Abstract

In reproductive studies of juveniles, puberty has to bedefined and decisions made on how to record it in both sexes. Our recording criterion for age at puberty in heifers is the date (and hence age) of first behavioural oestrus. This is a heritable trait under New Zealand grazing conditions, with a realised heritability estimate from a long-term AgResearch selection experiment of0.27 ± 0.04. Data published in Edinburgh by R. B. Land suggest that male puberty traits in mammals are also inherited and are related to female puberty traits, but the difficulty is in measuring puberty in males. Our indicator in the selection experiment mentioned above is scrotal circumference (SC), where single SC records had a heritability of 0.40 ± 0.03 and a repeatability over time of 0.67 ± 0.01. Our objective in the present study was to determine whether another indicator might be semen quality, derived from a serial assessment of semen samples over the likely puberty period, and to compare this with serial SC records. The selection experiment was established in 1984/85 with Angus cattle. Genetic selection was applied in one line (‘AGE-’) for lower age at puberty in heifers, and in a second line (‘AGE+’) for greater age at puberty in heifers. SC data were monitored each year in bulls from both lines. Young bulls from the 1997 and 1998 calf crops (25 per year) were also monitored for semen quality from about 9 to12 months of age. Semen samples were obtained by ampullary massage, and quality was assessed on a warmmicroscope stage, using a 0-5 scale. Semen score was plotted against SC from means for each breeding line and month, and the correlations over both lines were 0.98 in 1997 and 0.86 in 1998. The AGE- line was superior to the AGE+ line by 0.81 semen score units (P < 0.001) and by 4.0 cm (14%) in SC (P < 0.001). Results showed that semen quality was associated with SC, and that both had responded to heifer puberty selection. It was easier to measure SC as the male puberty trait.

CA, Morris, and NC Amyes

Proceedings of the New Zealand Society of Animal Production, Volume 67, Wanaka, 365-369, 2007