Doctoral Dissertations

Date of Award

6-1972

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Animal Science

Major Professor

Robert R. Shrode

Committee Members

J. B. McLaren, Karl M. Barth, Robert S. Dotson, Don O. Richardson

Abstract

Records from 669 yearling Angus and 871 yearling Hereford bulls from three central test stations and seven Land Grant University or USDA research herds were utilized in a study which was designed to determine needed refinements in procedures for performance testing young bulls. Generalized least-squares multiple regression analyses were conducted to determine the value of measures of fatness and measures of skeletal size in defining or explaining weight and size differences over and above the effects of age and age of dam, which are in common usage. Principal-component analysis also was employed in an effort to assess its useful-ness in evaluating bulls as to size and body shape. The study indicated that weight alone is not a sufficient measure of size and that selection for conventional adjusted yearling weight alone would tend to favor fatter animals and would produce animals of several different skeletal sizes and body shapes. A positive but variable association between fatness measures and final weight was found. Ultrasonically measured fat thickness was found to be more accurate as a measure of fatness per se than was subjective condition score, though both were of similar utility in explaining final weight differences. Condition scores tended to measure fatness on a relative basis and were apparently confounded with size and general body type. Wither height and body length were quite valuable as skeletal-size measures and explained large portions of variance in final weight. Either of these or subjective frame-size scores were valuable in explaining differences in body type and apparent differences in maturity rates. These skeletal-size measures tended to reduce the value of age in explain-ing weight differences. Shoulder width was highly related to weight differences but was influenced greatly by degree of fatness. Although there are apparently basic relationships among weight, skeletal size and fatness, existent individual-animal variations suggest that the accurate measurement of all three of these is essential in classifying bulls accurately as to potential breeding value. Weight and fatness vary on an individual-animal basis depending on maturity rate. Therefore it seems inappropriate to attempt to develop regression procedures based on group statistics for the adjustment of weight to a fat-constant basis. The first principal component was found to be useful in ranking bulls on general size. The second component indicates that two basic extremes in body shape apparently exist among all cattle and is useful in ranking animals between these extremes. It appears that performance testing programs should include mea-sures of fatness and skeletal size in addition to weight and age if potential sires are to be accurately evaluated. This study strongly suggests that post-weaning performance tests for estimation of breeding value should be terminated at a constant physiological age rather than at a constant chronological age. Fatness and skeletal-size measures appear useful in determination of physiological age.

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