Genetic Selection for Litter Size in Cattle

Efficiency of production in beef cattle is limited by their relatively low reproduction rate. In this paper, we present the results of a fourteen year breeding program aimed at increasing prolificacy in a mixed beef x dairy herd, grazing at Armidale in the New England region of NSW. The herd was established by purchasing cows with a repeated history of twinning and bulls from dams with unusually high prolificacy. For the 51foundation cows with at least one subsequent calving record, the average prolificacy was 1.11 +/- 0.05 (123 total records. For all cows born in the herd, the mean ovulation rate was 1.12 and the mean prolificacy was 1.03. The twinning rate was lower than expected based on reports from other experimental herds aimed at increasing prolificacy that have been established in the USA, New Zealand and France. The observed genetic trends for ovulation rate and prolificacy were essentially zero but were moderately positive for fertility, cow rearing ability and reproduction rate. The desired outcome of generating a positive genetic trend in reproduction rate was achieved, albeit to a limited degree, but not as a result of genetic response in ovulation rate or prolificacy, the traits under direct selection pressure.