Relationships among clean wool weight and its components. 2. The effect of maternal handicap and its influence on selection.

Abstract

The average difference in clean wool weight between handicapped (twins and the progeny of young ewes) and unhandicapped (singles, the progeny of adults) animals of both sexes, born in the same year, was -0.34 � 0.07 lb (365 days' growth shorn at 16 months of age). 87% of the difference in wool weight arose from a difference in total number of fibres (or follicles) per sheep, the difference in output per follicle being negligible. Body weight, wrinkle score, and follicle population density were all lower in the handicapped animals. The difference in total follicle population arose from a deficiency in secondary but not in primary follicles. If the handicapped animals are not identified at selection, they will suffer a heavier culling rate if selection is on fleece weight, and estimates of differential culling rates are given. The effect on genetic progress and current production is, however, negligible with a handicap of this size. In fact, when the handicapped animals are twins, there are more animals available for selection than there would have been if the births had been single, and the higher selection differential arising from the smaller proportion saved more than offsets any deficiency from the handicap. The main effect of not identifying handicapped animals at selection is that twins will be selected against, with a resultant lowering of the twinning rate. The suggestion is made that management might possibly overcome the handicap, at least in part.