Crossbred sheep production II. The repeatability of performance and the scope for culling

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Abstract
There are many flocks in Britain where first-cross ewes are mated with a Down ram in order to produce offspring of both sexes to be fattened for slaughter between 3 and 12 months of age. The ewes are normally bred out of hill ewes by rams of the special crossing breeds, the Border Leicester being the most popular example. Commonly the crossbred flock is maintained by the annual buying in of young sheep at 6 or 18 months old. These replace animals lost through normal wastage and those sold after a set number of lamb crops. In such a situation the flock owner relies on others to produce his crossbred ewes and can himself do nothing to improve their performance through selective breeding. He can only attempt to cull ewes with a poor performance. Any gains from such culling are, of course, temporary and must be maintained by a regular system of culling within each separate intake of females. The success of a culling scheme depends partly upon the extent to which future production is predictable from past performances. This is normally assessed by the correlation between an individual's successive records: the intra-class correlation or repeatability (t). The work reported here estimates the repeatability of various productive traits within two crossbred ewe flocks and considers the scope for culling within such flocks.