Response to partial selection on clean fleece weight in South Australian strong-wool Merino sheep

Abstract

This paper reports the results of an experiment carried out at Roseworthy Agricultural College from 1953 to 1966 comparing two bases of selection of rams: (1) visual appraisal (the "visual" method); (2) visual culling of 70% of rams followed by selection of breeding animals from the remaining 30% on the basis of clean fleece weight (the "index" method). No selection for fleece weight was practiced on ewes. Seven rams were used in each of the two flocks, and six of the seven were replaced each year by 16-month-old animals from the succeeding drop, the seventh being a year older.

Rams used for breeding in the Index flock were significantly higher in clean fleece weight than those of the Visual flock, and were higher, though not always significantly so, in greasy fleece weight. This led to a divergence between the flocks for clean fleece weight (P < 0.05), with an increase of c. 0.05 lb per year per head for the Index animals over the Visual animals. The Index rams were more wrinkly and lighter, on the whole, than the Visual rams, and the wrinkly effect carried over into their offspring with a slight divergence apparent, the Index animals becoming more wrinkled than the Visual (P < 0.05). In fibre diameter there were no differences between selected animals or between flocks. Visual rams tended to have more crimps per inch in their wool, and this was observed in the flocks also, but the divergence was not significant (P > 0.05). Index rams had longer wool than Visual rams, with the difference generally significant within years, and this tendency carried over in a lesser manner to the flocks. Fleece density was generally greater in the Index group, though not usually significantly so.

This experiment did not include a randomly bred control flock, but was rather designed to compare two practical methods of selecting rams. Merits of this scheme are discussed in the light of both groups' failure to show increases in fleece weight over the course of the experiment