Water, Sodium, Potassium, and Chlorine Metabolism
of Dairy Cows at the Onset of Lactation in Hot Weather
U. SHALIT; E. MALE,' and N. SILANIKOVE'
Agriculture Research Organization
The Volcani Center
Bet Dagan W250,Israel
A. BERMAN*
Hebrew University
Rehwot, Israel
ABSTRACT
Water, Na, K,and Cl balances, blood
plasma composition, and urine and fecal
outputs were studied in 5 high yielding
cows (>30kg/d milk) at 2 wk prepartum
and at 2 and 7 wk postpartum during the
summer in Israel. Cows were fed complete diets with electrolyte content exceeding dietary recommendations.
Plasma volume, as assessed by hematocrit changes, was greater postpartum,
probably due to increased heat load and
water turnover. Milk secretion markedly
increased electrolyte output, which was
compensated for only partially by increased intake. This was associated with
marked reduction of electrolyte losses in
excreta, particularly that of Na and C1.
On the basis of urea excreted in the
urine, it seems that the c m n t practice
of abruptly increasing protein content of
the diet at the onset of lactation might
reduce the efficiency of dietary protein
utilization, compared with efficiency of
protein utilization at a later stage of lactation. The need to excrete excessive N
also adversely affected the water and
electrolyte balances. At initiation of lactation, when DMI is still limited and hot
weather obstructs its rapid increase, the
current recommendations for electrolytes
as a percentage of the ration do not meet
the needs.
(Key words: electrolyte metabolism, wa-
Received March 8, 1990.
Accepted November 13, 1990.
'Institute of Agricultural Engineering.
*Department of Animal Sciences.
1991 J Dairy Sci 7418744883
ter metabolism, lactation)
Abbreviation key: WTO = water turnover.
INTRODUCTION
High producing dairy cows are among the
most productive mammals in terms of milk
yield. At peak lactation, milk may exceed 10%
of body mass per day. Because milk contains a
significant amount of water (ca. W%),the
onset of lactation is associated with large acceleration in water turnover (WTO).
During lactation, electrolytes (Na, K, and
Cl) are lost in milk, which puts an extra burden upon mechanisms regulating electrolyte
balance. From data available on goats, it can
be calculated that the amount of Na secreted
via milk is equal to that lost in urine (22, 28).
Available data on electrolyte balances for the
high producing dairy cow are meager. Due to
their high metabolic rate, dajr cows in subtropical, desert, and Mediterranean mas are
above their thermoneutral zone during the
summer (3). The resulting need for enhanced
heat dissipation increases electrolyte requirememts due to their losses by sweat and saliva
polypnea (2). Providing electrolyte requirements as a percentage of DM rather than daily
intake (21) may oversimplify a complex situation, especially at the onset of lactation. Addition of NaHCO3 and KCl benefited heatstressed dairy cows in terms of milk yield,
regulation of acid-base balance, and lowering
body temperatures (5, 25, 26, 27).
Supplying protein at levels greater than requirement increases water needed for urinary
disposal of waste products (8). This need is
aggravated by heat stress, which decreases protein utilization (1) but increases water loss for
thermoregulation.
1874
WATER AND EL.ECTRl DLYTE METABOLISM
1875
Dairy cows in Israel usually are transferred ing at 1300 h. The cows drank from individual
abruptly from a high forage, modest protein (8 drinking cups connected to flow meters (Arad,
to 12%) diet prepartum to a high concentrate Dalia Ltd.,Israek precision f 1.0 L). Cows
(60 to 70%), high protein (16 to 18%) diet were milked at 8-h intervals at 0500,1300, and
after parturition. The abrupt increase in protein 2000 h.
The following procedures were carried out
intake is not always accompanied by extra
at 2 wk before partutition (period 1) and at wk
protein output in milk.
The present study was to examine balances 2 (period 2) and 7 (period 3) postpartum.
of water, Na, K, and C1 in prepartum and early Rectal temperature of the cows was measured
lactation cows fed according to practice in every day at 1200 h as was ambient temperaIsrael to evaluate their requirements, their ture and relative humidity in the individual
adaptation to lactation, and the interaction be stall barn. For urine collection, a rubber device
was glued around the vulva (Medical Adhetween N and electrolyte metabolism.
sive, Dow Corning Corp., Medical Products,
MI) and connected via tubing to
Midland,
MATERIALS AND METHODS
containers placed on a lower level. Every
A trial was conducted during the summer morning for 7 d, containers were weighed, and
between July and September in five Israel a sample was taken and stored at -17'C until
Holstein cows approaching their second partu- analysis. Feces were weighed once daily over
rition. Noon temperature ranged from 28 to the same perid, homogenized, sampled, and
30'C and relative humidity 65 to 70%. Six dried in a forced-air oven at 6o'C to constant
weeks before expected parturition, cows were weight, ground (1 mm), and dehydrated at
transferred to individual indoor stalls (with no 105'C. Five samples of fresh fecal excretions
change in diet) in which feed and water intakes taken at different hours over the day served for
and excretions (feces and urine) were mea- determination of water and electrolyte content.
sured individually. Before and after parturition Milk samples from every milking were taken
cows were fed complete rations prepared daily and analyzed daily. The water balance
Vable 1). Half of the daily ration was given at described in Table 3 was defined as total in0800 h and the remainder after the noon milk- take minus the measured output avenues. In
addition to total values, the water output
avenues also may be presented as "milk-free
water balance", which is the total intake minus
TABLE 1. Ingredient allotment and chemical analysis of the water in milk. This represents the water
diets. Fed 2 wk before parturition (period I) and 2 and 7 needed for maintenance plus water cost of
wk @ a i d s 2 and 3. respectively) postparhun.
milk production. Obviously, for cows prepartum, milk-free water balance equals the total
P d d S
Ingredient
Paiodl
2and3
intake. Thus, when loss avenues are compared
prepartum and postpartum as a fraction of
(% DM)
milk-free water balance, trends of adaptation
Wheat silage
100
32.9
can be identified easily. Fecal electrolytes were
Concentrates'
...
53.8
cottonseeds
...
13.3
extracted by boiling in distilled water followed
42.5
65.O
DM, 96 as fed
by overnight equilibration (24).
8.0
16.3
CP
On the last day of each experimental period,
26.0
14.7
Crude fiber
cows were fitted with jugular catheters, and
8.01
6.92
Ash
.71
.32
Na
blood samples were withdrawn hourly from
K
4.08
1.83
each cow during 24 h. Blood samples were
a
1.37
.61
centrifuged immediately (4000 rpm), and
Ca
27
.60
plasma
was separated and stored in -17°C until
P
.10
.48
analyzed. Whole blood samples also were colvitamin complex2
...
152
lected into heparinized capillaries for the hem'Mixture of corn and soybean meal.
atocrit determination.
kontains 4, .8 and .75 million IU/kg vitamin A, D,
Sodium and K were analyzed by flame phoand E, respectively, and .36,6.6, 32. .08, 6, and .08 gfkg
tometry (Coming 480, Coming Medical and
I, Fe, Mn, Cu, Co, Zn, and Se, respectively.
-
Journal of D a q Science Vol. 74. No. 6, 1991
1876
SHALIT ET AL.
TABLE 2. Body weight,DMI. milk yield and body temperature at noon in 5 cows 2 wk before parturition (period 1) and
2 wk postpvhm (period 2) and 7 wk @eriod 3) ptpamm.
period1
X
sw.43
DMI
DMI, 46 of B W
MilL yield, kgld
Body temperature, 'C
683'
8.0b
1.1T
Paid 2
period
SD
fz
SD
21
.3
Sab
25
2.0
...
385b
.07
.2
14.2'
25ob
34.1
39.3'
.30
1.2
.1
X
54p
15.2'
2.78'
31.4
39.4'
3
SD
32
2.0
22
2.6
.3
4b*CMeanswithin rows having different rmpersCripts differ (P < .05).
Scientific, Corning Ltd.,Halsted. Essex) and
C1 by titration (Chloride Titrator CMT 10,
Radiometer, Copenhagen). Plasma and urine
osmolality were measured by a vapor pressure
osmometer (51OOC, Wescor, Inc., Logan, LIT).
Urea was determined colorimetrically following treatment with diacetylmonoxime (12).
Water content in milk was determined by dehydration at 105'C. The content of individual
electrolytes in drinking water samples were
measured and were less than 1 me@. Significance of differences between period means
was determined by paired t test (32) at P < .05
unless otherwise noted.
only .76 prepartum. The main reason for
this
was the reduction in water content of the diet
(Table 1). In addition, in period 3 (after peak
lactation) the cows increased their drinking
from levels in period 2.
Most of the water described as balance in
Table 3 represents evaporative and cutaneous
water loss. This conclusion could be drawn
from estimates of water retention, one of the
water balance components. Total water was
calculated to be 55 to 60% of postcalving BW
(if the cow is very fat) and 80 to 85% of BW
in the end of period 3 (if the cow is very thin).
Subtracting the former from the latter yielded
54 to 120 kg of water retention Over 7 wk,
which is 1.1 to 2.4 kg/d. This is a very small
fraction of the water balance (Table 3). In our
RESULTS
work, where the cows were neither very fat
postcalving
nor very slim at the end of pericd
Body Weight, DMI, Mllk Yield,
3, the water retention per day is even smaller.
and Body Temperature
Evaporative water loss (respiratory plus
Body weight dropped after pamuition and, cutaneous) was the main component of the
in period 3, decreased further (Table 2). Dry water output, accounting for -67 of the total
matter intake increased after parturition by intake prepartum and approximately .55 during
about 80%. rectal temperature increased mark- lactation. In terms of absolute values, and
edly, but ambient conditions did not differ. related to BW, evaporative water losses actuMilk yields, DMI, and rectal temperature were ally increased at beginning of lactation (period
not significantly different in the two postpar- 2) and increased further as lactation proceeded
tum periods. However, DMI as a percentage of (period 3). Nevertheless, when expressed as a
fraction of ndk-free balance (total intake miBW increased significantly.
nus water secreted in milk), the differences
between the periods diminished, although the
Water Balance
differen= between prepartum and each of the
postpartum periods remained significant. WaData on the water balance components are ter secreted through milk was the second
presented in Table 3. Total water intake during largest avenue for water loss, being .25 and .21
lactation (periods 2 and 3) was about twice of the total intake in periods 2 and 3, respecthat in the prepamm period. Milk-free water tively.
balance (total intake minus water in milk) in
Water loss in feces was the third largest
periods 2 and 3 was greater than in period 1. avenue accounting for .12 to .13 of the total
Drinking water accounted for approximately intake during lactation @eriods 2 and 3, re.9 of the total water intake during lactation but spectively) and .20 prepartum. Fecal water
Journal of Dairy Science Vol. 74, No. 6, 1991
1877
WATER AND J5LIXTROLY"I'E METABOLISM
TABLE 3. Water mtah and output in five cows 2 wk before calving (puiod 1). 2 wk postpamun @eriod 2). and 7 wk
postpartnm @eriod 3).
DrinLing
X
S D
Within
food'
-X
S D
Total
Inmilk
intake.
X
Fecal
Urinary
-X
SD
Balance2
SD
SD
-X
SD
Paiodl
ks
p/kp B W
49.4' 11.6
72c 16
15.F 2
1
e
64.p 11.8
94'
17
Fra~ti~n
Of milkfree
water
balance3
. ..
...
8.3b 1 5
12b 1
...
.13" .02
12.gb 1 5
2
1sC
2@
.03
43.3'
63'
.67b
11.6
17
.M
Period 2
kg
g/Lg B W
Fraction of milk-
103.Sb 17.8
183b 37
18.2b 2
32b 2
1.0 1 2 . p 1.9
21'
3
2
14.6b 2.1
Zb 3
65.2b
llSb
20.0
44
121.7b 17.8
21Sb 39
30.1'
53
129.9 18
237a 39
13"
.W
.16b .04
.71a
.08
Period 3
27.4b 2.4 10.4" 2.8
17.1' 2.4
75.1" 16.7
50
4
19 4
31'
5
137"
34
...
...
dree
water babe.
BW
Fraction of miIk-
lO9.b 16.3
199" 38
free
water balance
2O.P .6
38" 3
... ...
lob
.M
.17
.03
.73"
.OS
~bq'Means within cohurms for a given trait havisg different superscripts differ (P < .05).
lF'refomed plus metabolic. Metabolic wata was calculated assuming 1 kg H20/1 kg OM digsted (data not
presented).
%OW intake minus milk, Urinary, and fecal water.
3The fra~tionof output avutues in the t a intake. minus milk water.
loss, compared on the basis of milk-free water
balance, and fecal water content, which was
significantly reduced in period 3 (Table 4).
point toward the tendency for water conservation.
The water excreted via urine had the
smallest contribution to total water loss, accounting for .13, .lo. and .08of water intake in
periods 1,2, and 3, respectively, all differences
being statistically significant. The tendency to
conserve water duough this avenue was evident also when the data were compared on a
&-free
water balance. However, only the
difference between periods 1 and 3 was statistically significant.
Plasma, Ullne, and
Fecal Composltlon
Plasma. Lactation resulted in significant reduction m hematocrit, osmolality, and concentrations of Na, K, and Cl m comparison with
prepartum levels (Table 4). Except for osmola-
lity and K, these variables remained significantly lower during period 3 (postpeak) as
well. Urea concentration increased from 5.7
mM prep-,
to 8.8 m M in period 2, and to
11.4 mM during period 3 (Table 4).
U ~ WOnset
.
Of lactation al~0W ~ SC h m terized by a significant rise in urine volume
and parallel reduction in osmolality and in
Concentration of Na, K, and Cl. As lactation
progressed (period 3), evidence for significant
restitution could be observed for urine volume
and Na and K concentrations.
Feces. Lactation resulted in marked reduction in concentrations ( m e DM) of Na and
K in feces, a pattern that was enhauced in
period 3. The reduction in Na concentration
was larger than that of K reflected in the NxK
ratios in feces, which were .59, .49, and .54 for
periods 1, 2, and 3, respectively.
Sodlum Balance
The components of Na balance are p
sented in Table 5. Prepartum Na balance was
J o d of Dairy Science Vol. 74. No. 6, 1991
1878
SHALIT ET AL.
TABLE 4. plasma, urine and fecal composition in 5 cows 2 wk pqartum (period 1) and 2 and 7 wk (periods 2 and 3,
respectively) postpartum.
Mod 1
a
SD
Hematocrit, %
Osmolality, mOsM/kg
Na, meqR.
K, m+
CI, meqR.
urea, &/L
32.1'
284'
143'
3.9'
5.7=
.4
Output, kg/d
Osmolality, mOsM/L
Na, m@
K,
CI,
Urea, mM/L
8.3b
8Wb
1.2
88
20
31
31
31
Water, %
Output, kg DM/d
Na, meq/kg DM
K, me& DM
C1, meqkg DM
83.1a
2.4'
92'
1598
64'
106'
608
236'
81a
68b
1.1
4
4
.1
2
1.8
.2
24
40
I1
Period 2
Period 3
sz:
si
SD
Plasmal
26.3b
.8
273b
2
13gb
6
4.P
.3
103b
5
8.8b
1.0
Urine'
12.1'
1.9
152
790b
Nb
13
168b
23
lob
7
267'
66
Feces3
81.7*
15
3.5b
A
59b
18
22
107b
75&
19
SD
25.9
.4
281"
131'
3.8b
4.5
6
.3
4
.8
lW
11.4'
10.4'
1095'
54'
l8s"b
1Sb
114b
79.3b
4.48
51b
97b
9Ib
2.8
144
11
33
5
61
.7
.9
22
24
15
4b*cMeans within rows having different superscript differ (P < -05).
'Plasma composition based on 24 hourly samples.
'urine composition based on seven daily total co~ections.
3Fecal composition based on five fresh samples taken in different hours during the day.
positive 391 meq/d (.36 of the intake); 217
meq/d (.20 of the intake) were excreted
through the feces and 492 meqid (.45 of the
intake) excreted through urine. Increase in Na
output during lactation could be explained almost solely by Na excretion in milk (a.
635
meq/d or .52 of the intake), but Na intake
increased compared with the prepartum period
by approximately 140 meq/d. Consequently,
the Na milk-frze balance (intake minus milk
secreted) was approximately .56 of the prepartum intake.
In both periods, fecal and urine excretions
during lactation remained about the same as
prepartum excretions. The overall outcome
was a significant reduction in Na balance compared with prepartum Na, which apparently
became balanced in period 2 and slightly negative in period 3 (2 vs. 3, NS).
537 meq/d (.39 of the intake); 154 meq/d (.11
of the intake) were excreted through the feces
and 824 meq/d (.61 of the intake) via urine. An
increase in C1 output during lactation could be
explained solely by C1 excretion in milk, approximately 1100 meqid (.76 of the intake),
but C1 intake increased over prepartum levels
by 78 meq/d (NS)in period 2 and by 102 meq/
d in period 3. Consequently, the C1 milk-free
balance (intake minus milk secreted) was approximately .25 of the prepartum Cl intake.
Fecal excretion during lactation in both
periods increased compared with prepartum
excretion. Urine C1 excreted was reduced considerably during lactation.
The overall outcome was a significant reduction in C1 balance during lactation in comparison with the prepartum perid, C1 balance
was -89 meq/d in period 2 and -231 meq/d in
period 3 (2 vs. 3, NS).
Chlorine Balance
Potassium Balance
The components of C1 balance are presented
in Table 5. Prepartum C1 balance was positive
The components of K balance are presented
in Table 5. Prepamm K balance (positive) was
Journal of Dairy Science Vol. 74, No. 6, 1991
WATER AND ELECTROLYTE METABOUSM
I
1879
1382 meq/d (.38 of the intake) of which 387
meq/d (.lo of the intake) were excreted
through the feces and 1945 meq/d (.52 of the
intake) excreted via urine. The increase in K
output during lactation could be explained
solely by K excretion in milk, (ca 1540 meq/d
or .34 of the intake), but K intake increased by
417 mq/d (NS)in period 2 and by 780 meq/d
in period 3 relative to period 1. Consequently,
the K milk-free intakes (intake minus milk
secreted) were .68 and .81 of the prepartum
intake for periods 2 and 3, respectively.
Fecal and urinary excretion during lactation
remained about the same as prepartum in both
periods. The overall outcome was a signifcant
reduction in K balance in period 2. This was
followed by a significant increase in period 3,
but K was still lower (NS) than prepartum
levels.
DISCUSSION
Dairy cows are pregnant most of the lactation period. Pregnancy and lactation are accompanied by marked cardiovascular and endocrine adaptations. Increasing blood supply to
the fetal-placental unit seems to be essential
for successful pregnancy (23). and a large,
undisturbed blood supply to the mammary
gland is important for milk secretion (15). In
goats and sheep, plasma volume has been
found to increase during pregnancy (unaccompanied by lactation), and this expanded plasma
volume was maintained during subsequent lactation (23). Plasma volumes were similar in
lactating and in pregnant cows in thermal comfort (34). Hematocrit values in the present
study indicated that apparent plasma volume
was larger during lactation than in late pregnancy. This was associated with an increase in
rectal temperature; the latter probably was due
to both the higher metabolic heat load induced
by lactation and environmental conditions (3).
Increase in rectal temperature might be the
factor responsible for the plasma volume expansion during lactation in this study. It was
shown that the increase in thermoregulatory
requirements (30) and increase in WTO (16)
may increase plasma volume.
Lactation affects kidney function in goats
(17, 22) and cows (7). Accordingly, in the
present study, initiation of lactation increased
urinary output. The large volume, low concenJoaroal of Dairy Science Vol. 74, No. 6, 1991
1880
SHALIT ET AL.
trated urine in the 2nd wk of lactation is in
accordance with the high plasma volume and
activated renin-angiotensin system in this period (22). Increasing urine concentration capacity (7th wk), although plasma volume apparently remained high, enabling the cow to
improve its osmoregulatory capacity and save
water, indicates the trend of adaptation to conserve electrolytes and water.
At initiation of lactation, urea was the main
component contributing to urine osmolality,
whereas the Concentrations of Na, Cl, and K
were extremely low. After peak lactation, urea
concentration in urine was reduced, and electrolyte concentration was slightly increased.
The low electrolyte concentration (particularly
C1 and Na) in the very early phase of lactation
(i.e., between parturition and peak lactation)
may reflect either electrolyte deficiency, or the
need to excrete urea, or both. If protein is
consumed in supra-optimal quantities or if rumen ammonia concentrations are excessive due
to high dietary NPN or rumen degradable protein intake in that period, there is a need to
excrete large amounts of urea; because kidney
concentration capacity is limited, Na, K, and
C1 contents in urine were reduced.
The amount of protein catabolized postpartum, calculated from the amount of urea excreted, was equivalent to 1.44 kg protein in
period 2 (47% of the intake) and .53 kg in
period 3 (16% of the intake), although feed
intake was even higher in period 3 and milk
yield was similar. Thus, excessive provision of
protein to dairy cows was excreted as urea in
urine (14). In this work, prepartum cows were
fed protein below the 12% current recommendations (21). Should this be the case, it is
possible that adjustment to 16.3% dietary protein would be more efficient. However, the
recommendation for the first 3 wk in lactatim
is 19% (21). Thus, the difference between
prepartum and postpartum CP recommendations is 7% but in OUT work 8.3%. Therefore, it
is likely that similar results regarding protein
metabolism would have been achieved if the
cows were fed a ration with protein density as
recommended. The apparent supra-optimal
protein consumption may explain earlier nutritional studies showing lack of response (in
terms of milk production) to in&
protein
content in feeds in early lactation (11, 14, 33).
The current nutritional recommendations regarding protein intake of lactating cows (21)
Journal of Dairy Science VoL 74. No. 6, 1991
differentiate between degraded and undegraded
intake protein. This might be of substantial
importance at onset of lactation. Protein
degradability according to components (21) in
the ration of this study was estimated to be
56.7%. Providing a source of rumen bypass
protein (high undegraded:degaded intake protein ratio) in the first few weeks of lactation
may be useful to increase amino acid availability in the lower tract and thus allow lower
rumen ammonia and blood urea values. It is
possible that the reason for inefficient protein
utilization indicated in our work and others is
the outcome of limited increase in metabolism.
Metabolism could be higher if thermoregulation would have been optimal and not obstructed by lack of electrolytes for sweating, as
demonstrated by the balance calculations.
Higher electrolytes intake could provide the
means for optimal thermoregulation, thus enabling not only the successful utilization of
protein intake, in our case, but even higher
amounts, resulting from higher DMI.
The conclusion that cows were Nadeficient
in the present work also was supported by
electrolyte fecal content. Most Na in feces is of
endogenous origin (29). The amount of Na
secreted by saliva to the gut exceeded by 15
times that consumed by feed, and its absorbability is very high. When Na is deficient, its
content in the saliva decreases and that of K
increases (6). This trend should be reflected in
reduced Na:K ratio in feces as well, as indeed
was found in this trial. In addition, the marked
reduction in Na, K, and Cl concentrations in
urine and of Na and K in feces could be a
result of elevation of the renin-angiotensinaldosterone system, which is activated when
Na is deficient (4). The deficiency in electrolytes may be an additional factor that contributes to their reduced content in plasma during
lactation in comparison with the prepartum
period. Reduction in plasma Cl under conditions of Cl deficiency was described for early
lactation a).It should be noted that the dietary
concentrations of Na, K, and Cl in this work
were higher than those recommended even for
heat-stressed cows (21). With recommended
concentrations and the cows eating as much as
they did in this study, deficiency would have
been bigger.
This study adds more information to the
longestablished fact that lactation imposes a
large acceleration in the water metabolism in
WATER AND ELECTROLYTE METABOLISM
dairy cows in terms of WTO. The increase in
the milk-free water balance probably reflects a
response to an increase in energy metabolism
during lactation (16, 18, 34). Indeed, fmm data
on lactating goats (from different environments), it seems that the ratio of milk-free
WTO to digestible energy intake is similar in
lactating and nonlactating periods (31).
Postpartum water conservation in the kidneys and gut, indicated by trend of losses when
expressed as fractions of milk-free water balance, is consistent with the parallel trend for
Na and C1 Conservation and OUT expectation of
elevation of the renin-angiotensin-aldosterone
system. This is because water reabsorption
from the kidney and absorption from the gut
usually are associated with net movement of
water in the same direction. However, because
of the relatively small proportion of excreta
water in the total output, the contribution of
the enhanced water conservation in kidney and
feces during very early lactation to the total
water balance was rather small. From analysis
of the results presented by Flatt et al. (lo), it
seems that this situation may be changed toward late lactation when WTO is r e d u c e d considerably. In lactating desert goats, at the end
of lactation the &-free water balance was
more efficient than in prepartum goats (17,
18).
Proportionally (i.e., as fraction of &-free
water balance), the cows’ increase in evaporative output at onset of lactation was rather
small, although the thermoregulatory demands
of the cows when lactating obviously were
larger than during prepartum; this may be
related to the significant increase in body temperature. However, unlike the situation with
energy in early lactation, the inadequate increase of water intake to support the increased
demand for evaporative cooling is unrelated to
physical or physiological constraints. According to prediction equations (19), cows’ ingestion capacity is higher than that measured in
this study. The limiting factor probably was
deficiency of Na and Cl, which limited sweating (see below) and perhaps even milk production. The positive balance of K, C1, and Na in
the prepartum period is a reflection of their use
for sweating. The K, Cl, and Na are major
constituents of the nonwater fraction of sweat
(13), and sweating is a major avenue for cooling in hot conditions such as those that
prevailed in the present experiment (3).
1881
Milk secretion induced a large increase for
the demand of Na, K, and C1. Even after
allowing a significant proportion of the electrolyte balance during the prepartum period for
retention (placental fluid increase and fetus
growth), the marked reduction in the balance
of Na and Cl and, to a lesser extent, K
represents a reduction in their availability for
sweating and consequently limits evaporatory
cooling through this avenue. Indeed, adding
NaCl to the diet of lactating cows in early
lactation in hot summer improved their thermoregulatory ability (5). Based on the reduction of electrolyte balances, we can assume
that their content and composition in sweat
were influenced markedly by initiation of lactation. The possibility is supported by the work
of Jenkinson and Mabon (13) showing electrolyte concentration variability correlating with
sweating rates. The minimum electrolytes required for sweating in this study probably were
mobilized from the systemic fluids.
Providing Na, K, and C1 according to current dietary recommendations as a percentage
of the ration (20,21) is limited by the DMI of
the cow. Obviously in OUT work this was the
case. The DMI increases gradually from parturition to reach a maximal value following peak
milk production. In this work, we expected the
DMI to be 3% of BW or more at wk 7. If DMI
had been 3 or 3.5% of BW, the cows would
have gained 8.3 and 26%, respectively, more
electrolytes. It is a wellestablished fact that in
Israel during the summer, DMI is considerably
lower than during other seasons, which also is
reflected in milk production. It is also known
that summer calvers produce less milk than
winter calvers (9). Several factors might be
involved in disturbing the DMI increase to the
optimal values in this work. Stage of lactation
and unfavorable environmental conditions required optimal thermoregulation, which was
obstructed by insufficient electrolyte intakes.
The abrupt increase in dietary protein immediately postparnun imposed an additional burden
in adapting to the physiological changes that
accompanied the onset of lactation. However,
this explanation for insufficient electrolyte intakes does not hold for the 2nd wk p o s t p m
since DMI of 2.5% of BW is not considered
low in this period. This fact points toward the
possibility that initiation of lactation probably
is, in itself, a factor that causes the elecJournal of Dairy Science Vol. 74, No. 6, 1991
1882
SHALJT ET AL.
mlytes’ depletion, which is enhanced by environmental heat load.
CONCLUSIONS
The negative effect of elevated body temperature on productive and reproductive cows
is well established. Great effort is devoted to
reduce this negative effect by applying different means of cooling (3). This research indicated that when DMI was not above 2.5% of
BW at wk 2 and was below 3 to 3.5% at wk 7
postpartum, thennoregulatory capacity might
be damaged because of lack of electrolytes
when they are supplied as a percentage of the
ration according to current recommendations
(20). In addition, the current trend to increase
abruptly the protein content of the cow’s diet
immediately postpartum resulted in unnecessary excretion of large amounts of N in urine,
which also had a negative stressful effect on
the water and electrolyte balance.
ACKNOWLEDGMENTS
The authors wish to thank H. Leherer and
M. Nikbaht for technical assistance and A.
Borut, S. L. Spahr, and I. H. Clark for their
useful comments.
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Jomnal of Dairy Science Vol. 74, No. 6. 1991