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Effects of Inbreeding on Growth and Conformation of Beef Cattle¹

South Dakota Agricultural Experiment Station, Brookings

Abstract

Data collected on 860 purebred Hereford calves born in a continuing inbreeding project initiated in 1952 were used to evaluate the effects of inbreeding on growth and conformation and to evaluate present methods of adjusting for inbreeding effects. Two mating systems (MS) each with four sires were established. In one, mating was restricted to four single sire lines, and in the other, matings were restricted to a relationship less than half-sib. The two systems were formed from the same foundation animals by mating related animals for the foundation of the inbred system and by mating the same animals in unrelated pairs for foundation of the noninbred system. Inbreeding of the calves has reached the 30% level while inbreeding of the dams has reached the 25% level.

Inbreeding effects were studied first by the use of within year-line-sire subclass regressions of trait on inbreeding. Inbreeding of calf and inbreeding of dam effects appeared to be more important on weaning traits than on post-weaning 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 between MS variation. The major exception was weaning weight.

Differential responses by the two sexes appeared in several of the different analyses. These results are discussed in relation to possible explanations of the presence of greater heterosis in the homogametic sex.

From a practical standpoint the results indicate that breeders could close their herd and use as few as four initially unrelated bulls without suffering severe inbreeding effects. Even in the one sire lines, inbreeding effects which were severe at weaning for the female had partially declined by the yearling age. In breeding appeared to affect conformation even less than growth. Therefore, the mating system should be useful in purebred herds where it holds other advantages and where the herd is of sufficiently high merit to justify its use.

¹ Published with the Approval of the Director of the South Dakota Agricultural Experiment Station as Publication No. 730 of the Journal Series. In Cooperation with the Agricultural Research Service, U.S.D.A., and Regional Project Nc-1, Improvement of Beef Cattle through Breeding Methods.