paired t-test. In experiment
2, mean values were calculated for further analysis when more than
one value was obtained for a given parameter during the data collection periods (e.g., daily milk production). Due to the small number of
animals (n = 6), the data were often not normally
distributed. Therefore, the nonparametric Wilcoxon test was used for
the comparison of baseline and restriction period values of the same
animals. All analyses were done with SYSTAT 7.0 (SYSTAT, Evanston, IL). Data are presented as means ± SE. P values <0.05 were
considered significant.
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RESULTS |
Experiment 1.
Baseline drinking water intake in experiment 1 was 59.8 ± 1.9 (5-day individual means ± SE) and 59.7 ± 3.3 l/day
for the cows subsequently subjected to 25 and 50% drinking water
restriction, respectively. Note, however, that total water intake
(drinking water plus water in food) was reduced only ~22 and 44%
with 25 and 50% water restriction, because food intake was reduced
less than 25 and 50%, respectively. During the 8 days of water
restriction, body weight decreased in cows with 50% restriction, but
not in cows with 25% restriction (interaction group × day:
P < 0.05, Fig. 1). Body
weight stabilized around 99% (25% restriction) and 95% (50%
restriction) of baseline values after 4 days of water restriction. On
rehydration, the body weight of both restriction groups immediately
increased above the baseline level and was higher (P < 0.05) than during baseline on all rehydration days. Daily food intake
decreased in relation to the water restriction level (interaction
group × day: P < 0.05, Fig.
2) during the first 3-4 days of
water restriction and fluctuated around an 11 and 21% (with 25 and
50% water restriction, respectively) lower level for the remainder of
the restriction period. Food intake immediately returned to the
baseline level on rehydration. Meal pattern analysis revealed a
particularly strong effect of water restriction (50 and 25%
restriction) on the size of the first meal after the presentation of
fresh food in the morning (Fig. 3). The
first meal during water restriction started at about the same time as
during baseline, but, from the third restriction day onward, it was
>50% smaller (effect of day: P < 0.05) and shorter
(effect of day: P < 0.05, data not shown) than
baseline meals. Note that the size of the first meal was not affected
on the first water restriction day because at that time the cows had
not yet experienced any water restriction. On the first restriction day
they ran out of water (reached their allotted amount) at 1200 (50%
restriction) and 1700 (25% restriction), respectively. From the third
day of 50% water restriction onward, the cows consumed all the
allotted drinking water within the first 60 min of access. With 25%
water restriction, they consumed about 65% of the allotted amount
within the first 60 min. A more detailed analysis of feeding and
drinking patterns during the first 60 min of access to drinking water
showed that the cows irregularly alternated between drinking and
feeding. On rehydration, the size of the first meal rapidly reached the baseline level again. The mean size of the second and of all following meals decreased by about one-third with 50% water restriction (effect
of day: P < 0.05) and scarcely so with 25% water
restriction. Meal frequency increased with 25 and 50% water
restriction to reach a maximum on the third day of restriction
(day 8, significantly higher than all baseline days, Fig.
4). On the first day of the rehydration
period after 50% water restriction, meal frequency decreased
(day 13 vs. day 14: P < 0.05) to
approximately the baseline level. After 25% water restriction, meal
frequency stayed as high as at the end of restriction. Milk yield was
lower with 25 and 50% water restriction (effect of day:
P < 0.05) than during baseline, decreasing with a
delay of 1 day (Fig. 5). Milk yield also
recovered with a 1-day delay during the subsequent rehydration period.
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Fig. 1.
Body weight changes (relative to the initial weight) of
lactating dairy cows during 25 and 50% drinking water restriction
(relative to baseline intake) and rehydration (experiment
1). Data are means ± SE of 9 (25% restriction) and 8 (50%
restriction) cows. See RESULTS for further details.
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Fig. 2.
Food intake of lactating dairy cows during baseline, 25 and 50% drinking water restriction (relative to baseline intake), and
rehydration (experiment 1). Data are means ± SE of 9 (25% restriction) and 8 (50% restriction) cows. See
RESULTS for further details.
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Fig. 3.
Size of the first meal and mean size of all subsequent
(subs) meals during baseline, 25 and 50% drinking water restriction
(relative to baseline intake), and rehydration (experiment
1). Data are means ± SE of 9 (25% restriction) and 8 (50%
restriction) cows. For the values of the subsequent meals, individual
means were calculated first. See RESULTS for further
details.
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Fig. 4.
Meal frequency of lactating dairy cows during baseline,
25 and 50% drinking water restriction (relative to baseline intake),
and rehydration (experiment 1). Data are means ± SE of
9 (25% restriction) and 8 (50% restriction) cows. See
RESULTS for further details.
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Fig. 5.
Milk production of dairy cows during baseline, 25 and
50% drinking water restriction (relative to baseline intake), and
rehydration (experiment 1). Data are means ± SE of 9 (25% restriction) and 8 (50% restriction) cows. See
RESULTS for further details.
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Experiment 2.
With 50% drinking water restriction (37.7 ± 1.8 vs. 75.4 ± 4.5 l, P < 0.05), food intake during the 5 last
days of the restriction period was ~20% lower than during the water
ad libitum (baseline) period (25.7 ± 1.2 vs. 32 ± 1.2 kg/day, P < 0.05). The total water-to-food ratio
[(intake of drinking water + water in food)/food DM (kg/kg)] decreased from 5.6 ± 0.2 (mean ± SE) in the baseline period
to 4.3 ± 0.1 (P < 0.05) in the water restriction
period. Body weight, measured on the last day of each period, decreased
from 601 ± 23 kg (mean ± SE) to 535 ± 18 kg
(P < 0.05). Compared with baseline, ~50% fewer
feces (P < 0.05) were produced in the water
restriction period, but their DM content was markedly
(P < 0.05) higher (Table 1). Similarly, the total volume of urine
produced decreased (P < 0.05) during the water
restriction period by 43%, but the urine was more concentrated
(P < 0.05, Table 1). Milk production decreased (P < 0.05) during the water restriction period by
~27% from 18.7 to 13.6 kg/day, but milk composition did not change
significantly, except for lactose and urea content, which were higher
during the water restriction than during the baseline period (Table 1). The total water-to-milk ratio [(intake of drinking water and water in
food)/milk (kg/kg)] decreased from 4.9 ± 0.2 (baseline) to 3.8 ± 0.1 (restriction period) (P < 0.05). Water
balance did not change between baseline and restriction periods (Table
2). The decreases (P < 0.05) in water excretion via urine, feces, and milk paralleled the
reduced intake so that the water excretion-to-intake ratio remained
<1, because water loss via body surface and lung was not considered in
the balance calculation. Organic matter digestibility was higher during
the restriction than during the baseline period (0.74 ± 0.01 vs.
0.70 ± 0.01, P < 0.05). Digestibility of the
crude fiber tended to be higher in the restriction than in the baseline
period (0.70 ± 0.01 vs. 0.66 ± 0.02, P > 0.05).
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Table 1.
Composition of feces, urine, and milk of 6 lactating dairy cows with ad
libitum access to water (Baseline) and with 50% water restriction
(Restriction) in experiment 2
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Table 2.
Water balance of 6 lactating dairy cows with ad libitum access to water
(Baseline) and with 50% water restriction (Restriction) in experiment
2
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The nitrogen balance became negative during the water restriction
period (P < 0.05, Fig.
6). Expressed as percent of nitrogen intake, the proportion of nitrogen excreted with milk and especially with urine was higher (P < 0.05) during the
restriction than during the baseline period (Fig. 6). Therefore,
nitrogen utilization was less efficient during water restriction than
during the baseline period (0.53 vs. 0.65), and the cows had 44.6%
less nitrogen available for production (retained N) than during the
baseline period (P < 0.05, Fig.
7).
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Fig. 6.
Nitrogen balance of 6 lactating dairy cows with ad
libitum access to water and 50% drinking water restriction
(experiment 2). Data are means ± SE. *Significant
(P < 0.05) difference between water ad libitum and
50% restriction values. See RESULTS for further details.
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Fig. 7.
Nitrogen utilization levels of 6 lactating dairy cows
with ad libitum access to water and 50% drinking water restriction
(experiment 2). Data are means ± SE. The numbers
represent the coefficients of utilization from one level of nitrogen
utilization to the next. *Significant (P < 0.05)
difference between water ad libitum and 50% restriction values. See
RESULTS for further details.
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The energy balance did not change significantly with 50% water
restriction (Fig. 8), because the
decrease in absolute energy excretions with water restriction largely
paralleled the decrease in energy intake. Furthermore, a reduction
(P < 0.05) in the proportion of ingested energy lost
through feces compensated for the increase (P < 0.05)
in the proportion of energy excreted in urine during the same period
(Fig. 8). Plotting the various stages of energy utilization during the
baseline and restriction periods (Fig. 9)
reveals that the difference (P < 0.05) between both
periods at the level of gross energy intake became gradually smaller
and disappeared at the level of retained energy. The coefficients of
energy utilization tended to be higher during the restriction than
during the baseline period, but this difference reached significance (P < 0.05) only for digestibility (0.71 vs. 0.67). On
the basis of the reduced (P < 0.05) gaseous exchange
during the restriction period, absolute heat production decreased
(P < 0.05) by 21.7% (Table
3), but again, the proportion of heat
production relative to intake did not (Fig. 8). Heat production can be
divided into heat related to milk production [calculated with kl = 0.6 (30)] and heat related to maintenance needs. Under
the assumption of an unchanged efficiency of utilization of
metabolizable energy (kl), heat production for maintenance was reduced
(P < 0.05) by 17.8% during the restriction period
(Table 3). In line with the trend observed in energy balance, the cows
mobilized less (P < 0.05) fat during water
restriction than during baseline (Table 3).
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Fig. 8.
Energy balance of 6 lactating dairy cows with ad libitum
access to water and 50% drinking water restriction (experiment
2). Data are means ± SE. *Significant (P < 0.05) difference between water ad libitum and 50% restriction values.
See RESULTS for further details.
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Fig. 9.
Energy utilization levels of 6 lactating dairy cows with
ad libitum access to water and 50% drinking water restriction
(experiment 2). Data are means ± SE. The numbers
represent the coefficients of utilization from one level of energy
utilization to the next. GE, gross energy; DE, digestible energy; ME,
metabolizable energy; RE, retained energy. *Significant
(P < 0.05) difference between water ad libitum and
50% restriction values. See RESULTS for further details.
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Table 3.
Gaseous exchange, heat production, and body fat balance of 6 lactating
dairy cows with ad libitum access to water (Baseline) and with 50%
water restriction (Restriction) in experiment 2
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The plasma concentrations of urea and sodium as well as the hematocrit
were higher (P < 0.05) during the water restriction than during the baseline period (Table
4). Plasma concentrations of lactate,
BHB, and chloride, as well as plasma osmolality, tended to be higher
during the water restriction period; plasma glucose, FFA, and glycerol
tended to be lower (Table 4), but all these differences did not reach
significance. Plasma concentrations of insulin, cortisol, glucagon,
triglycerides, protein, and potassium, did not change during water
restriction (Table 4). The plasma concentrations of some metabolic
hormones and metabolites showed great individual variations due to one
animal that was in estrus during the baseline period.
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Table 4.
Composition of the blood plasma of 6 lactating dairy cows with ad
libitum access to water (Baseline) and with 50% water
restriction (Restriction) in experiment 2
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DISCUSSION |
This study shows that even lactating dairy cows are able to cope
with a sustained 50% restriction of drinking water intake (~44% of
total water intake) and identifies some of the mechanisms involved. In
demonstrating that water restriction primarily reduces meal size, the
results of the first experiment confirm and extend previous studies of
our group in which ruminants were completely deprived of drinking water
for shorter periods of time (25, 34). The new finding is
that this reduction in meal size was also present with sustained water
restriction and that it was particularly pronounced for the first meal
on every day. This is surprising because the cows consumed substantial
amounts of water around the time of this first meal. It is unlikely
that meal size was somehow limited by the competing drinking behavior or by rumen space, because on the first day of rehydration after 25%
water restriction, the size of the first meal was much bigger than
during water restriction (see Fig. 3), although the cows ingested
38 l of water during the first hour of access, i.e., exactly the
amount that was consumed in the same time with 50% restriction.
Despite the substantial reduction in its size, the first meal was still
the biggest meal of the day during water restriction. As rumen
hypertonicity has been proposed to be a major control of meal size in
ruminants (12), it is possible that the intake of a
substantial amount of water before and together with the first meal
allowed the cows to eat more in this meal before a level of rumen fluid
or perhaps systemic osmolality was reached that limited meal size. In
particular, with 50% water restriction, this level would be reached
much sooner during subsequent meals, when no more water was available.
Results from another study in which we measured rumen fluid osmolality
in relation to spontaneous meals in cows with unlimited access to water
and during water restriction (39) are consistent with the
idea that an abnormal prandial increase in rumen fluid osmolality
contributes to the meal size reduction during water restriction. Other
possible factors include systemic hypertonicity (39) and
cardiovascular and vascular volume effects of feeding related to the
copious production of saliva in ruminants (3, 13, 37),
which could not be detected with the design of the present study.
Despite the marked reduction of the first meal's size, it should be
noted that the food intake suppression that occurred with 50% drinking water restriction was mainly due to the consistent reduction of the
size of subsequent meals, because this effect, similar to the feeding
suppressive effect, was not observed with 25% water restriction.
Most of the parameters measured in the first experiment, in particular
body weight, reached a new and remarkably constant level after 3-4
days of water restriction. Moreover, when water was offered ad libitum
again, there was no sign of a compensatory increase in food intake.
Thus the cows appeared to reach a new equilibrium after 3-4 days
of water restriction. The second experiment was based on the assumption
that the cows were in stable energy balance during the balance
measurement periods and that representative mean values could be
calculated over the last 5 days within the water restriction period.
The unchanged water excretion-to-water intake ratio during the baseline
and restriction periods in experiment 2 confirmed that a new
equilibrium was reached.
The observed increases in hematocrit and plasma osmolality are signs of
a reduced plasma volume (17) in response to water restriction. Also, the rumen water volume was certainly reduced, and
this presumably accounted for a major part of the total water loss. Use
of gut water helps to attenuate the rise in blood plasma osmolality
during dehydration (37). Active sodium absorption from the
rumen (21), which usually drives fluid absorption, presumably contributed to the observed hypernatremia. Increased sodium
retention in the kidney also helps to conserve water. It reflects the
activation of compensatory endocrine mechanisms (renin-angiotensin system, aldosterone, vasopressin) by the reduction in plasma volume and
the increase in plasma osmolality (5, 8, 23). With an
increase in plasma sodium, renal sodium excretion eventually increases
again (29, 31) and stabilizes plasma sodium at a higher
level. Plasma sodium is also recycled and concentrated in the saliva
(36).
Water restriction caused an increase in the apparent digestibility of
organic matter and energy, which helped to maintain energy balance. It
is unlikely that the better digestibility during water restriction was
an artifact of the short collection period of only 5 days, because
others reported similar results with longer adaptation and collecting
periods (4, 35). One reason for the better digestibility
is probably a longer mean retention time of the digesta in the
gastrointestinal tract, in addition to a decrease in the size of
particulate matter in the rumen (14). So, the marked
osmotic changes that presumably occurred every morning when the cows
quickly consumed the allotted amount of water did not seem to inhibit
the fermentation capacity of the rumen microorganisms (6,
11). In another similar experiment from our laboratory
(39) in which drinking water intake was restricted by 65%
of ad libitum intake, rumen fluid osmolality in fact never reached 400 mosmol/kgH2O, i.e., a level above which cellulose
degradation appears to be inhibited (6).
The apparent digestibility of nitrogen was not improved, and, in fact,
the nitrogen balance became negative when water was restricted. It is
not clear why the apparent digestibility of nitrogen did not change.
Brosh et al. (10) found the dehydration-induced increase
in nitrogen digestibility to be inversely related to the quality of the
diet. But the quality of our ration was not high enough for such an
explanation. Perhaps enhancing effects of water restriction on nitrogen
digestibility at the site of the rumen were compensated by the
incomplete digestion of the rumen microbial protein synthesized from
the additional ruminally fermented organic matter. It is of course also
possible that changes in endogenous nitrogen excretion masked changes
in true nitrogen digestibility. Further studies are necessary to
clarify this question. The negative nitrogen balance was due to the
relative increase in nitrogen excretions (in % of intake) in form of
urinary urea and, partly, milk urea as a result of the elevated plasma
concentration of urea. Tissue protein breakdown and the subsequent
increase in amino acid catabolism was the most likely source of the
increase in plasma urea concentration. Recycling urea in the kidneys
helps to reduce urinary volume and, hence, to conserve water
(28). The increased plasma urea content was unlikely to be
a consequence of the reduced nitrogen needs for milk production because
water restriction increases plasma urea also in nonlactating animals (e.g., Ref. 33).
During the water restriction period, the cows still had ~81% of the
baseline metabolizable energy available. This energy was used for
maintenance and milk production (neglecting the needs for early
pregnancy). When water is limited, milk production declines according
to the water and nutrient shortage (15), thus decreasing the energy needs for milk production to a certain extent.
Interestingly, the results suggest that the energy needs for
maintenance also declined. A similar observation was reported by Brosh
et al. (9) in nonlactating, infrequently watered,
desert-adapted goats. These authors based their conclusion on the
reduced O2 consumption under these conditions and on the
fact that the goats did not lose body mass during dehydration except
for body water. In the dairy cows of our study, the absolute heat
production for maintenance was reduced by water restriction. According
to the nitrogen and fat balance calculations, the cows may have lost
~60 g protein and 80 g fat per day during water restriction.
This appears to be at odds with the numerically positive energy balance
and the unchanged body weight, but all these changes were very small
and are probably within the error range of the methods employed.
Therefore, the present data do not allow us to judge whether the cows
lost some solid substances during water restriction or not. In
experiment 1, the cows completely regained their body weight
during the first day of rehydration, suggesting that they had mainly
lost water during the water restriction period. Silanikove and Tadmor
(37) showed that body water loss accounted for ~89% of
the total weight loss of nonlactating beef cows during 3 days of water
deprivation. If a similar relationship held for the lactating dairy
cows in the present study, they would have lost ~59 kg of water and 7 kg solid substances. This would have negligible effects on net energy
for maintenance when using the following equation (2): net
energy maintenance = (0.53 × body wt/1.08)0.67 + 0.0091 × body wt. The energy requirements for maintenance include
the energy needed for gut metabolism (eating, food processing and
absorption), which was found to decrease with the decrease of food
intake in non-desert-adapted animals (40). This could
contribute to the decrease in energy requirements for maintenance in
the cows of the present study. All in all, it remains unsolved whether
and to which degree reduced maintenance requirements and/or an
increased efficiency of use of metabolizable energy for milk production was responsible for the slight increase in overall use of metabolizable energy during water restriction (0.34 vs. 0.32).
The results of the blood metabolite measurements fit the roughly stable
energy balance. The lack of a change in plasma fat and carbohydrate
metabolites indicates that there was no fat mobilization and, hence, no
energy deficit during water restriction. The lack of changes in the
plasma levels of metabolically active hormones is also consistent with
this interpretation. In addition, the constancy of the plasma cortisol
concentration across the baseline and water restriction periods
suggests that the cows were not continuously stressed by the water
restriction. This is also interesting from an animal welfare point of
view. In line with our findings, others (33) recently
reported that also in South African indigenous goats several blood
parameters changed during only the first few days of a 50% water
restriction period and returned to baseline levels thereafter. Thus, in
principle, it could be possible to extend the water restriction period
for a longer period of time without additional effects, as did Little
et al. (27) during 3 wk, but with a lower level of water
restriction (40%).
To our knowledge, this is the first report in which calorimetry
measurements in respiration chambers were used to critically examine
the changes in metabolism as well as energy and fat balance during
water restriction. The results reveal that dairy cows are able to cope
with a sustained restriction of total water intake to almost 50%,
i.e., the decrease in milk yield and food intake, with the concomitant
decrease in heat production (metabolism) and improved water retention
combined to save sufficient water to reach a new equilibrium at a lower
water turnover level. The body weight and food intake data show that
the cows reached a new balance after 3-4 days of water
restriction. The food intake suppression was primarily due to a
reduction in meal size, which fits the idea that an abnormal prandial
increase in rumen fluid osmolality contributes to the food intake
suppression during water restriction (39). The lower
energy intake was compensated by a lower milk production, a higher
digestibility, and, apparently, a more efficient metabolic energy
utilization. Further studies should also critically examine nitrogen
metabolism during water scarcity because a loss of nitrogen, which
might occur based on our nitrogen balance calculations, could become
limiting for long-term adaptation to water scarcity.
Perspectives
In many areas of the world, ruminants have to cope with temporary
water shortages; even in industrialized countries, water availability
may at times be insufficient. For example, in young animals shortly
after weaning, when it takes days before the level of water intake
reaches that of milk. Water intake may also be limited during early
lactation in high-yielding dairy cows, when fluid balance is pushed to
its limits. Moreover, pathological situations such as diarrhea, rumen
acidosis, and other diseases can cause systemic dehydration. The
present results demonstrate the amazing ability of ruminants to cope
with water shortages. Initially, much of that ability is related to the
water reservoir function of the rumen (36); with
continuing water shortage, however, other mechanisms, including the
food intake suppression, come into play. Eating less during water
restriction helps to maintain osmotic balance, because smaller meals
reduce the impact of an osmotic load (food) not balanced by adequate
drinking water intake. If ruminants failed to decrease food intake
during dehydration, it might even compromise the osmotic buffer
function of the rumen, because it might increase rumen fluid osmolality
so much as to prevent the use of rumen water to alleviate the systemic
hypertonicity of dehydration. Thus the suppression of food intake
during water scarcity represents a compromise between the needs to
ingest nutrients and to maintain osmotic balance of body fluids and
reflects a homeostatic mechanism that minimizes the negative
consequences of dehydration.
| |
ACKNOWLEDGEMENTS |
We thank the staff of the ETH research station Chamau, in
particular Dr. Hans Leuenberger, for help organizing and performing the
experiments, and Myrtha Arnold and Anthony Moses for the plasma analyses and Meret Gebert for the analyses of food, milk, feces, and urine.
| |
FOOTNOTES |
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Address for reprint requests and other correspondence: W. Langhans, Physiologie und Tierhaltung, Institut für
Nutztierwissenschaften, ETH-Zentrum/LFW B 55.1, Universitätstr.2,
CH-8092 Zürich, Switzerland (E-mail:wolfgang.langhans{at}inw.agrl.ethz.ch).
Received 14 January 2000; accepted in final form 22 September 2000.
| |
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