Simulated efficiency of range beef production. II. Fertility traits

A modified version of the Texas A&M University Beef Cattle Production Model was used to simulate the effects of changes in potential for age at puberty (AAP), potential for probability of conception (PCA) and winter feed levels on biological and economic efficiency of beed production in a northern plains, range environment. Two management systems were simulated: a weanling system in which all calves except replacement heifers were custom fed in a feedlot immediately post-weaning; and a yearling system in which calves were kept on the ranch through their second summer, then custom-fed. Biological efficiency was defined as the ratio of TDN input to product output, and economic efficiency was defined as the ratio of total dollar cost to 100 kg product output. A simulated increase in AAP from 365 to 425 d resulted in slightly decreased economic efficiency under a yearling system of management. An increase in PCA from .75 to .85 caused decreased biological efficiency under both weanling and yearling management systems, suggesting biological inefficiencies associated with maintaining mature cows. Simulation results indicate that optimal supplementation levels and corresponding levels of observed fertility depend on the value of product derived from cull cows relative to the value of product derived from fed animals and on the costs of developing replacement heifers relative to the costs of maintaining mature cows. Decreased fertility causes change in the sources of products, not product loss per se. For this reason, survivability may be a more important aspect of reproduction than fertility.