Doctoral Dissertations

Date of Award

12-1973

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Animal Science

Major Professor

James B. McLaren

Committee Members

Robert R. Shrode, Karl M. Barth, Robert S. Dotson, Don O. Richardson

Abstract

The objectives of this study were (1) to measure the effects of certain environmental factors on growth and body dimensions, (2) to obtain estimates of heritability of these traits, (3) to determine the phenotypic and genetic correlations between traits and (4) to study the potential of principal-component analyses as a means of identifying lambs of different sizes and shapes for selection purposes. Data were obtained from 257 Awassi lambs sired by 13 rams in the Agricultural Experiment Station sheep flock at the University of Mosul, Iraq, during 1966, 1967 and 1970. The traits studied were rate of gain, body weight, selected body measurements and certain principal components at birth, at weaning and at a year of age. A total of 14 traits of each lamb at each age were considered. Environ-mental effects studied were year of birth, sex of lamb, type of birth, age of dam and month of birth. Most of the environmental factors significantly (P < .05) affected all of the dependent variables. However, the effects of age of dam and month of birth varied with age of lamb. Some factors were important at early ages and decreased as lamb age increased. Other environmental factors influenced various traits only during the post weaning period. Constants which reflected the average effect of age of dam, sex, type of birth and month of birth, were calculated and used to adjust the individual records when the effects had a definite (P < .05) impact on the dependent variable. Estimates of heritability were calculated from paternal half-sib correlations calculated from the adjusted data. These estimates at birth, weaning and a year of age, respectively, were: body weight, 0.19, 0.06 and 0.20; body length, 0.16, 0.04 and 0.60; heart girth, 0.13, 0.19 and -.05; wither height, 0.31, -.06 and 0.59; hip width, 0.12, -.08 and 0.82; shoulder width, 0.15, 0.34 and 0.60 and hip width 0.11, 0.16 and 0.35. Most of the estimates of genetic and phenotypic correlations between pairs of traits were high. Some of these estimates approached or exceeded 1.0, indicating that large sampling errors were associated with these estimates. Principal-component analyses were preformed using six body measurements and body weights at birth, weaning and one year of age. The first three components accounted for 87, 89 and 94 percent of the total variation in the correlation structure of all body size measures at the three ages, respectively. The first principal component re-flected general size and the second and third components were measures of shape. However, the shape components tended to contrast different body dimensions at the various ages. Estimates of heritability of the first three principal components at the three different ages were low to medium in magnitude. These results indicate that principal-component indexes could be used to describe size and shape in lambs more efficiently than weights and body dimensions per se. They suggest also a genetic basis for these indexes and that genetic progress would result from a selection program based on the use of principal-component indexes.

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