Date of Award
Master of Science
Haley Jamison, Don Richardson
The data used in this study were the Tennessee Beef Cattle Improvement Program weaning records of 18,393 Angus and Hereford calves accumulated over the nine year period, 1964 through 1972. The calves were classified according to weaning age (within the range of 120 to 300 days inclusive), sex (bulls, heifers, steers), age of dam (by years from two to 10 years inclusive and 11 years and over), month of birth, management (creep or non-creep fed), year and breed in preliminary analyses. The purpose of this study was to determine the effect of various methods of adjusting weaning weight for environmental variation and various methods of calculating genetic parameters on the magnitude of these estimates.
Nine combinations of four methods of adjusting weaning weight to an age-constant basis and four methods of adjusting age-constant weights fixed environmental effects were used to generate nine sets of adjusted weights. A nested analysis of variance procedure was used to analyze the nine sets of adjusted 205-day weights and weaning type score and to produce variance and covariance components to be used in estimating heritability parameters for the traits in question. It was determined by Barlett's Test for Homogeneity of Variances that the records of Angus and Hereford calves could be combined in a single analysis. However, the residual mean squares of creep and non-creep fed calves were heterogeneous and in all final analyses the two management groups were analyzed separately. Sire and herd were found to significantly effect (P < .01) both weaning weight and weaning type score. The effect of herd had a tremendously pronounced effect upon both traits regardless of the management group (creep or non-creep fed).
Estimates of heritability and genetic correlations were calculated from components of variance by two different methods—inter-herd and intra-herd. The intra-herd estimates were calculated by the standard paternal half-sib methods. The inter-herd estimate was calculated by adding the component of variance of herd within year to the denominator of the standard paternal half-sib formulas for calculating heritability and genetic correlation.
Estimates of heritability calculated by the inter-herd method were similar to those reported in other studies when the estimates were calculated from commercial herds. The mean estimate of heritability for adjusted 205-day weight (average of nine methods of adjusting) was .424 and the estimate for weaning type score was .401 in non-creep calves. The estimates were .343 and .293 respectively, in creep-fed calves. Estimates in both management groups tended to be higher than those reported for experimental herds. The estimates calculated on an inter-herd basis tended to be closer to the estimates reported for these traits which were determined in experimental herds. The estimates for 205-day weight (average value) and weaning type score were .288 and .298, respectively, in non-creep calves and .177 and .180, respectively, in creep-fed calves.
Genetic correlations between weaning type score and adjusted 205-day weight were positive and fairly large in magnitude. The inter-herd method tended to increase the genetic correlations between different methods of correcting weaning weight in both the non-creep and the creep-fed calves. However, the intra-herd correlations between the traits (weaning type score and adjusted 205-day weight) were higher for non-creep-fed calves than were the inter-herd correlations.
Comparison of these results with estimates derived in experimental herds suggests that the component of variance for herd in a nested analysis of variance contains both genetic and environmental variation. The true estimates of the genetic parameters probably lie somewhere between the intra-herd and the inter-herd estimates calculated in this study.
Brown, Dan T., "Estimating genetic parameters in commercial beef cattle populations. " Master's Thesis, University of Tennessee, 1975.