Skin folds and Merino breeding. 1. The net reproductive rates of flocks selected for and against skin fold

Abstract

Single character selection flocks-Folds Plus (selection for high fold score) and Folds Minus (selection for low fold score)-were established at Trangie in 1951 from a Peppin Merino flock. The Trangie environment over the period 1951 to 1960 has been described relative to the management of these experimental flocks. An analysis of the vital statistics for the first nine joinings showed a net reproductive rate (the number of 19 month old replacement ewes produced by a ewe in her lifetime) for Folds Plus of 1.248 compared with 2.270 for Folds Minus. This resulted from the Folds Minus flock having a higher percentage of ewes wet and twins mothered, from it having lower mortality rates of young sheep between birth and mothering, between mothering and weaning, and between weaning and 19 months, as well as from it having lower death and casting rates of mature ewes. An analysis of the relative importance of each component of net reproductive rate (N) in contributing to the difference between the Folds flocks (percentage deviation technique) showed :- 1. Twenty eight per cent of the difference could be ascribed to wet ewes (W) and nine per cent to twinning (M) a total of 37 per cent from lambs born per ewe mated (L). 2. Differential survival rate of lambs between birth and 19 months (S) contributed 36 per cent, made up of 17 per cent birth to mothering (Sm), 11 per cent mothering to weaning (Sw), and 8 per cent weaning to 19 months (Sj). 3. The remaining 27 per cent was contributed by the annual wastage of breeding ewes as this affected the number of times a ewe is joined in her lifetime (J). This component was further divided into 16 per cent from death rate (D) and 11 per cent from casting rate (C).