Pregnancy to Artificial Insemination in Beef Cows as Affected by Body Condition and Number of Services1,21This research was supported by Select Sires, Inc., Plain City, OH.2Authors gratefully acknowledge J. O. Sanders and D. K. Lunt for supplying animals and facilities.
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1998 American Registry of Professional Animal Scientists. Pluriparous, lactating (L) Angus (n=173) and nonlactating (NL) Angus (n=78) and Hereford (n=34) cows were used in this trial to determine the effect of body condition score (BCS) at the start of a 45-d AI period on pregnancy rates and to compare fertility between females conceiving at first AI service and those requiring more than one AI service. Pregnancy rates to first AI service and overall pregnancy rates were not affected by lactational status or by breed among NL cows. Probability analyses indicated that cows with BCS of five or six should have the highest pregnancy rates. Overall pregnancy rates and pregnancy rate to first AI service did not differ among NL cows with BCS of five or six, but L cows with BCS of five had lower (P<0.025) overall pregnancy rates than L cows with BCS of six (70.1 vs 85.9%, respectively). Probability analyses further indicated that pregnancy rates for females requiring more than one AI service would be 35 to 49 percentage points lower than in females conceiving at first service. In cows that could display estrus more than once before the end of AI breeding, actual pregnancy rate to first service was 77.3% compared to 35.7% to second AI service (P<0.005). Additional analyses indicated that cows displaying estrus in the first 21 d of the AI period had pregnancy rates of 81.8% compared to 58.5% (P<0.005) for those displaying their first estrus after that period. These data indicate that BCS of six at the start of breeding may be advantageous in achieving pregnancy in a 45-d AI period, fertility is low in females requiring more than one AI service, and that cows displaying estrus in the first 21 d of the breeding period will have higher pregnancy rates than cows whose first estrus occurs later in the breeding period.
