Optimum Number of Bull Sires In Dairy Cattle Breeding

Abstract

In future efficient progeny testing systems based on selection for estimated breeding value, the selected elite sires and their daughters will be used to breed the next generation of bulls. The bull breeding part of the population is then like a dispersed nucleus breeding group and largely determines the genetic response rate in the whole population. The inbreeding rate per generation is approximately 1/(8Me), where Me is the effective number of bull sires per generation; inbreeding rate is four times that in a conventional four-path progeny testing system. Inbreeding leads to reduction in genetic variance and to inbreeding depression. Methods are given to estimate the effective number of bull sires per generation to maximize the net response in economic merit, the response adjusted for these effects of inbreeding. The optimal number increases with increased inbreeding depression and number of generations considered. The optimum is flat, especially in the use of more sires, and high proportional responses are obtained for a range of two to three times the optimal number. The flexibility afforded by the range may be restricted in practice, for AI breeding industry groups have to maximize their short-term competitive position. For progeny testing systems, a conservative recommendation of a minimum effective number of 10 bull sires per generation for the whole breed is made, as this will allow both high genetic and high net genetic responses and will allay concern about inbreeding.