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