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Evaluating Relationships among Immature Measures of Size, Shape and Performance of Beef Bulls. I. Principal Components as Measures of Size and Shape in Young Hereford and Angus Bulls¹

University of Arkansas, Fayetteville 72701 and U.S. Department of Agriculture, Knoxville, Tennessee

Abstract

Nine skeletal measures and body weight were recorded on 267 Hereford (H) and Angus (A) bulls at 4, 8 and 12 months of age. A total of six principal component analyses, one for each breed and age, were obtained. In addition to a better understanding of the dependence structure among skeletal dimensions at a given age, the principal components estimated in these analyses measured differences in size and shape. The first principal component (PC1) at each of the three ages accounted for 56 to 68% of the variation in the 10 measurements and provided a linear function of size with nearly equal emphasis on all 10 standardized traits. The second principal component (PC2) for each of the three ages contrasted tall, narrow bulls with short, wide bodied bulls and this contrast in shape accounted for more than 10% of the variation in the dependence structure of the system of ten variables. Therefore, nearly 75% of the variation in the dependence structure of nine immature skeletal dimensions and body weight was explained either on the basis of overall size or by the shape contrast of short, wide vs. tall, narrow bulls. The consistency and importance of the shape contrast provided by the second principal component at 4, 8 and 12 months suggests that tall, narrow bodied and short statured, wide bodied bulls represent two basic shapes in cattle at preyearling ages. The correlations between PC2 at 4, 8 and 12 months indicated that bulls of these two body shapes remained relatively unchanged over the ages examined. The failure of body length to consistently receive a weighting equal to height or width in the second component contrasting shape suggests there was more phenotypic latitude for length in cattle of different heights and widths than there was in width for cattle of different heights.

The genetic and phenotypic correlations between principal components at different ages indicated that the complex of genes influencing size or shape tended to have a similar effect at the ages of 4, 8 and 12 months. However, the correlations also indicated that, when equal weighting was given to each of the 10 measures of size as in the first principal components, bulls selected at one age on this single composite value could be quite different in shape at later ages. This result pointed out the likelihood that unrestricted selection based on a composite character such as weight, size or gain would produce animals of variable dimensions or shape.

¹ Published with the approval of the Director of the Arkansas Agricultural Experiment Station.

² Department of Animal Sciences, University of Arkansas, Fayetteville.

³ Animal Science Research Division, A.R.S., University of Tennessee, Knoxville.

⁴ This study supported in part by a grant from Agricultural Research Service, U.S. Department of Agriculture.