A simulation study of selection methods to improve mastitis resistance of dairy cows.

Mastitis problems were assumed to decrease profitability of dairy cows through milk price, treatment cost, and involuntary culling cost. Milk price decreased through a stepwise function of SCC (actual French conditions). A continuous latent variate was supposed to trigger other costs through appropriate thresholds. The relative weights for one genetic standard deviation in the selection objective were 1, -.07, and -.14 for yield, SCC, and mastitis liability, respectively. Annual genetic gains were predicted for a conventional breeding scheme using statistical parameters from the literature. Selection on yield and log SCC, with or without mastitis culling rate, increased genetic gains for the overall profitability of .7 or .9%, respectively, compared with selection on yield only. Increases of log SCC and mastitis problems because of selection were substantially reduced (40 to 60%). Consequences from constraint of the genetic trend for mastitis liability to zero depended on the method used to assess mastitis problems. Use of log SCC had a significant and variable negative impact (-8.9 to -36% according to parameters) on overall efficiency compared with the relevant unconstrained selection. Simultaneous use of log SCC and culling rate had a moderate effect (-4.9 to -7.4%) on overall efficiency compared with that from unconstrained selection.