Abstract

Fourteen herd-years of data, comprising a total of 2169 records, were obtained from beef heifers involved in the Ruakura Genetics Section's long term breeding trials. The contributions of breed-cross, weight selection , herd year, birth date and age of dam, to variation in live weight, proportion exhibiting oestrus and proportion pregnant were studied. For heifers reaching first oestrus by about 15 months of age, means ages and weights at puberty were also calculated. Later-born calves were younger and lighter at puberty than average. Younger dams produced heifer offspring which reached puberty when older but lighter than average. Breed and location differences were greater for proportions of heifers expressing oestrus before joining than for proportions pregnant. At Goudies, the proportion achieving oestrus among Jersey x Angus heifers was 0.72, Friesian-cross 0.61, Hereford x Angus 0.47, Hereford 0.38 and Angus 0.25. Corresponding proportions diagnosed pregnant after 8 weeks of mating were 0.83, 0.93, 0.83, 0.78 and 0.73. Where there were major feed shortages over an extended period between 5 and 15 months of age, live weights and pubertal age were more affected than pubertal weight. In contrast, long term weight selection increased pubertal weight by 11%, while pubertal age was little changed. Preliminary estimates of heritability for pubertal age and weight were 0.31

AH, Kirton, CL Feist, and DM Duganzich

Proceedings of the New Zealand Society of Animal Production, Volume 46, , 59-62, 1986