Doctoral Dissertations

Date of Award

5-1975

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Animal Science

Major Professor

Robert R. Shrode

Committee Members

J. B. McLaren, C. C. Chamberlain, W. W. Overcast, C. C. Melton

Abstract

The objective of this study was to assess the relative importance of sire differences, sex differences and sire by sex interaction influence on variation among individuals in various recorded variables and to estimate the heritability of these variables. Data were obtained from 858 Angus calves sired by 85 sires over a period of 6 years (1968-1973) from the purebred Angus herd at the Plateau Experiment Station of The University of Tennessee. The traits recorded on each animal at weaning (at an average age of 231 days) and after post-weaning feeding (at an average age of 385 days) were Body Weight, Type Score, Condition Score, Average Daily Gain, Heart Girth, Body Length and Hip Width. Lifetime (to a year of age) average daily gain also was calculated. Restricted feeding of heifer calves and full feeding of bull calves were followed during the post-weaning period. Heifers were restricted to prevent the excessive fattening which can occur on a high plane of feeding from jeopardizing their future reproductive and milking abilities. Nested within-year analyses were performed on these data. Least-square estimates of the effects of sex, sire (within year) and sire-sex (within year) interaction on the various traits were obtained. Results showed highly significant sire effects on all the traits studied. Sex effects also were highly significant (P < .01) with respect to all traits but two. Type Score measured at both stages and Hip Width at weaning. Sire by sex interaction effects were non-significant in all traits except for Type and Condition Scores measured at weaning and Average Daily Gain at post-weaning. These interactions, however, were probably due to sampling because of a small number of sires or a small number of progeny per sire. Bartlett's test of homogeneity of variance was conducted in order to justify statistically the pooling of the data over the six-year period. Results of the test indicate homogeneity of variances of all traits except one, Condition Score at post-weaning. Level of significance of differences in variances between sexes was tested by a simple F-test, using variance components of pooled data for each sex. This test showed highly significant (P < .01) sex differences with respect to variances due to differences between offspring of the same sire (s²W̳). However, some of the sex differences in variances due to sire differences (s²S̳) were not significantly (P > .05) different. All of the sex differences in total variance (s²) were highly significant (P < .01). Heritability estimates were calculated for each sex separately, at weaning and at a year of age, using the method of half-sib correlations. The results indicate higher heritability estimates in heifers than in bulls which suggests that heifer progeny provide more effective genetic discrimination between sires than would bull progeny at early ages of the calves and that selection among females may be more nearly as effective as that among males than is usually believed, the higher heritability in females offsetting, to some extent, the lower intensity of selection among females as compared to that among males. In general, data from heifers yielded higher heritability estimates, especially at weaning. These differences, however, were smaller at post-weaning and even reversed in the case of body dimension traits for which post-weaning data from bulls yielded higher estimates of heritability than did post-weaning data from heifers.

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