Response to selection in Australian Merino sheep. IV.* Reproduction rate in groups selected for high clean wool weight, with a ceiling on degree of skin wrinkle and either fibre diameter or crimp frequency

Abstract

Reproduction rate (number of lambs weaned per ewe joined) and its components (proportion of ewes lambing, lambs born per ewe lambing, and lamb survival rate) are compared for 2-year-old and adult Australian Peppin Merino ewes in a random control group and two groups under selection since 1950 for high clean wool weight, with limits on high wrinkle score and either high fibre diameter or low crimp frequency. The conclusion is that there has been no correlated response in reproduction rate to selection for high clean wool weight.

The difference between the selected and control groups was expressed as a percentage deviation from the control. These percentage deviations for all four characteristics fluctuated considerably and there was no consistent trend. Mean percentage deviations were calculated for ewes born in an initial period of years, soon after selection began, and a final period, at the end of the time for which data were available. Any difference between such means would indicate a selection response.

For both 2-year-old and adult ewes in both selection groups, means of the percentage deviations in number of lambs weaned were negative and in most cases large during the initial period (–19.4 and –3.4 for 2-year-old and adult ewes in one group, –17.6 and –11.9 in the other), but smaller and sometimes positive in the final period (+ 4.6 and –3 30 in the first group, + 3.3 and –8.7 in the second). The main contributor to the low number of lambs weaned for 2-year-old ewes in the initial period was a low proportion of ewes lambing. For adult ewes, the number of lambs born per ewe lambing (twinning rate) was lower in the final compared with the initial period, but showed no signs of continuing to fall. The fall was compensated by an increase in lamb survival rate in the selected groups compared with the control, which was not due solely to the lower incidence of twins but was present when only single lambs were considered. Possible reasons for a lower incidence of twinning and a higher lamb survival rate are discussed.

The conclusion that reproduction rate has shown no consistent change under selection for high clean wool weight would have been predicted from the zero genetic correlation between the two characteristics previously estimated in the same flock. Comparisons are made with the negative estimates of genetic correlation reported by other authors, and reasons for differences discussed.

Although no negative correlated selection response has been found in these groups, it is recommended that reproduction rate should always be considered when selecting for high clean wool weight in commercial practice.

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*Part III, Aust. J. Agric. Res., 21: 955 (1970)