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San Joaquin Experimental Range
- Research Bulletin -
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Effects of Bull Exposure on Postpartum
Reproduction in Fall-Calving, First-Calf Beef Cows
by
Randy C. Perry
and
Dennis E. Jones
CATI Publication #940803
© Copyright August 1994, all rights reserved
Abstract
One hundred and three crossbred, fall-calving, two-
year-old beef cows were used to evaluate the effects of
bull exposure on postpartum reproduction. Similar trials
were performed during three consecutive years with 42
cows used the first year (Trial 1), 36 cows the second year
(Trial 2), and 25 cows the third year (Trial 3). Cows were
allotted by calving date into either a bull exposure or
control group. Cows that were exposed to bulls (BE) were
placed in a pasture with a penile deviated bull from
shortly after calving until the first day of the breeding
period. Cows in the control group (C) had no visual or
tactile contact with mature bulls during the same period.
Pastures available to both groups of cows had similar
amounts of native forage and cows received similar levels
of supplemental feed during the treatment period. Two
blood samples were collected 10 days apart before the
start of the breeding period to determine cyclicity based
on progesterone concentrations at the start of the breeding
period. Cows were exposed to fertile bulls for
approximately a 70-day breeding period. Ultrasonography
per rectum was used to determine pregnancy, and calving
dates were recorded the subsequent year. Cows were
scored for body condition on the first day of the breeding
period. Cows in both groups were similar in terms of body
condition at the start of the breeding period in all three
years.
Cyclicity at the start of the breeding period was
similar for BE and C cows, respectively, in each of the three
trials (55 and 63 percent in Trial 1; 86 and 100 percent in
Trial 2; and 96 and 94 percent in Trial 3). In addition, BE
did not influence pregnancy rate, with pregnancy rates of
91 and 94 percent (Trial 1), 85 and 94 percent (Trial 2),
and 85 and 83 percent (Trial 3) for BE and C groups,
respectively. Mean calving date the subsequent year was
similar for both groups during the first two years of the
study (October 20 ± 4 days and October 18, 1991
± 4 days, Trial 1; October 17 ± 3 days and October
9, 1992 ± 3 days, Trial 2; for BE and C groups,
respectively). However, bull-exposed cows in Trial 3
calved earlier (P = .03) the subsequent year than C cows
(October 8 ± 3 days versus October 19, 1993 ± 3
days, respectively). In summary, bull exposure failed to
influence cyclicity status at the start of the breeding
period, pregnancy rates after breeding, and subsequent
calving date in fall-calving, first-calf beef cows.
KEY WORDS: Bovine, Bull Exposure, Postpartum,
Reproduction
Introduction
First-calf cows that calve as two-year-olds in many
beef operations are the most difficult group of females to
rebreed. These females experience a higher degree of
dystocia than older females, with increased nutritional
requirements related to growth and lactation. The length
of postpartum anestrus influences rebreeding efficiency in
these young females. Postpartum anestrus is influenced by
many factors including dystocia (Brinks et al., 1973;
Bellows et al., 1982); level of milk production (Whitmore
et al., 1974; Bartle et al., 1984); suckling (Short et al.,
1972); season of parturition (Hansen, 1985; Smeaton et al.,
1986); body condition (Rutter and Randel, 1984); and level
of nutrition (Bellows and Short, 1978; Dunn and Kaltenbach, 1980).
Exposure of postpartum females to males prior to the
start of the breeding period, referred to as bull exposure,
has been shown to reduce postpartum anestrus in first-
calf (Custer et al., 1990) and mature (MacMillan et al.,
1979; Zalesky et al., 1984; Alberio et al., 1987; Naasz and
Miller, 1987; Roberson et al., 1990) beef cows. This
practice offers potential as a management tool to increase
rebreeding efficiency in postpartum beef cows. Beef
producers that use short breeding periods should
experience economic benefits through increased pregnancy
rates and more cows calving earlier in the calving season if
bull exposure is used to shorten the length of postpartum
anestrus and to increase the number of cows that are
cycling at the start of the breeding period.
The mechanism(s) by which bull exposure decreases
postpartum anestrus is (are) still unknown. It seems
logical that male exposure stimulates the hypothalamic-
pituitary axis in a way that influences gonadotropin
secretion because of the involvement of gonadotropin
secretion in initiating cyclicity following parturition. This
was exhibited in other species. Introduction of males
increased serum luteinizing hormone concentrations in
prepubertal mice (Bronson and Desjardins, 1974) and
increased pulse frequency of luteinizing hormone in
anestrous ewes (Martin et al., 1980; Poindron et al., 1980).
Exposure to males reduced age of puberty in cattle (Pennel et al., 1986; Roberson et al., 1991) and other
species including mice (Vandenbergh, 1967) and swine
(Zimmerman et al., 1974). These findings are significant
since mechanisms associated with the onset of puberty are
similar to those associated with the return to cyclicity
after parturition.
The objective of this research was to
evaluate the effect of exposing fall-calving, first-calf beef
cows to mature bulls from parturition to the start of the
breeding period on postpartum reproductive performance.
Materials and Methods
TRIAL 1. Forty-two first-calf crossbred beef
cows (2-year-olds) were allotted by calving date into
either a bull-exposed (BE) or control (C) group during the
fall of 1990. Cows in the BE group were exposed to a penile
deviated bull from shortly after calving until the first day
of the breeding period. Cows in the C group had no visual
or tactile contact with mature bulls during the same
period. Similar amounts of native forage and supplemental
feed were available to both groups during the treatment
period.
Blood samples were collected on the first day of the
breeding period and 10 days prior to the start of breeding
to determine concentrations of progesterone in serum.
These values were used to determine which cows were
cycling at the start of the breeding period. Cows were
determined to be cycling if they had >= 1.00 ng/ml of
progesterone in serum at either of the two blood sampling
times. Cows were scored for body condition on the first
day of the breeding period by two independent evaluators
using a scale from 1 to 9 (1 = emaciated; 9 = obese).
Cows in both treatment groups were combined into a
single group during a natural breeding period of 60 days.
Pregnancy was determined using ultrasonography per
rectum, and calving dates were recorded the subsequent
year.
TRIAL 2. Thirty-six first-calf crossbred beef
cows (2 year-olds) were used during the fall of 1991. All
materials and methods were similar to those used in Trial
1 except that the natural breeding period was 69 days in
length.
TRIAL 3. Twenty-five first-calf crossbred
beef cows (2 year-olds) were used during the fall of 1992.
Materials and methods were similar to those discussed in
Trial 1, except that the natural breeding period was 73
days in length.
Results and Discussion
Body condition score, cyclicity status, pregnancy rate
and subsequent calving date for all three trials are
presented in table 1. Average calving date
the initial year (the year cows were allotted to their
respective treatments) was similar between the BE and C
groups in all three trials. Average body condition score at
the start of the breeding period was also similar for BE and
C cows in all three years. Similarities in body condition
insured that cows were on similar planes of nutrition
during the treatment period. Level of nutrition, as a result,
should not have influenced the results presented here.
Table 1. Effects of Bull Exposure
Treatment Group
------------------------ Treatment
Characteristic Bull Exposure Control Effectsa
Number of head
Trial 1 21 21
Trial 2 20 16
Trial 3 13 12
Body Condition Scoreb
Trial 1 4.9±.1 5.1±.1 NS
Trial 2 5.1±.1 5.2±.1 NS
Trial 3 5.2±.1 5.1±.1 NS
Cyclicityc %
Trial 1 55% 63% NS
Trial 2 86% 100% NS
Trial 3 96% 94% NS
Pregnancy Rated %
Trial 1 91% 94% NS
Trial 2 85% 94% NS
Trial 3 85% 83% NS
Subsequent Calving Datee
Trial 1 Oct. 20, 1991 Oct. 18, 1991 NS
Trial 2 Oct. 17, 1992 Oct. 9, 1992 NS
Trial 3 Oct. 8, 1993 Oct. 19, 1993 .03
a NS = no significant differences.
b Scale = 1 to 9 (1 = emaciated and 9 = obese). Measurements taken at the
start of the breeding period.
c Percentage of females cycling at the start of the breeding period.
d Percentage of females pregnant following the natural breeding period.
e Standard errors of mean calving dates for both treatment groups were ±4 days
for trial 1, and ±3 days for trials 2 and 3.
Bull exposure had no influence on cyclicity at the
start of the breeding period or on pregnancy rate after the
natural breeding period in any of the three trials. These
results contradict earlier findings (Macmillan et al., 1979;
Zalesky et al., 1984; Alberio et al., 1987; Naasz and Miller,
1987; Custer et al., 1990; Roberson et al., 1990) which
indicated that exposure to bulls during the postpartum
period reduced the length of postpartum anestrus in beef
cows. These results agree, however, with earlier findings (Berardinelli et al., 1978; Roberson et al., 1987) which
suggest that exposure to males failed to influence onset of
puberty in beef heifers.
Bull exposure favorably influenced subsequent
calving date in Trial 3 but had no influence on this
measure of reproductive efficiency in the first two trials.
It is difficult therefore to draw any conclusion from this
finding since bull exposure failed to influence other
measures of reproductive efficiency that were evaluated.
In conclusion, bull exposure failed to influence cyclicity at
the start of the breeding period and pregnancy rate after
breeding, with little influence on subsequent calving date
in fall-calving, first-calf beef cows.
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