Effects of Sire Breed, Breed Type and Age and Weight at Breeding on Calving Rate and Date in Beef Heifers First exposed at Three Ages

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Abstract
Calving rate and calving date were studied in 859 Angus-, Brahman-, Charolais- or Hereford-sired straightbred or crossbred heifers exposed to calve first at 24, 30 or 36 mo of age. Angus-, Charolais- and Hereford-sired crossbred heifers had calving rates similar to those of contemporary straightbred heifers, whereas Brahman-sired crossbred heifers had higher (P < .01) calving rates than straightbred Brahman heifers (64 ± 4 vs 28 ± 6%). Heifers calving first at 24 and 36 mo had similar calving rates (73 ± 7 and 79 ± 10%), which were slightly higher (P > .10) than for those heifers calving at 30 mo (61 ± 7%). Calving rates for Angus- and Charolais-sired heifers were highest at 24 mo, whereas calving rates for Brahman- and Hereford-sired heifers were highest at 36 mo. Heifers that were older at the start of breeding season had higher calving rates (b = .41 ± .14%; P < .01) at 24 mo, but not at 30 or 36 mo. Breeding season weight adjusted for heifer age influenced (P < .01) calving rate at 30 (b = .32 ± .07%) and 36 mo (b = .23 ± .08%) but not at 24 mo. Brahman-sired heifers calved later (P < .01) during the calving season (63 ± 3 d) than heifers sired by Angus, Hereford or Charolais bulls (45 ± 2, 48 ± 1 and 50 ± 2 d). Heifers that were older and weighed more at the start of the breeding season calved earlier when exposed to calve at 24 mo (b = −.30 ± .08 d for heifer breeding age [P < .01]; b = −.11 ± .05 d for breeding season weight [P < .05]). These data indicate that, under conditions of our study, heifer sire breed and the interaction of sire breed with first calving opportunity influenced calving rate and date. Heifer age influenced calving rate, and heifer age and weight affected calving date when heifers were managed to calve at 24 mo. Heifer breeding season weight influenced calving rate for heifers exposed to calve at 30 and 36 mo. Copyright © 1989. American Society of Animal Science . Copyright 1989 by American Society of Animal Science

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