The effects of slaughter age on growth and carcass traits in an intensively managed crossbred beef herd

The purpose of this study was to determine and describe the effects of slaughter age (at constant weights) on pre- and post-weaning growth rate and carcass traits of unselected steer and heifer progeny from a crossbred beef herd. The data were obtained from 340 heifers and 377 steers weaned at 305 days of age and fed a high-energy post-weaning diet. Heifers and steers were slaughtered at individual unshrunk weights of 420 and 470 kg, respectively, and divided into slaughter age classes based on 25-day intervals. There were differences (P < 0.01) between slaughter age classes for all growth traits including 305-day (weaning) weight, post-weaning daily gain, days on feed, and weight/day. Mean 305-day weight of steers in the youngest slaughter age class (347.8 kg; 376–400 days of age) was 28% greater than the mean of steers in the oldest age class (272.2 kg;>501 days of age). The youngest steer class averaged 63% more for daily gain than the oldest steer class (1.462 vs. 0.894 kg); the corresponding value for heifers was 64%. Post-weaning time in the feedlot ranged from 84.7 days for the youngest steer to 215.6 for the oldest steer class. Calculations based on mean differences between age classes in 305-day weight, daily gain and days on feed indicated that variability in weaning weight and in daily gain were of similar importance in determining slaughter age. Marbling score was greater (P < 0.01) for older steer classes, but was not different in the heifer data. Fat thickness of the oldest steer class was 25% more than the youngest steer class (1.89 vs. 1.51 cm); the corresponding percent in heifers was 30%. With the exception of differences in untrimmed rib and chuck percents in steers (P < 0.05), and rib specific gravity in heifers (P < 0.05), carcass quantity traits were essentially unaffected by slaughter age class when marbling score and fat thickness were included as continuous independent variables. None of the organoleptic traits (tenderness, juiciness and flavour) were affected by slaughter age class.