Genetic parameters, maternal ability and related selection criteria for unsupplemented Hereford range cows

Data on 5,130 unsupplemented Hereford range cattle were used to evaluate genetic and phenotypic parameters of growth to 2 yr of age under extensive range conditions. From those data, records on 769 heifers saved as replacements were used to evaluate the relationship between growth and subsequent productivity expressed as most probable producing ability (MPPA). Variation in weight largely was due to the year effect. Also, age of dam, the interaction between age of dam and year and the regression on day of birth significantly affected weaning weight. Heritabilities among males and females, respectively, were: birth weight, .53 +/- .09 and .52 +/- .09; weaning weight, .05 +/- .03 and .18 +/- .05; 12-mo gain, .24 +/- .10 and .10 +/- .04; 20-mo gain, .62 +/- .18 and .29 +/- .08; 24-mo gain, .45 +/- .16 and .17 +/- .07. The traits evaluated may have been a response to nutritional stress as well as gainability. The genetic correlation between gain from weaning to 12 mo (a period of weight loss) and gain from 12 to 20 mo (greatest weight gain) was -.93 +/- .45. Metabolic processes favoring growth in a good nutritional environment may result in greater weight loss in a stressful nutritional environment. The genetic correlation between a heifer's gain from weaning to 12 mo and her subsequent MPPA was .47 +/- 28, whereas the correlation between gain from 12 to 20 mo and subsequent MPPA was -.55 +/- .23.(ABSTRACT TRUNCATED AT 250 WORDS)