Factors affecting growth and maternal traits of sheep evaluated using the National Sheep Improvement Program

This project consisted of three studies. The data for each study included birth and weaning weights from six breeds of sheep enrolled in the National Sheep Improvement Program. All analyses were conducted by breed type. The large breed type included the Columbia, Hampshire, and Suffolk breeds and the small breed type included the Dorset, Polypay, and Rambouillet breeds. The data consisted of 13,501 birth weight records, 3,721 30-d, 10,988 60-d, and 3,285 90-d weaning weight records, respectively. The primary objective of the first study was to evaluate the effectiveness of standard age adjustment procedure for each weaning weight. Models included effects for breed, contemporary group, sex by age-of-dam by type-of-birth/rearing (B/R), and linear and quadratic regressions of actual and age adjusted weight, on age at weaning. Results indicated that the standard age adjustment did not effectively remove age effects for the small breed type at 30 days and the large breed type at 60 days. In the second study, multiplicative and additive adjustment factors were estimated for birth weight and 30-d, 60-d, and 90-d weaning weight. Factors were estimated for sex, type-of-birth and rearing (B/R), and age-of-dam. Analyses were conducted, by breed type, using a least squares model which included effects for breed, contemporary group, sex, age-of-dam, and B/R. Subclass means and variances were proportional for sex and B/R with increased weight associated with increased variation for birth weight and each weaning weight. Consequently, multiplicative adjustment factors were recommended. The subclass variances for age-of-dam were not heterogeneous within breed type, consequently, additive adjustment factors were recommended. In the third study, adjustment factors for number-of-lambs-born (NLB) and kilograms-of-lamb-weaned (KLW) were estimated for each breed type, by age-of-ewe. Analyses were conducted, within each breed type, using a least squares model which included effects for breed, contemporary group, and age-of-dam. Age-of-ewe subclass means and variances were heterogeneous for NLB, but, not heterogeneous for KLW at 30-d, 60-d, or 90-d. However, subclass means and variances were proportional in most analyses, therefore, multiplicative adjustment factors were recommended for NLB and KLW