Growth and carcass data were collected during 10 yr at the Fort Keogh Livestock and Range Research Laboratory in Miles City, MT from a Hereford herd selected at random. The objective was to compare genetic parameter estimates from this randomly selected herd with estimates from published selection studies. Only bulls were included in the data set. Bulls were slaughtered between 14 and 17 mo of age. Genetic parameters were estimated by the paternal half-sib procedure. Data were subdivided into three subgroups: 1) all bulls that had birth weights, 2) bulls with complete records for growth traits, and 3) bulls with complete carcass and growth records. The heritability estimate for birth weight on 736 progeny from 86 sires was .18. The second group included 616 progeny representing 85 sires and heritability estimates were weaning weight, .17; daily gain from birth to weaning, .20; final test weight, .42; and daily gain on test, .47. The third group included 401 progeny from 75 sires and heritability estimates were carcass weight, .38; longissimus muscle area, .51; marbling, .31; slaughter weight, .42; and dressing percentage, .25. Thus, these estimates of heritability were in quite good agreement with previously published values, with a tendency for these estimates to be lower for early growth weights. Phenotypic and environmental correlations were in good agreement and genetic correlations were in general agreement with the data reviewed. Estimates of heritabilities from this herd selected at random were not larger than previous estimates from selected herds.

Growth rates and weights at weaning, 365 d, and at slaughter were obtained on 616 bulls in a nonselected Hereford herd over a 10-yr period beginning in 1978. Carcass data were obtained for 401 of these bulls at 16 mo of age and on 101 that were sires or alternates and slaughtered at 30 mo of age. Fifty-five bulls slaughtered at 30 mo of age sired 301 male offspring on which growth data were obtained and 30 sired 169 male offspring on which carcass data were obtained. Bulls gained an average of .75 kg/d preweaning and 1.16 kg/d postweaning on a 168-d feed test. Rate of daily gain from the end of feed test to slaughter ranged from .7 to 1.2 kg/d. Time from the end of the feed test to slaughter ranged from 48 to 140 d. Slaughter weight, marbling score (Small = 12, Traces = 6), longissimus muscle area, fat covering over the 12th rib, percentage of kidney, pelvic and heart fat (KPH), and dressing percentage for bulls slaughtered as yearlings were 470 kg, 7.6 score, 82.5 cm, 8.2 mm, 1.0%, and 58.8%, respectively. The 30-mo-old bulls were slaughtered directly from range pastures. Marbling was devoid or practically devoid and fat covering over the 12th rib and KPH fat were insufficient to measure or estimate accurately. Sufficient variation was not available for statistical analyses of these traits. Slaughter weight, longissimus muscle area, and dressing percentage of 30-mo-old bulls were 583 kg, 91.8 cm, and 54.0%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)