Optimal structure for a cooperative nucleus breeding system.

Abstract

Genetic gains in an established co-operative nucleus breeding system are maximized when the number of nucleus ewes is between 5 and 10 per cent of the total number of flock ewes, and nucleus ewe replacemnts are approximately half nucleus- and half flock-born. The optimum age structure (at a 70 per cent lambing rate) is 2 and 3 ram age-groups in the nucleus and flocks respectively, and 4 and 5 ewe age-groups, respectively. The total number of ewes in the system, and the number of flocks over which they are spread, do not influence genetic gains provided flocks or nucleus size is not small. I.