Trying to improve beef cow pregnancy rate by genetic selection within a breed is generally considered to be slow and difficult. We have investigated a promising option which involves selecting to change age at puberty in heifers and then exploiting the genetic correlation with lifetime pregnancy rate. Angus herds were established from a common foundation population in the early 1980s, and were then selected from 1984 for reduced age at puberty (AGE-) or increased age at puberty (AGE+), with a control herd for comparison. Relative to the AGE+ herd, heifers born in the AGE- herd in 1993 and 1994 were 83 days (=20%) younger at puberty (P<0.001), and their half brothers had 2.0 cm (=6.6%) greater scrotal circumference (P<0.001). Corresponding live weights in the AGE- herd were 17% less at puberty (P<0.001), not significantly different between herds at birth, weaning or 8 months of age, and 3.5% higher as yearlings (P<0.01) than in the AGE+ herd. Conception rates for the 1992-95 matings were 91.2 and 71.2% (P<0.001) for AGE- and AGE+ yearling heifers, but not yet significant at 89.6 and 85.0% respectively for older cows. Restricted maximum likelihood estimates of heritability for age at puberty and scrotal circumference (mean of 3 measurements) were 0.27±0.04 and 0.48±0.05, with a genetic correlation of -0.29±0.10. Phenotypic standard deviations for age at puberty in the AGE- and AGE+ herds were 51 days for each herd. However, within-herd heritabilities for age at puberty were 0.32±0.11 and 0.13±0.14 for the AGE- and AGE+ herds, respectively, which suggests to us that different modes of inheritance should be investigated in each herd.

CA, Morris, and JA Wilson

Proceedings of the New Zealand Society of Animal Production, Volume 57, , 9-11, 1997
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