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Effects of Sire, Age of Dam and Gestation Feed Level on Dystocia and Postpartum Reproduction¹

Livestock and Range Reasearch Station, Miles City, MT 59301

Abstract

A 2³ factorial study was conducted with 48 primiparous heifers and 54 multiparous cows (all Angus Hereford reciprocal crossbreds) that had been bred to one of two Charblais sires selected to produce calves of either moderate (M) or heavy (HV) birth weight. Dams were placed in drylots 90 d before die predicted calving date and received diets of either 3.6 (L) or 6.8 (H) kg TDN daily until calving. The postcalving diet consisted of 8.6 kg TDN daily until all dams were placed on range pastures. Cows were larger than heifers in pelvic height (P<.05), width (P<.01) and area (P<.01), and heifers experienced more dystocia (P<.01). The H diet resulted in more rapid gains (P<.01) by the dam, but the effect on birth weight was nonsignificant. Calves from cows weighed more at birth (P<.01) than those from heifers, as were six of seven body measurements of the neonate (P<.10 and P<.01). Gestation diet had no significant effect on calf dimensions, but calves sired by the HV sire were heavier (P<.01) at birth with measurements exceeding (P<.05 to P<.01) tliose of the M-sired calves in five of the seven dimensions. Male calves experienced more severe dystocia than females (P<.05), were heavier at birth (P<.01) and had larger (P<.05 to P<.01) body dimensions in six of the seven measurements. The L diet had a detrimental effect on day of year at first estrus (P<.01) and postpartum interval ,(P<.05). Heifers had poorer postpartum reproductive performance than cows, as measured by day of first estrus (P<.01), postpartum interval (P<.01), percentage in estrus by the beginning of the breeding season (P<.01) and pregnancy rate following the 45-d artificial insemination period (P<.05). Dam age x gestation diet interactions indicated that the L diet was more detrimental to heifers than to cows for all aspects of postpartum reproduction studied (P<.10 to P<.01). Dams nursing female calves had a higher pregnancy rate (P<.05) than dams nursing male calves. The study suggests sires might be selected to produce calves that will have lower birth weights and dystocia but which will grow rapidly from birth to weaning, resulting in weaning weights essentially equivalent to those of calves produced by high birth weight-high dystocia sires.

¹ USDA, ARS and Montana Agr. Exp. Sta., Cooperating.

² Biometrician, USDA, ARS, Fort Collins, CO.

³ Authors express appreciation to the American International Charolais Association, Houston, TX, for financial assistance for portions of this work.

⁴ This study was conducted as a contribution to Western Regional Project W-112, Improving Reproductive Performance in Cattle and Sheep. The paper is published with the approval of the Director of the Montana Agr. Exp. Sta., Journal Series No. 1186.

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