365 Feeding Cattle – Optimal Days on Feed: Rates of change of economically important performance and carcass characteristics

Abstract Feedlot research data from 7 experiments with beef steers, 6 experiments with beef heifers, and 2 experiments with Holstein steers, representing a total of 687 pen observations, were analyzed to determine the rate of change in selected performance and carcass measurements over extended feeding periods. All cattle were fed high-grain diets and managed under typical industry conditions. The 15 experiments included extended days on feed as a factor (steers: 0 to 62 d; heifers: 0 to 42 d; and Holsteins: 0 to 56 d), with several experiments including beta-agonist and implant program comparisons. Variables of interest were dry matter intake, average daily gain, final shrunk body weight, hot carcass weight, and measures of carcass fatness, yield, and quality. Beta-agonist and implant programs effects and their interaction with extended days on feed, as well as the overall rate of change in performance and carcass measurements with extended days on feed were evaluated using mixed-model statistical methods adjusting for random intercept effects of studies. Effects of zilpaterol or ractopamine and implant programs were consistent with the published literature, and for analyses across steer and heifer data, interactions between beta-agonist/implant program and extended days on feed were detected for only 2.5% of the variables evaluated. Slope values in the overall analyses for all 3 classes of cattle were generally significant (P ≤ 0.03 for 85% of the variables analyzed), reflecting increased final shrunk body weight and hot carcass weight, greater carcass fatness, and shifts toward higher quality and yield grades with extended days on feed. Producers marketing cattle on a carcass basis could use the slope data from these analyses to estimate changes in carcass weight along with potential premiums and discounts for use in economic projections.