Doctoral Dissertations

Will T. Butts

Date of Award

12-1966

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Animal Science

Major Professor

C. S. Hobbs

Committee Members

Harold J. Smith, James T. Miles, Charles C. Thigpen

Abstract

The generally detrimental effects of inbreeding on performance are of considerable interest to the beef cattle industry, since they are a major part of the price a breeder must pay for the use of inbreeding as a tool. The theoretical benefits possible with various mating systems involving inbreeding are widely accepted. However, actual progress through the employment of such systems has been difficult to demonstrate empirically. A relatively long generation interval and low reproductive rate argue effectively against expecting the same degree of success through hybridization in cattle as that achieved in certain of the plant species. While evidence of heterosis has been found for most performance traits of beef cattle, some doubt remains that this heterosis is a result of non-additive genetic variance. It has been pointed out that evidence of heterosis can be explained on purely additive grounds. A more general problem in the beef cattle industry is the mild inbreeding resulting from a buildup of relationship in a herd for purposes of increased uniformity, fixation of desired characteristics, or inability to locate satisfactory replacement animals outside of the herd. This increase in relationship fundamentally results from the relatively small numbers typical in large animal operations. From a breed standpoint, the increased use of artificial insemination may contribute to this problem in the future by decreasing the effective number of sires. This study was undertaken to estimate the effects of inbreeding on birth weight, average daily gain, type score, and condition score at approximately 120 days of age and at weaning. It was felt that these data offered an excellent opportunity to obtain relatively independent estimates for the effects of inbreeding of calf and inbreeding of dam, since the correlation (0.23) between the two inbreeding coefficients was lower than that usually encountered in closed inbred lines. In addition to the usual fixed environmental effects, an effort was made to consider cow size and the relationship between weaning condition score of the calf and the maternal contribution of the dam. Heritability estimates from these data were also of interest.

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