Genetic parameters and trends in a selection experiment for increased clean fleece weight involving South African Merinos

Abstract

Summary. Data of 3603 to 7385 progeny of a Merino line selected for an increased clean fleece weight with a limitation on fibre diameter in sires and an unselected control group were used to estimate heritability for liveweight and wool traits. Estimated breeding values were obtained by backsolution and averaged within birth years to obtain genetic trends. Direct additive heritability (± s.e.) estimates were 0.29 ± 0.02 for clean fleece weight, 0.52 ± 0.03 for liveweight, 0.68 ± 0.02 for clean yield, 0.35 ± 0.02 for staple length, 0.63 ± 0.02 for fibre diameter and 0.42 ± 0.03 for total fold score. Fitting maternal additive effects to the model of analysis resulted in an improvement (P≤0.05) in the log likelihood ratio for clean fleece weight and liveweight. These effects were relatively small in magnitude, resulting in maternal additive effects (± s.e.) of 0.04 ± 0.01 for both traits. Genetic and phenotypic correlations of clean fleece weight with the other traits were positive. Genetic trends for fleece traits in the control group were less than 0.1% of the overall phenotypic mean per year. A substantial increase, depicted by a regression (± s.e._b) amounting to 0.14 ± 0.01 kg/year (0.28% of the phenotypic mean) was, however, found for liveweight (r = 0.91). The direct response in clean fleece weight in the selection line amounted to 0.039 ± 0.001 kg/year (r = 0.99), or 0.97% of the overall phenotypic mean. Correlated responses ranged from 0.78% of the overall phenotypic mean (r = 0.97) for liveweight to 0.17% per year (r = 0.51) for clean yield. The genetic change in mean fibre diameter (the trait under observation because an increase was unwanted) amounted to 0.020 ± 0.002 m/year (r = 0.69) or 0.10% of the overall phenotypic mean. The relaxation of the limitation on fibre diameter in sires used during 1986–90 probably contributed to this trend. A substantial increase in clean fleece weight was thus possible while unwanted increases in fibre diameter were limited.