Performance of Hereford and crossbred Hereford cattle in the subtropics of New South Wales: genetic analyses of pre-weaning performance of first-cross calves

摘要

First-cross calves of Simmental (S), Friesian (F), and Brahman (B) sires were compared with straight- nbred calves of Hereford (H) sires over 5 years at Grafton, N.S.W. There was a total of 775 calves nsired by 205 bulls with complete records of performance from conception to weaning. Analyses nwere conducted to determine sire breed and sire breed x environment effects, and to provide paternal nhalf-sib estimates of heritabilities and genetic and phenotypic correlations. Various models were nused to ascertain the sensitivity of genetic parameters to the removal of different sources of variation. nAmong females, S x H were heaviest at birth, while among males, B x H were heaviest. The nregression coefficient of birth weight on gestation length was significantly greater for B x H calves nthan for other crosses. All crosses grew significantly faster to weaning and were significantly heavier nat weaning than H x H calves. However, there were a number of significant interactions between nsire breed and environmental variables for measures of preweaning growth. S x H and F x H ncalves appeared most sensitive to environmental variation, while B x H appeared least sensitive. nThe significance of this is discussed. nEstimates of heritability for most traits were high. Values derived by using simple models were: ngestation length, 0.68; birth weight, 0.56; average daily gain (ADG) to weaning, 0.47; weaning weight, n0.54; eyelid pigmentation, 0.50. Heritabilities of birth weight and ADG during the early preweaning nperiod appeared most sensitive to the amount of environmental variation removed in the model. nGestation length was positively correlated with size at birth but negatively correlated with growth nto weaning. There were large positive genetic correlations among various measures of size at birth, nand among various measures of growth to weaning. When compared with ADG to weaning, weaning nweight was more strongly correlated with birth weight, but less strongly correlated with gestation nlength. The results also indicated that selection for gain over a fixed time period, immediately nprior to weaning, would be as efficient as direct selection for total ADG to weaning.