Date of this Version
Determining the optimal weight for breeding replacement heifers has been a goal of researchers for decades. It is an important question because replacing cows in the beef herd is expensive.
Finding the answer is a complex problem because of the inter-related nature of the economic and physiological variables. Patterson et al. (1992) may have expressed the nature of the problem best when he said, “Although puberty in the female and the events that precede its onset may be fixed from a physiological standpoint, the animal and management environment in which it is placed are in constant transition.” This indicates that the optimum depends on economic, as well as the biological factors.
Recent studies conducted by University of Nebraska researchers (Funston and Deutscher 2004, and Martin et al. 2007) at the West Central Research and Extension Center (WCREC) have shown the fertility rates between groups of heifers fed to different average sizes prior to breeding were not statistically different, while the feed costs for the heavier heifers was greater than that of the lighter ones. Thus, the economic analyses in these studies were based on the development cost of cattle groups. This study also suggested that optimal heifer size was less than the traditionally accepted 64 to 65 percent mature body weight (PMBW).
The data collected in the above WCREC studies was used to conduct a rigorous economic analysis of optimal breeding weight using classical economic optimization theory. A profit equation was the central tool, as suggested by Fuez (1991). This profit equation consisted of five revenue equations and three cost equations, all of which were affected by the heifer’s pre-breeding size, PMBW. The complex nature of the profit equation required a numerical methodology to derive the optimal PMBW.