Effects of extended days on feed on rate of change in performance and carcass characteristics of feedlot steers and heifers and Holstein steers

We analyzed data from feedlot studies in which beef steers and heifers and Holstein steers were fed for extended days beyond the typical slaughter date with the objective of determining the rate of change in selected performance and carcass measurements over the extended feeding periods. Performance and carcass measurements from 7 experiments with beef steers, 6 experiments with beef heifers, and 2 experiments with Holstein steers, representing a total of 687 pen observa- tions, were analyzed. Cattle were fed high-grain diets and managed under industry-standard conditions. All experi- ments included extended days on feed as a factor (0- to 62-d range for steers; 0- to 42-d range for heifers; and 0- to 56-d range for Holsteins), and several experiments included β-agonist and implant program comparisons. Key variables analyzed included DMI, ADG, final shrunk BW, hot carcass weight, and various measures of carcass fatness, yield, and quality. Mixed-model statistical meth- ods were used to evaluate β-agonist and implant programs 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, adjusting for random intercept effects of studies. Feeding control diets versus zilpaterol (2 steer studies) or ractopamine diets (2 steer studies) resulted in effects consistent with the published literature. Interactions between extended days on feed and β-agonist or implant program treatments were noted for less than 5% of the variables examined. Similarly, for the overall analyses within class of cattle, interactions between extended days on feed and dietary or implant treatments were rare (2.5% of 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 BW and hot carcass weight, greater carcass fatness, and shifts toward higher QG and YG with extended days on feed. Producers selling cattle on a carcass basis could use our slope data to es- timate changes in carcass weight with extended days on feed, as well as potential premiums and discounts that could be used in profit–loss projections.