Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Animal Science


Birth weight and weaning weight records collected by the American Angus and American Hereford Associations were used to determine sire direct and sire maternal effect variance components. Data from 20 Angus and 25 Hereford herds were selected for study. A total of 25,586 Angus birth weight records resulted from 718 sires and 1,319 maternal grandsires. For Angus weaning weight analyses, 34,190 records were used from 941 sires and 1,576 maternal grand- sires. Hereford data contained 14,436 birth weight records from 566 sires and 1,134 maternal grandsires. For weaning weight analyses, 46,616 Hereford records resulted from 1,366 sires and 2,169 maternal grandsires;Two mixed models were developed that accounted for fixed contemporary group, as well as random sire and/or maternal grandsire effects on the calf's phenotype. Approximate solutions to Henderson's mixed model equations were computed by inverting a 2 x 2 matrix which included the diagonal elements of a bull's direct and maternal equations and the corresponding off-diagonal elements form the coefficient matrix after absorption of fixed effects. Quadratic forms and their expectations were developed from these solutions. Estimates of either direct and maternal effect variances and the covariances (Analysis I) or sire and maternal grandsire variances (Analysis II) were obtained iteratively. Convergence occurred quickly and to the same point despite a wide range of priors tested;For weaning weight, Analysis I yielded direct heritability estimates ((')h(,A)('2)) of .24 for Angus and .18 for Herefords. Maternal weaning weight heritabilities ((')h(,M)('2)) were .18 and .23 for each breed, and the direct-maternal covariance (r(,A(.)M)) was .16 and .25, respectively. Analysis II produced values for (')h(,A)('2) of .40 and .36, and the heritability of total maternal grandsire effect was estimated as .23 and .30;For birth weight, negative sire direct variances were estimated by Analysis I for both breeds. If r(,A(.)M) were restricted to equal zero, the Hereford analysis resulted in a negative estimate for maternal birth weight variance, but iterative estimates of (')h(,A)('2) = .57 and (')h(,M)('2) = .04 were obtained for Angus. Analysis II produced estimates of (')h(,A)('2) = .61 and .58 for sire direct effect, but negative estimates were obtained for (')h(,mgs)('2). Results indicate that almost no genetic maternal variance exists in either breed for birth weight.



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Brad Richard Skaar



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121 pages