FLOCK STRUCTURE AND GENETIC GAIN IN A SELF-REPLACING

The rate of genetic gain for a specific trait is determined by the age at which sheep are cast-for-age, heritability and the variability of the trait in the flock. In a self-replacing flock that breeds its own rams, it is managerially more convenient to breed rams from a nucleus or elite group of ewes. This reduces the number of entire males present at classing and lessens the performance-recording required. There is a lack of extension-level guidelines to rationalise the conflict between increas-ing selection intensity and reducing generation interval in a breeding flock. There is also a lack of guidelines to determine the optimal nucleus breeding structure. METHODOLOGY A program has been written for a microcomputer (BASIC language) which calculates the response to selection for one character and the steady-state flock age structure in the nucleus and flock given the following inputs: Number of breeding ewes in the flock. Lambing percentage. Deaths until classing. Deaths in flock ewes. Culling rate of older ewes. Age at which ewes are cast-for-age. Age at which rams are cast-for-age Ram percentage. Fraction of flock in the nucleus. Fraction of nucleus ewes born in the base. Trait heritability. Trait variance or range from a subpopulation. Trait average. If required, the program can be used to predict total productivity of a wool-growing flock from these inputs. The pattern of change in reproduction rate with age, the influence on wool production of ewe’s age, type of birth, age of dam, pregnancy, and lactation are all taken into consideration as outlined by Turner et al. (1968). The calculation of genetic gain has been based on the theory outlined by James (1977) for open-nucleus breeding systems. If there is no nucleus t e gain is calculated by standar