Genotype X Environmental Interactions on Reproductive Traits
of Bovine Females. II. Postpartum Reproduction as Influenced by
Genotype, Dietary Regimen, Level of Milk Production and Parity
P. J. Hansen, D. H. Baik, J. J. Rutledge and E. R. Hauser
J ANIM SCI 1982, 55:1458-1472.
The online version of this article, along with updated information and
services, is located on the World Wide Web at:
http://jas.fass.org/content/55/6/1458
www.asas.org
Downloaded from jas.fass.org at University of Florida on June 10, 2011
GENOTYPE • ENVIRONMENTAL INTERACTIONS ON REPRODUCTIVE
TRAITS OF BOVINE FEMALES. I1. POSTPARTUM REPRODUCTION AS
INFLUENCED BY GENOTYPE, DIETARY REGIMEN, LEVEL OF MILK
PRODUCTION AND PARITY 1'2,3
P. J. Hansen 4, D. H. Baik s, J. J. Rudedge and E. R. Hauser
University o f Wisconsin 6, Madison 53706
Summary
PP3. Cows fed H diets had significantly shorter
IE and IC in PP1 and PP2, but n o t in PP3.
Interval to conception was not recorded in
PP3. There were breed x diet interactions
for IE in PP1 (P<.06) and PP2 (P<.10) as
differences in IE between L and H diets were
more pronounced for the Holsteins than
for the Herefords. Though not significant,
Herefords fed L diets tended to have shorter
intervals between estrus and conception than
other groups in PP1 and PP2. In Exp. 2, breed
of sire did n o t significantly affect any p o s t partum reproductive traits. Cows fed the H
diet had shorter (P<.10) intervals to first
estrus in PP1 and shorter (P<.05) intervals
to first ovulation (I0) in PP2 than L - f e d cows.
Interval to first ovulation was also shorter
for H - f e d cows in PP1, b u t the differences
were not significant. The differences in IE
between L and H diets tended to be greater
for A F and HF cows than for SF and CF
! Meat and Animal Science Paper No. 786. Reanimals for PP1, but the interaction was n o t
search supported by the College of Agricultural
significant. The HF cows fed L diets had
and Life Sciences, Univ. of Wisconsin, Madison and
longer IE in PP2 than HF cows fed H diets,
Science and Education Administration, USDA. These
experiments were contributions to North Central
hut the other b r e e d - o f - s i r e groups were
Regional Coop. Proj. NC-1, "The Improvement
n o t influenced by dietary regimen, resulting
of Beef Cattle Through Breeding Methods."
in a breed of sire x diet interaction (P<.10).
2 Semen was provided by the following organizations: American Breeders Service, DeForest, WI; Multiple regression analysis indicated that
American Int. Charolais Assoc., Houston, TX; Carna- cows fed L diets that produced more f a t tion Genetics, Hughson, CA; Curtiss Breeding Service,
corrected milk and consumed less total
Cary, IL; East Central Breeders Coop., Waupun, WI; digestible nutrients (TDN) from 0 to 56 d
Midwest Breeders Coop., Shawano, WI; Noba, Inc.,
Tiffin, OH and Tri-State Breeders Coop., Westby, postpartum had longer IE during PP1. F a t corrected milk production, TDN intake, weight
Wl.
aThe authors thank E. Hoch, P. Bringle and
at calving and change in b o d y weight from
J. Kane for their excellent technical assistance; J.
0 to 56 d postpartum were not significantly
Grass, who collected some of these data, and S. related to IE for cows receiving H diets during
Kading, W. Winkler, M. Connors, C. Keuler and J.
Busby for their assistance in the preparation of this PP1. Data from PP2 and PP3 were pooled
across diet groups for multiple regression
manuscript.
4Trainee of the Endocrinology - Reproductive analysis. None of the factors included in
Physiology Program.
the model were significantly related to IE.
s Present Address: Dept. of Anim. Sci., College
We concluded that the influence of diet on
of Agriculture, Jeonbak National Univ., Jeonju 520,
postpartum reproductive function interacted
Korea,
6Dept. of Meat and Anim. Sci.
with breed and parity and that the influence
1458
JOURNAL OF ANIMAL SCIENCE, Vol. 55, No. 6, 1982
The interaction between dietary regimen
and breed or breed of sire on postpartum
reproduction was examined in two experiments. Exp. 1 was a 2 • 2 factorial involving
two breeds (Hereford and Holstein) and two
dietary regimens, high (H) and low (L). In
Exp. 2, females out of Holstein dams and
sired by bulls of four breeds, Angus (AF),
Hereford (HF), Simmental (SF) and Chianina
(CF) were used in a 4 • 2 factorial with two
dietary regimens. Postpartum traits were
observed for three consecutive postpartum
periods (PP1, PP2, PP3), beginning after first
calving. In Exp. 1, Holsteins tended to have
longer intervals to first estrus (IE) and had
longer intervals to conception (IC, P<.025)
than Herefords in PP1 but not in PP2 or
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FACTORS AFFECTINGPOSTPARTUMREPRODUCTIONIN COWS
of level of milk production on IE varied with
dietary level and parity. Because diet influenced
reproduction more in Holsteins than Herefords,
the use of dairy breeds in beef production
may necessitate diets higher in energy to
achieve maximum reproductive performance.
(Key Words: Postpartum Cow, Nutrition,
Breed, Milk Production.)
Introduction
Calving interval is determined by interval
from parturition to first estrus, interval from
first estrus to conception and gestation length.
Variation in the interval to first estrus accounted for 63.8% of the variation in the
interval from parturition to conception (Wiltbank, 1955) and only about 1% of the variation in calving interval was due to gestation
length (Touchberry et al., 1959). Breed differences were noted in duration of the postpartum anestrus (Foote et al., 1960; Rakha
et al., 1971; Holness et al., 1978) and in gestation length (Preston and Willis, 1970). Cows
fed low energy diets had longer intervals
to first estrus (Wiltbank et al., 1962, 1964;
Dunn et al., 1969), longer gestation periods
(Kress et al., 1971a) and lower fertility (Wiltbank et al., 1964; Folman et al., 1973) than
cows fed diets higher in energy. Intervals
to first estrus were longer for cows with high
milk production than for low producers (Menge
et al., 1962; Saiduddin et al., 1968). Primiparous cows had longer intervals to estrus
(Tervit et al., 1977) and conception (de Kruif,
1975) than multiparous cows. Fertility was
lower when cows were bred soon after calving
(Shannon et al., 1952; Whitmore et al., 1974)
compared with those bred later. Postpartum
reproductive traits have been reported to
be influenced by genotype x nutrition interactions (Dunn et al., 1969; Kropp et al., 1973;
Whitmore et al., 1974; Bellows and Short,
1978).
Our objectives were to determine interactions between breed or breed of sire and
energy density of diet on postpartum reproductive traits and to determine the influence
of body weight, total digestible nutrient (TDN)
intake, weight gain and fat-corrected milk
production on the reproductive function
of postpartum cows on two nutritional
regimens. The crossbred groups were chosen
to reflect types o f cattle that have potential
use in areas where dairy cattle make u p a
1459
sizable portion of the bovine population
and where a terminal sire system of crossbreeding for beef production might be implemented.
Materials and Methods
Exp. 1 was a 2 x 2 factorial, with two
breeds (Holstein and Hereford) and two dietary
regimens. Data were obtained from one member of each set of 31 pairs (15 Holstein and
16 Hereford) of identical or fraternal twins.
At 210 d of age, the heifers were started
on one of two diets of differing TDN content,
High (H) and Low (L). Diet changes were made
at 365 and 449 d of age to adjust for changes
in the animals' nutrient requirements. Total
digestible nutrient percentages of the diets
and feeding management were detailed by
Grass et al. (1982). The TDN intake/28-d
interval after calving was estimated as individual feed consumption per 28 d x TDN
percentage of the diet. Heifers were artificially
inseminated at their first estrus after 15 mo
of age and at all subsequent estrous periods
until conception. After the first (PP1), second
(PP2) but not third calvings (PP3), animals
were bred at their first estrus and all subsequent estrous periods until conception. Holsteins were bred with semen from Polled
Hereford bulls and Herefords were bred with
semen from Holstein bulls. The cows were
palpated per rectum at weekly intervals beginning at first estrus after calving for
detection of corpora lutea.
All cows were suckled for 224 d postcalving.
Calves were allowed with cows at all times
except at feeding times and on the days that
milk production was estimated. Milk production of the cows was estimated weekly by a
method described by Kress et al. (1971b).
Milk production levels were adjusted to 4%
fat content by the equation: fat-corrected
milk = .4 (milk production) + 15 (butterfat
content).
Exp. 2 was a 4 X 2 factorial, with four
breeds of sire and two dietary levels. The
heifers were out of Holstein dams and sired
by Hereford (HF, n=15), Angus (AF, n=14)i
Simmental (SF, n=16) and Chianina (CF,
n=13) bulls. The sire breeds were chosen
to provide two small ( H F and AF) and two
large crossbreds (SF and CF), with high (AF
and SF) and low milk production (HF and
CF) in each size category.
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1460
HANSEN ET AL.
Procedures were the same as in Exp. 1,
with the following exceptions. The experimental diets were started when the heifers
were 168 d of age. Dietary changes were
made at 308 and 420 d of age. Estrus was
determined as before, but vasectomized bulls
were present in each lot. Monogamous matings
were made by inseminating each female at
her pubertal estrus and all subsequent estrous
periods until conception with semen from
one of 58 Jersey bulls. After each calving,
animals were bred with semen from one of
58 Charolais bulls at all estrous periods until
conception.
The cows were palpated weekly beginning
at d 14 postpartum for detection of ovulation
and pregnancy. When a corpus luteum was
detected before an observed estrus, day of
ovulation was defined as 21 d before the
subsequent estrus. In the event that the estrus
after a quiet ovulation was not observed,
first ovulation was presumed to have occurred
10 d before detection of the first corpus
luteum.
Statistical Analysis. The effects of breed
or breed of sire and dietary level on gestation
period and intervals from parturition to first
ovulation, from parturition to first estrus
and from parturition to conception were
analyzed b y least-squares analysis of variance.
The model included effects of season of calving
(winter = October to March, and summer =
April to September). Reproductive data were
initially analyzed with all interactions included,
then reanalyzed without partitioning the
sum of squares for some nonsignificant
interactions (breed or b r e e d - o f - s i r e • season
and breed or b r e e d - o f - s i r e • diet x season).
Nonreproductive traits were analyzed by
least-squares analysis of variance omitting
the effect of season of calving. When b r e e d o f - s i r e effects were significant, mean separation was carried out by Duncan's new multiple
range test.
Breed or b r e e d - o f - s i r e effects on service
interval (interval between first estrus and
conception) were tested by pooling across
dietary regimen and applying the KruskalWallis test. Diet effects were tested by pooling
across breeding groups and applying Wilcoxon's
t w o - s a m p l e test. Because these tests do not
allow the testing of interactions, breed or
b r e e d - o f - s i r e effects were also tested within
diet groups using the Kruskal-Wallis test.
Spearman's correlation coefficients were calcu-
lated to determine the relationship between
interval to first estrus and service interval.
The statistics described in this paragraph
are detailed by Steel and Torrie (1960).
In Exp. 2 the following multiple regression
model was used to p a r t i t i o n the variance
of interval to first estrus:
y -- i + b t B W + b2ABW + b3TDN +
b4FCM + bsCDI + e, where
i = the y intercept,
bl to bs -- net partial regression coefficients,
BW = b o d y weight at calving,
ABW = change in body weight from 0
to 56 d postpartum,
TDN = TDN intake from 0 to 56 d
postpartum,
FCM = f a t - c o r r e c t e d milk production
from 0 to 56 d postpartum,
CDI = calving date index where 0 =
July 2 and 182 = January 1
and December 31 and
e
=
error.
Standard partial regression coefficients were
calculated from the regression analyses.
Results
Exp. 1. The effects of breed and dietary
regimen on reproductive traits for PP1, PP2
and PP3 are presented in tables 1, 2 and 3,
respectively. Holsteins tended to have longer
intervals to estrus (P>.10) and had longer
intervals to conception (P<.025) than Herefords during PP1, but breed did not affect
these intervals in subsequent postpartum
periods. Cows fed H diets had shorter intervals
to estrus and conception in PP1 and PP2,
but not in P P 3 ) T h e r e were breed • diet
interactions for interval to estrus (P<.06)
in PP1 and PP2 (P<.10) and for interval to
conception (P<.001) in PP1; the differences
in mean intervals to estrus or conception
between H and L groups were greater for
Holsteins than for Herefords.
Though not significant (P>.10), Herefords
fed L diets tended to have shorter service
intervals than the other groups in PP1 and
PP2. Spearman's correlation coefficients between intervals to first estrus and service
interval for PP1 and PP2 were - . 4 6 (P<.025)
and - . 2 1 (P>.25), respectively. Correlations
between intervals to estrus and conception
for PP1 and PP2 were .77 (P<.001) and .70
(P<.OOl).
Body weight, TDN and FCM were signffi-
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FACTORS A F F E C T I N G P O S T P A R T U M R E P R O D U C T I O N IN COWS
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1461
1462
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FACTORS AFFECTING
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1463
1464
HANSEN ET AL.
cantly greater for Holsteins than Herefords
and for H than L cows in all postpartum
periods (tables 1, 2 and 3). Holsteins lost
weight from calving to 56 d postpartum during
PP1 and PP3, while Herefords gained weight
during the same periods. These differences
were significant. During PP2, both groups
gained weight. Diet did not significantly affect
ABW during any postpartum period.
There were breed • diet interactions for
FCM in PP1 (P<.10), PP2 (P<.05) and PP3
(P<.05). Differences between H and L groups
were greater for Holsteins than for Herefords.
The Herefords receiving L diets were probably
producing milk at a level near their maximum
potential while Holsteins responded to the
higher level of TDN in the diet by increasing
milk production. Milk production is determined
partially by amount of milk removed from
the udder (Schmidt et al., 1964). Therefore,
the capacity of a calf to consume milk could
have affected the amount of milk produced.
Holsteins fed H diets were the only group
that did not increase in FCM production
between PP2 and PP3 (tables 2 and 3), perhaps
because the level of milk production in this
group was determined by the capacity of their
calves. Cows in other groups may have p r o duced below their calves' ability to consume
milk.
Exp. 2. There were no significant effects
of breed of sire on intervals to first ovulation,
first estrus or conception in PP1 (table 4).
Cows fed L diets had longer (P<.lO) intervals
to first estrus than H cows and longer intervals
to first ovulation, though this effect was
not significant. There were no significant
breed-of-sire x diet interactions, but the
influence of diet on intervals to first ovulation
and estrus tended to be greater for HF and
AF than for SF and CF. During PP2, breed
of sire did not significantly influence intervals
to ovulation, estrus or conception (table 5).
Interval to ovulation was longer (P<.05)
for L than for H cows. There was a breedof-sire x diet interaction (P<.10) as the
interval to estrus was longer for L than H
cows in the HF breed-of-sire group, but
not in other breed-of-sire groups. There
were no significant effects of breed of sire,
diet or breed-of-sire x diet interactions
in PP 3 for any traits.
Service intervals were not significantly
influenced by breed of sire or diet in any
of the postpartum periods, though they tended
to be greater for H- than L - f e d cows in PPI
and PP3. Spearman's correlation coefficients
between service interval and interval to first
estrus for PPI, PP2 and PP3 w e r e - . 4 9
(P<.O01), - . 3 5 (P<.01) and - . 1 6 (P<.25),
respectively. Correlations between intervals
to first estrus and conception for PP1, PP2
and PP3 were .59 (P<.O05), .23 (P<.IO) and
.45 (P< .005), respectively.
Breed of sire significantly influenced BW
in all postpartum periods (tables 4, $ and 6);
CF were heavier than AF and HF in PP1,
PP2 and PP3 and heavier than SF in PP2,
and SF were heavier than HF in PP1 and
PP2. Cows fed the H diets were heavier at
calving than L - f e d cows for PP2 (P<.10)
and for PP3 (P<.01), but not for PP1 (P>.10).
Change in body weight was not significantly
affected by breed of sire in any postpartum
period, but tended to be greater in H - f e d
cows than L - f e d cows in PP1 (P<.08), PP2
(P<.06) and PP3 (P>.10). The TDN intake
was greater (P<.001) for SF and CF cows
than for HF and AF cows in PP1 and PP3
(tables 4 and 6). In PP2, SF cows consumed
significantly more TDN than HF and AF
cows, and CF cows consumed more TDN
than HF cows. Cows fed the H diet consumed
more TDN than L - f e d cows in all postpartum
periods. There were breed- of- sire x diet
interactions on TDN intake in PP1 (P<.025)
and PP3 (P<.08). In PP1, H - f e d cows consumed more TDN than L - f e d cows except
in the SF breed-of-sire group, in which
the opposite occurred. In PP3, H - f e d cows
consumed more TDN than L - f e d cows in
all breed-of-sire groups, but the differences
between dietary regimens were greatest in
CF cows and least in AF cows.
Fat corrected milk was not influenced
by breed of sire in PPl (table 4) or PP2 (table
5), but was greater (P<.05) for SF than for
HF cows in PP3 and greater in H- than L - f e d
cows for PPl (P<.01) and PP3 (P<.025).
In PP2, there was a breed-of-sire x diet
interaction (P<.01); FCM was greater in Hthan in L - fed cows in all breed - of- sire groups
except CF, in which the opposite occurred.
The results for PPl of partitioning the
variance of interval to first estrus in a multiple
regression analysis are presented in table 7.
Because diet level influenced interval to first
estrus, the data were analyzed within diet
group. Cows fed the H diet that calved at
dates further from July 2 had longer intervals
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FACTORS A F F E C T I N G POSTPARTUM REPRODUCTION IN COWS
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1465
1466
HANSEN ET AL.
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FACTORS AFFECTING POSTPARTUM REPRODUCTION IN COWS
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1467
1468
HANSEN ET AL.
TABLE 7. RELATIONSHIPS BETWEEN TRAITS AND INTERVAL TO ESTRUS AFTER FIRST CALVING:
REGRESSION AND CORRELATION COEFFICIENTS (EXP. 2) a,b
High diet
Traits from 0 to 56 d postpartum
Body weight Change in
at calving body weight
Type of coefficient
(BW)
Partial regression
Standard partial regression
(ABW)
TDN intake
Fat- corrected miik'
production
(TDN)
(FCM)
.03
.07
.02
.08
-.05
-.21
- . 12
-.14
-.47**
.73****
.59***
-.05
.03
-.62****
--.23
.24
-.26
-.30
-.11
-.55 **
-.64" *
--.26
-.12
-.19
.31
.22
CDI c
R2
.29*
.44*
.25
.25
.50***
.63***
.62***
.63***
Correlation
ABW
TDN
FCM
CDI
.11
Low diet
Partial regression
Standard partial regression
.13
.19
.20* * *
.49" * *
Correlation
ABW
TDN
FCM
CDI
--.38 f
.79****
.30
.30
-.07
aRegression equation: Interval to first estrus -- intercept + b t BW + b 2 ABW + b 3 TDN + b 4 FCM + b s CDI +
error.
bThere was heterogeneity of regression between high and low dietary levels (P<.05).
CCalving date index, where 0 = July 2 and 182 = January 1 and December 31.
"['P<.10.
*P<.05.
**P<.025.
***P(.O1.
****P<.001.
to first estrus (P<.05). No other factors considered were significantly related to interval
to first estrus in the H group and the coefficient
of determination was small. Cows fed the L
diet that consumed less TDN, produced more
milk and calved at dates further from July 2,
had longer intervals to first estrus. Body
weight at calving could have influenced d u r a tion of the postpartum anestrus in the L group
indirectly because TDN intake and BW were
positively correlated (table 7). Total digestible
nutrient intake was probably related to interval
to estrus in L, but not H - f e d cows because
the higher energy density of the diets fed
the latter group ensured feed intake adequate
for optimal reproduction for most cows.
Data from PP2 and PP3 were pooled across
diet level because interval to first estrus was
not significantly influenced by diet. None
of the regression coefficients in either equation
were significant and coefficients of determination were small (R 2 = .09 for PP2 and .16
for PP 3).
The average lengths of gestation periods
of the first, second and third calves (G1, G2,
G3) are presented in table 8. The first gestation and G2 tended to be the longest for
CF cows, followed by HF, SF and AF, but
the differences were not significant. There
was a significant influence of breed of sire
on G3; CF and HF cows had longer gestation
periods than A F cows. The effect of diet
on G1 was not significant, but G1 tended
to be shorter for H than for L cows. There
was a breed - o f - sire x diet interaction (P<.05)
for G1 because HF and SF crossbreds fed the
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FACTORS AFFECTING POSTPARTUM REPRODUCTION IN COWS
1469
TABLE 8. INFLUENCE OF BREED OF SIRE AND DIET ON GESTATION LENGTH (EXP. 2) a
.
.
.
.
.
',
'
,
.',T'
Gestation period (d)
Breed of sire
Dietary level
First
Second
Third
Hereford
Angus
Simmental
Chianina
High
High
High
High
277.2
276.3
274.0
284.2
+
+
+
•
2.6 d
2.3
2.3
2.6
286.0
286.6
288.7
288.2
~
•
+
•
3.5
3.3
3.4
4.0
290.5
281.4
285.8
295.9
+- 2,0
+ 1.8
+ 1,9
+ 2.9
Hereford
Angus
Simmental
Chianina
Low
Low
Low
Low
282.2
276.1
280.$
277.6
•
+
+
+
2.2
2.7
2.3
2.6
289.9
280.8
287.0
289.4
• 3.6
• 4.2
:!: 3.3
• 3.8
288.5
285.6
286.5
295.3
+
+
+
+
Breed
Hereford
Angus
Simmental
Chianina
279.7 b
276.2 b
277.3 b
280.9 b
287.9 b
283.7 b
287.8 b
288.8 b
289,5 c
283.5 b
286.2 bc
295.6 c
Diet
High
Low
277.9 b
279,1b
287.4 b
286.8 b
288.4 b
289.0 b
1.9
2.2
2.0
3.9
aLeast-squares means + SE.
b,CMean s with different superscripts differ at (P<.005). All other ~lifferences were nonsignificant.
dThere was a breed • diet interaction (P<.05).
low diets had longer gestation periods than
cows fed the high diets. Cows fed the L diet
had shorter gestation periods for CF crossbreds, and AF crossbreds were not affected
by dietary regimen. Diet did not significantly
affect length of G2 or G3.
Discussion
In Exp. 1, the differences between Hand L-fed cows in intervals from calving
to first estrus were greater for Holsteins than
for Herefords. Similar breed x nutrition interactions have been observed comparing
Holsteins to Herefords and Hereford x Holsteins (Kropp et al., 1973) and high milkproducing to low milk-producing Holsteins
(Whitmore et al., 1974). Breed differences
between Holsteins and Herefords other than
milk production could also have influenced
the interaction of breed with diet. No interactions were observed in Exp. 2 when
comparing breeding groups chosen for high
and low milk production, but in PP1, cows
with greater milk production had longer
intervals to first estrus for L- fed, but not for
H-fed cows, indicating a diet x milk production interaction.
An influence of level of milk production
on interval to first estrus has been reported
for dairy cows (Menge et al., 1962; Saiduddin
et al., 1968). Our results suggest that level
of milk production also influenced duration
of postpartum anestrus in cows suckling calves
when they were fed a diet of chopped hay.
It was not possible to determine whether
increased milk production affects return
to estrus by increasing nutrient demands,
by increasing frequency of suckling (Randel
and Welker, 1977) or by some other mechanism. These interactions in reproductive
function indicate that the adaptability of
breeds varies with environmental conditions.
When nutrients are limited, cows with genotypes for high milk production will have
longer postpartum anestrous periods relative
to cows with genotypes for low milk production. Ample nutrient availability will shorten
interval to first estrus for high milk producing
COWS.
In Exp. 2, breed x diet interactions were
not significant in PP1 for intervals to ovulation
and estrus, but the influence of diet tended
to be greater in the small (HF and AF) breedof-sire groups than in the large (SF and CF)
breed-of-sire groups. In PP2, interval to
first estrus was influenced by dietary regimen
only for HF cows. The ability of the large
breeds to consume more TDN may have ac-
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1470
HANSEN ET AL.
counted for the interaction because TDN
intake seemed to influence reproductive f u n c tion only on the low diet.
Our results differed from others on the
influence of nutrition (Wiltbank et al., 1962,
1964; Dunn et al., 1969; Kropp et al., 1973;
Whitmore et al., 1974; Bellows and Short,
1978; Holness et al., 1978) in that diet did
not affect reproduction as the animals became
older. This was probably in part due to different methods employed to alter levels of
nutrition. We fed chopped hay-grain diets
essentially ad libitum and controlled TDN
content of the diets, while in the studies
mentioned above feed intake was controlled.
As the number of parities and age increased,
TDN intake also increased as the animals
continued to grow in size. Hohenboken et al.
(1972) found that cows fed in the same manner
as in our experiments, required the same
percentage of TDN for maintenance from
calving to 240 d postpartum, regardless of
parity. This would indicate that as TDN consumption increased with parity, more TDN
would be available for uses other than maintenance, including reproduction. As a consequence, the L diet may have provided sufficient
TDN for the cows to function reproductively
similar to H - f e d cows in PP2 and PP3. Greater
availability of TDN for uses other than mainte nance as parity and age increased was indicated
by greater weight gains after calving for L - f e d
cows in Exp. 2 during PP2 and PP3 as compared with PP1.
The greatest difference in TDN composition of the diets existed prior to 449 (Exp. 1)
or 420 d (Exp. 2). Because heifers in Exp. 2
were bred at puberty, those reaching puberty
at younger ages received rations higher in
TDN during a greater part of their fir.ct gesta tion than heifers reaching puberty at older
ages. Thus, differences between TDN content
of the rations between H and L groups may
have been greater during the gestation preceding first calving than differences between
diets in the gestations preceding subsequent
calvings, because H heifers were younger
at first breeding. Diet may also have had a
greater effect after first calving than after subsequent calvings if animals bred at younger ages
calved in a different season than animals bred
at older ages because season of calving affected
reproduction during PP1 (P. J. Hansen and
E. R. Hauser, unpublished data). In Exp. 1,
the last diet changes were made before first
breeding and most animals gave birth to their
first calf in one season. However, it is possible
that diet before first breeding may have affected reproduction after first and second
calving, but not later calving, through mechanisms other than the ones mentioned here.
Neither body weight at calving nor changes
in body weight postpartum were significantly
related to duration of the postpartum anestrus
in Exp. 2. Failure to find relationships between
these traits and interval to estrus differs from
findings of others (Menge et al., 1962; Whitman
et al., 1975; Holness et al., 1978; Stevenson
and Britt, 1980).
In our experiments, service interval was
not affected by diet, in contrast to findings
(Wiltbank et al., 1964; Folman et al., 1973)
that cows on a high plane of nutrition had
higher conception rates than cows on a low
plane of nutrition. The tendency in our experiments was for the service interval to be
shorter in L - f e d than H - f e d cows when
dietary regimen influenced interval to estrus,
probably because H - f e d cows were bred
sooner postpartum. Our data and those of
others (Van Demark and Salisbury, 1950;
Shannon et al., 1952; Whitmore et al., 1974)
demonstrated that breeding early in the postpartum period w a s associated with lower
fertility, but the high correlations between
intervals to first estrus and conception suggest
that cows that exhibited estrus earlier conceived earlier, even though their fertility
was lower.
Intervals from calving to first estrus were
longer for cows that had calving dates further
from midsummer. A function of calving date
was included in the multiple regression analysis
to determine the influence of other traits
independent of season of calving.
Gestation appeared to be longer for CF
cows than for other crossbred groups in Exp. 2.
A survey by Preston and Willis (1970) indicated
long gestation periods for Chianinas. Cows
fed L diets had longer average G1 for HF
and SF cows. Kress et al. (1971a) noted cows
fed low levels of nutrition had longer gestations
than cows fed high levels of nutrition. The
first gestation (G1) was shorter than G2 and
G3, possibly because of age or parity effects
or because the breeds of sire of the calves
being gestated differed. Positive correlations
of gestation length with age of dam have been
reported by Jafar et al. (1950) and Burfening
et al. (1978).
Downloaded from jas.fass.org at University of Florida on June 10, 2011
FACTORS AFFECTING POSTPARTUM REPRODUCTION IN COWS
In conclusion, diet interacted with breed
and age (or parity) to influence postpartum
reproduction; the difference between H and L
groups was greater for Holsteins than for
Herefords, tended to be greater for HF and
AF than SF and CF cows and was greater
following first calving than after subsequent
calvings. Level of milk production influenced
postpartum reproduction when nutrition was
limited. The large populations of dairy cows
offers the opportunity to produce beef x
dairy crossbreds that can be used as beef cows
because dairymen, at times, breed first-calf
heifers or low-producing cows to beef bulls.
The breeding of a portion of the dairy cattle
population in this manner would allow beef
producers to take advantage of an earlymaturing, high milk-producing, crossbred female in a terminal siresystem of crossbreeding,
1471
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