Factors influencing first-service conception and overall pregnancy rates in commercial beef heifers
Factors influencing first-ser vice conception and overall pregnanc y rates in commercial beef heifers
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Factors influencing first-service conception and overall pregnancy rates in
commercial beef heifers
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S.D. Utter, P.L. Houghton, L.R. Corah, D.D. Simms, M.F. Spire, and M.D. Butine
Thi s Research Report article is available in Kansas Agricultural Experiment Station Research Reports: http://newprairiepress.org/
FACTORS INFLUENCING FIRST-SERVICE
CONCEPTION AND OVERALL PREGNANCY RATES
IN COMMERCIAL BEEF HEIFERS 1
Commercial beef heifers (n=1863) from
16 different sources were used to evaluate
the influence of management practices and
biological traits on first-service conception
and overall pregnancy rates. Frame score,
initial weight, overall ADG, body condition
score, reproductive tract score, source, AI
technician, and AI sire signifi c antly influenced
first-service conception. Overall pregnancy
rates were influenced by frame score, body
weight, and ADG.
(Key Words: Beef Heifers, First-Service
Conception, Pregnancy Rates.)
Proper evaluation, selectio n , and
management of replacemen t heifers are critical to the
production and longevity of females in the
cow herd. Heifers that c o nceive early in their
first breeding season have a greater potential
to rebreed as cows, ensuring continued
production in the herd. Our objective was to
evaluate the influence of management and
biological parameters on first-service
conception and overall pregnancy r ates in heifers.
To evaluate the effect of various manage
ment practices and biological traits on
firstservice conception and overall pregnancy
rates, 1,863 heifers from six states and 16
sources were developed postweaning at a
commercial heifer development facility in
Southwest Nebraska. Initial body weight was
measured when heifers entered the facility in
early to mid-winter. A high roughage ration
was limit-fed at a level s o heifers would reach
65% of their projected mature body weight
before the breeding season began.
Production data collected 35 to 40 days
prebreeding included frame score, reproductive
tract score (1=hi ghly developed, 6=infantile),
and body condition score (1= thin, 9=obese).
The heifers were place d on a higher plane
of nutrition 33 days before breeding. That
plane was maintained through the first 21
days of the breeding season. Heifers were
synchronized by fee ding .5 mg/hd/day
melengesterol acetate (MGA) for 14 d ays, followed
by a subcutaneous injection of prostaglandin
(Bovilene®) 14 days after MGA withdrawal.
A breeding body weight was measured at the
time of injection. Heifers were inseminated
artificially approximately 12 hours after
estrus was first detected. Nonresponders
were reinjected 9 days afte r the first injection.
If no estrus was observed a f ter a third
prostaglandin injection, heifers were inseminated at
1Sincere appreciation is expressed to Heartland Cattle Company for providing the data set
used in this analysis.
2Heartland Cattle Co., McCook, NE.
3Department of Clinical Sciences.
4Department of Statistics.
72 hours post injection. First-service
conception and overall pregnancy rates were
determined by ultrasonography 35 to 45 days
after the end of the 45-day breeding season.
A final body weight was measured at that
time. The difference between initial weight
and final weight was used to calculate overall
average daily gain (ADG).
A specialized statistical analysis
(contingency tables) was used that allowed us to
examine the influence of a single management
factor or biological trait, while others were
held constant. Because of the significant
influence of heifer source, a separate analysis
of heifers from a single source (n=507) was
performed. A second statistical approach
(mixed-model analysis) was used for the
entire population (n=1863) to reduce the
influence of source, technician, and sire.
Results and Discussion
As frame score increased, first-service
conception (P=.16) and overall pregnancy
rates (P<.10) tended to decrease (Figure 1).
The contingency table analysis showed that
heifers that gained 1 to 1.5 lbs/day had the
highest first-service conception rates
(Figure 2) and overall pregnancy rates
(Figure 3). Daily gains above and below this
range produced lower (P<.01) pregnancy
rates. A quadratic response was observed as
body condition score increased (P<.10), with
a decline in first-service conception in either
extremely thin or fat heifers (Figure 5).
Reproductive tract score tended (P=.17) to
influence first-service conception but not
overall pregnancy rates (Figure 4). Heifers
with infantile tracts, designated by a score of
6, had the lowest first-service conception
rates when compared to those with scores of
1 through 5. The mixed model analysis found
the fixed effects of body condition score
(P<.0l; Figure 4) and reproductive tract
score (P<.0l; Figure 5) to be significant.
Interactions among ADG, frame score,
breeding wt, prebreeding wt, body condition
score, and reproductive tract score were
significant, illustrating the impact o f
numerous factors on first-service conception.
Overall, heifers with frame score, body
wt, body condition score, and ADG in the
“moderate” range were more reproductively
efficient. In addition, several factors interact
to influence reproductive efficiency,
demonstrating the importance of managing all
contributing factors, rather than focusing on
Figure 5 . Influence of Reproductive Tract Score on First-Service Conception 110