Age and weight at puberty (first behavioural oestrus) are known to be heritable traits in experimental beef and dairy cattle. The recording required is usually labour-intensive, and may also be expensive if serial blood samples are taken to estimate progesterone concentrations. Our objective in this study was to ascertain whether a less labour- intensive approach for estimating age at puberty (AP) would still provide enough statistical power to show genetic variation among sire groups of Friesian and Jersey heifers grazing in large groups. Commercial heifers (n = 797) were born in the late winter/early spring of 2002, and were predominantly the result of artificial insemination (AI) matings. They were reared to weaning in six herds, and then grazed in five herds through 2003. Tailpaint was applied to heifers at visits to the grazing herds at 4-6 week intervals, and tailpaint loss was recorded at each visit to determine approximate date at puberty. Birth dates were known, and hence AP was calculated. The heritability of yearling live weight (W12) was used as a control measure. From four of the herds (n = 453), there were animals by 13 widely-used AI sires (9 Friesian and 4 Jersey), and 10 other sire groups were represented with at least 5 daughters each. The phenotypic standard deviations for AP and WP were 48 days and 34 kg, similar to that found in New Zealand experimental beef cattle herds. The pooled within-breed heritability estimates for AP, WP and W12 were 0.09 ± 0.10, 0.21 ± 0.14 and 0.24 ± 0.15, respectively. Confounding of AI sire group with date of birth of heifer was encountered. The breeding-value difference in AP between the top 7 and bottom 6 of the widely used AI sires was 15 days, or 1.05 genetic standard deviations. In conclusion, recording AI calves in large grazing herds may be a practical method of getting access to puberty data, but more frequent visits to each herd would probably be needed to reduce the noise, and increase the heritability for AP and WP.

CA, Morris, and SM Hickey

Proceedings of the New Zealand Society of Animal Production, Volume 64, Hamilton, 115-117, 2004
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