This study was undertaken to investigate the genetic stability of an unselected Control Group of South African Merino sheep and genetic change in a flock selected for increased clean fleece mass (Selection Group) under the same environmental conditions. Data regarding 14-17 months clean fleece mass (CFM) and fibre diameter (FD) of 5 867 progeny from these groups (3186 and 2681 individuals in the Selection and Control Groups respectively) were analysed to investigate genetic change over the period 1969-1989. A smaller data set, involving 5273 progeny (2782 and 2491 individuals in the respective groups) born in 1971-1989, was used to investigate change in 16-17 month live mass (LM). An animal model was used to obtain predicted breeding values (PBVs) for all individuals by mixed model analysis (MMA). Average PBVs of Selection and Control group progeny within birth years were taken as a measure of genetic change, rendering genetic trends  independent from environmental bias. Genetic change in the Selection Group was also obtained by deviating average PBVs from those obtained in the Control Group. This approach is analogous to the method of expressing genetic change as least squares deviations of Selection Group progeny from Control Group contemporaries, and will be affected by genetic change in the Control Group. Prior heritability estimates, derived for the MMA by paternal halfsib procedures, were within ranges reported in the literature. Year-to-year variation, as derived from environmental trends from the MMA, appeared to be less for FD when compared to LM and CFM. Average PBVs for Control Group progeny increased (P ≤0.05) with time. When expressed in relation to the overall environmental mean, these genetic trends ranged from 0.09 (R² = 0.62) % p.a. in the case of FD to 0.25 (R² = 0.72) % p.a. in the case of LM. Owing to the slight genetic drift in CFM in the Control Group, the genetic trend derived from PBVs for Selection Group individuals alone (1.11% p.a.), differed (P ≤ 0.05) from that obtained by deviating average PBVs of the Selection Group from those in the Control Group (0.99% p.a.).  Similar results were obtained for LM; the genetic trend based on average PBVs within birth years amounting to 0.86% p.a. in comparison with 0.63% p.a. when deviations from the Control Group were used (P ≤ 0.01). The genetic drift in FD in the Control Group led to a marked underestimation (P  ≤ 0.01) of genetic change derived by deviating average PBVs in Selection Group progeny from those in the Control Group (0.056% p.a.; R² = 0.16) compared to the trend based on average PBVs of the Selection Group only (0.15% p.a.; R² = 0.68). While the underestimation of genetic change in the Selection Group may not be too serious in the case of CFM, it is important to note that the trend in FD would have remained undetected if not for MMA. Selection experiments should be interpreted with caution in the absence of family relationships enabling analysts to separate genetic and environmental effects using mixed model methods.

Keywords:  Clean fleece mass, control group, fibre diameter, genetic trend, live mass, selection