Genetic studies of South Australian Merino sheep. I. Genetic change in fleece and body traits

Abstract

During the years 1953-67 one flock of South Australian Merinos was selected on clean fleece weight (the Index flock) and another on the basis of visual appraisal (the Visual flock). A large number of qualitative and quantitative traits were recorded. Linear regressions of differences between flock means on time were estimated for each trait; not many of the regressions were significant. However, greasy and clean fleece weights of Index animals increased significantly faster than those of Visual animals, and body weights of Visual animals increased significantly faster than those of Index animals. Realized selection differentials were estimated for all traits. There were significant selection differentials for greasy and clean fleece weights and yield in the Index flock, and for body weight and greasy fleece weight in the Visual flock. The response to selection was estimated for all traits by using two methods; regression of the trait on an individual's generation number, and comparison of the deviation from the overall mean of the mean of the progeny of a 3+-year-old sire with the deviation of the progeny of the same sire as a 2+-year-old. The estimate of genetic change in body weight was higher and genetic changes in clean and greasy fleece weights lower in Index animals, which was in contrast with an earlier result. However, when the least-squares means of these traits were compared, it was concluded that the Index flock did, in fact, achieve significantly greater responses in greasy and clean fleece weights and a significantly lower response in body weight than the Visual flock. It was therefore considered that, in these particular experiments, both methods of estimating genetic change had serious limitations.