THE INTERACTION OF COW SIZE WITH GROWTH POTENTIAL OF THE SERVICE SIRE

Abstract

Weaning weight and individual feed records for 343 calves produced by 84 cows during an 8-yr period were utilized to evaluate the effects of high- and low-growth-potential sires on the efficiency of large and small cows. Cow efficiency was expressed as a ratio of cows' and calves' TDN consumption for a year divided by the weaning weight of the calf. The design allowed the mating of small and large cows of each of four breed groups to unrelated low- and high-growth-potential bulls. The effects of cow size on cow efficiency were further evaluated by omitting cow size group and the interaction of cow size with sire group from the model and replacing them with the linear and quadratic effects of cow weight and subclass regressions of efficiency on cow weight within sire groups. Results indicated no interaction of cow size with growth potential of the bull, and no effect of cow size group but a significant effect of growth potential of the bull. The inclusion of milk production as a dependent variable indicated that cows produced 0.6–0.8 kg d−1 more milk when bred to the high-growth-potential bull group. None of the quadratic or linear regression coeficients were significant but the subclass regression of cow efficiency on cow weight within the high-growth-potential bull was significantly different from zero and from the coefficient for low-growth-potential bulls. These effects were in the direction which agrees with the concept of complementarity but their values were rather small. Further experimental evaluation of this kind with greater range in weight of sire and dam would be desirable. Key words: Cow efficiency, complementarity, beef cattle