Genetic trends in South African terminal sire sheep breeds
Genetic trends were constructed for early growth traits in the Dormer and Ile de France terminal sire sheep breeds. The traits that were considered were birth weight, pre-weaning weight, weaning weight and post-weaning weight. However, pre-weaning weights were only available for the Ile de France breed and post-weaning weights were only available for the Dormer breed. Regressions of average annual breeding values on birth year indicated significant genetic gains in all early growth traits during the evaluation period. The average predicted direct breeding value of birth weight decreased by 0.055% per annum whereas weaning weight increased by 0.12% per annum and post-weaning weight improved by 0.32% per annum in the Dormer breed. The Ile de France registered an undesirable increase in the predicted direct breeding value of birth weight which amounted to 0.025% per annum. Predicted direct breeding values for pre-weaning weight increased at an annual rate of 0.23% and that of weaning weight by 1.21% per annum. Although adequate genetic variation for substantial genetic progress was available, only modest rates of progress were attained for all the traits in both breeds. The only possible exception was weaning weight in the Ile de France breed, which was improved at > 1% per annum. At least all changes were in the desired direction, barring the direct genetic trend for birth weight in the Ile de France breed. Breeders should be encouraged to record data consistently, as one of the major shortcomings in the data for both breeds were a lack of continuity in the submission of data to the NSIS. More informative analyses ought to be feasible if this requisite is met.
Keywords: Predicted breeding value, regression coefficient, evaluation, least square mean, significant.