Selection schemes for improving both reproduction rate and clean wool weight in the Australian Merino under field conditions

Abstract

Theoretical consideration has been given to 28 selection schemes which might readily be applied for the improvement of both number of lambs born and clean wool weight per head in Australian Merino sheep under field conditions. Two sets of relative weights were used, one lamb being considered equivalent to 5 or 10 lb of clean wool, and when both characters were selected in one sex, equal selection pressure was applied to each. Quality of wool is assumed to be safeguarded by preliminary culling of off-types.

The schemes were investigated at two levels of twinning (15 and 30%). At both levels, the optimal method of ram selection was to select on clean wool weight among twin-born rams. At the lower level, the optimal method of ewe selection was to save twin-born ewes and to make up numbers by selecting on clean wool weight among those born as singles. At the higher level, this scheme was slightly exceeded in total genetic gain by one involving selection of ewes on their own 3-year-old reproduction rate.

After 10 years of selection under the optimal scheme, an increase of about 40 lambs born per 100 ewes joined would be expected in reproduction rate, and about 1.2 lb clean wool in the cut per head. The gain in wool weight will be less (about 1 lb) if more rigorous culling on crimp is practiced, but the relative efficiency of the schemes near the optimum will not be changed.

The tables presented could be used in flocks of any breed in which clean wool weight and reproduction rate were uncorrelated and to which approximately the same estimates of heritability, variances, and relative weights of lamb and wool could be applied.