Divergent selection for postweaning feed conversion in Angus beef cattle: I. Mean comparisons.

Each year from 1979 through 1983, 35 Angus bull calves were selected from a herd at the Eastern Ohio Resource Development Center to be individually fed in a 140-d postweaning performance test. From these 35 individually fed bulls, the three highest and three lowest for feed conversion (feed:gain) were selected and randomly mated to approximately 20 cows each. A different set of high vs low feed conversion sires was used each year. Four replicates (403 progeny) from high vs low sires were evaluated by sire groups for subsequent postweaning and carcass performance. Progeny were slaughtered when estimated by ultrasonic measurement to have 8.9 mm or more of subcutaneous fat at the conclusion of a 140-d postweaning performance test. Progeny with less than 8.9 mm of subcutaneous fat were fed for additional 28-d periods until they reached the required minimum. No differences were found between high and low feed conversion progeny for 140-d feed intake (P less than .30) although high feed conversion progeny gained .09 kg/d more weight (P less than .01) during the 140-d postweaning test. Differences tended to exist between high and low feed conversion progeny for unadjusted (P less than .15) and maintenance-adjusted (P less than .15) feed:gain ratios. Progeny of the high feed conversion group had greater subcutaneous fat (P less than .05) at the end of the 140-d postweaning test and when slaughtered (P less than .05), indicating a genetic difference for composition of BW gain between high- and low-sired progeny. However, no significant differences existed for any other carcass traits evaluated. Bulls had more desirable unadjusted (P less than .001) and maintenance-adjusted (P less than .001) feed:gain ratios than heifers with increased 140-d ADG (P less than .001) and pen feed intakes (P less than .001).