Effects of Inbreeding on Growth and Conformation of Beef Cattle

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Abstract
Data collected on 860 Hereford calves born in a continuing inbreeding project consisting of 2 mating systems (differing in rate of inbreeding) each with 4 sires were used to study inbreeding effects by within year-line-sire regressions and by direct comparison of the 2 systems. Inbreeding of the calves had reached the 30% level while inbreeding of dam effects had reached the 25% level. Inbreeding of calf and inbreeding of dam effects appeared to be more important on weaning traits than on postweaning traits, although significant linear and quadratic effects of inbreeding were found at both ages. The inbred-noninbred comparisons indicate that for most traits the estimates of inbreeding effects obtained by within subclass estimation of regression coefficients accounted for most of the mating system variation. The major exception was weaning weight. Differential responses by the 2 sexes appeared in several analyses. These results are discussed in relation to possible explanations of the presence of greater heterosis in the homogametic sex.

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