Factors Affecting Pregnancy Rate of Angus, Zebu and Zebu-cross Cattle

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Abstract
A three-year study was conducted to determine factors influencing estrus and pregnancy rate from mating Angus (A) and Brahman (B) sires to A, B, Brangus (BR) and Africander-Angus (AF) cows; Brangus sires to BR cows and AF sires to AF cows. All heifers were placed in the breeding pastures as 2-year-olds and had a 6.8% lower (P<.05) pregnancy rate after two services and became pregnant 4.2 days later (P<.01) than older cows. Three-year-old lactating cows had a 15.1-day longer (P<.01) interval from calving to first estrus, a 10.6-day longer (P<.05) interval from calving to pregnancy and a 10.9% lower (P<.05) pregnancy rate than older lactating cows. Breeds differed (P<.05) in percentage of cows showing estrus. Straightbred A and AF matings were similar (P>.05) for all factors studied except pregnancy date. Both breeds conceived earlier (P<.05 to P<.01) and were superior to B (P<.05 to P<.01) in pregnancy rate at first service and at 75 days. Straightbred Brangus matings were intermediate between the Angus and Brahman in pregnancy date and pregnancy rate at first service but were similar (P>.05) to the Brahman in pregnancy rate for 75 days. Cows bred to A sires had higher pregnancy rates at first service (P<.05), for two services (P<.05 to P<.01) for 75 days (P<.05) and became pregnant earlier (P<.01) than cows bred to B sires. Due to low reproductive performance of the B, the A ✕ B and B ✕ A crossbreds exceeded the purebred A and B average by 9.2% for pregnancy rate at first service, 2.4% for pregnancy rate for two services, 7.5% for services required for conception and 2.9% for pregnancy rate for 7 5-days. Copyright © 1979. American Society of Animal Science . Copyright 1979 by American Society of Animal Science.