ENVIRONMENTAL AND GENETIC EFFECTS ON PREWEANING PERFORMANCE OF CALVES FROM FIRST-CROSS COWS. I. CALVING EASE AND PREWEANING MORTALITY

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Abstract
Calving ease and calf mortality were studied for 4034 calves produced in second and later parities at two locations, Brandon and Manyberries, by contemporary populations of first-cross dams mated to four terminal sire breeds, Charolais (C), Chianina (Chi), Limousin (L) and Simmental (S). The dams represented 10 F1 crosses, the Hereford × Angus (HA) and nine crosses produced by mating C, L and S Bulls with Hereford, Angus and Shorthorn cows. Calving difficulty and mortality were higher among males (P < 0.0001) than females. Calving ease was not influenced by location, but postnatal mortality, particularly of males, was substantially less at Brandon (P < 0.0001). Differences in calving ease among terminal sire progeny groups were not significant when the four breeds were mated to HA dams, but all paired or three-way comparisons involving the Limousin indicated the highest proportion of unassisted births (P < 0.0001) and the lowest incidence of difficult births (P < 0.006) for this sire breed. Terminal sire differences in postnatal mortality were not present among progeny from the HA dams, nor were they observed in the paired comparisons involving progeny of the Chi vs. those of the other three terminal sire breeds. For this trait, however, the paired comparisons among progeny from C, S and L sires indicated a ranking of C > S > L (P < 0.04). Breed of dam's dam (Hereford, Angus, Shorthorn) had no influence of calving ease or mortality, but breed of dam's sire (C, L, S) was important with progeny from L-sired dams having the highest incidence of unassisted births (P < 0.01) and the highest incidence of postnatal mortality (P < 0.06). The proportion of unassisted births increased as the experiment progressed (P < 0.001) and may have been associated with advancing age of the dams. There was, however, a large location × year interaction for this trait (P < 0.002) with Manyberries recording substantially greater calving difficulty than Brandon during the first 3 yr. Interactions involving calf sex, year of birth, breed of terminal sire and location were identified for calving ease. Calf mortality generally increased with increasing calving difficulty but the relationship between calving ease and mortality was influenced by the degree of precalving environmental stress on the dam. Key words: Beef cattle, crossbreeding, calving ease, mortality