Direct selection for 13-month live weight in NZ Angus cattle for 15 years has resulted in 17% heavier yearlings and 12% heavier adults when compared with unselected control animals. The mean daily food intake of growth-selected bulls is higher on a per head basis but is similar when adjusted for differences in live weight. The objective of this paper is to model the efficiency of a breeding cow herd, including replacements, derived from selected and control lines. The index of efficiency used is weight of calf weaned per unit weight of winter feed required. Published performance data from the 2 herds are used to quantify calf output and feed requirements are based on published growth patterns in the 2 herds. Our analyses show that, for the same winter feed requirement, the selected herd when compared with the control herd would have: 88 vs 100 cows wintered; 65 vs 72 calves weaned, and mean calf weaning weight would be 174 vs 158 kg per head. Efficiency was estimated to be 99.8 vs 100 units. Theses results indicate that while selection for yearling weight has resulted in heavier calves and breeding cows, the calculated extra winter feed costs associated with the heavier selected herd makes them no more efficient than the randomly bred control herd. For the selected herd to be more efficient, either a reproductive advantage well in excess of the 3 percent point advantage achieved would be required, or the metabolic efficiency of the selected cows would need to increase. In economic terms, the selected herd may be more efficient than the control herd for 2 reasons. Firstly, since herd size is lower, total herd costs are likely to be lower. Secondly, if a premium exists for heavier calves, the calves from the selected herd are likely to be worth more per head.

W, Abell

Proceedings of the New Zealand Society of Animal Production, Volume 52, , 107-110, 1992
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