Genetic and economic responses to breeding programs that consider mastitis.

Eight breeding programs were evaluated with a simulation study using discounted gene flow. Discounted economic responses and annual genetic changes were calculated. The evaluation was over 25 periods of 13 mo and the population modeled was DHI cows served by AI sires in the US. A breeding program without regard for mastitis gave annual genetic increases of 53.5 kg milk, 2.24 kg fat, .020 cases of mastitis, and an economic response of $98.24. Including SCC or mastitis in selection indexes that maximize economic gain reduced the rate of improvement in yield traits by 1 to 2% but did not reduce clinical mastitis. Instead these indexes slowed the rate of increase in clinical mastitis by 20 to 25% and increased response in economic merit by less than 1%. Restricted selection, which permitted no genetic change in mastitis, reduced the rates of improvement by up to 27% for yield and 17% for economic response. Direct selection against mastitis is unrealistic due to lack of field data on clinical mastitis. Indirect selection would be less efficient than predicted here, due in part to incomplete enrollment in SCC testing program. Simultaneous improvement of both yield and mastitis apparently is not economically desirable due to the antagonistic genetic correlation between yield and mastitis and to the relatively greater economic value and higher heritability of yield.