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. Mil k price decreased through a stepwise function of SCC (actual French co nditions). A continuous latent variate was supposed to trigger other c osts through appropriate thresholds. The relative weights for one gene tic standard deviation in the selection objective were 1, -.07, and -. 14 for yield, SCC, acid mastitis liability, respectively. Annual genet ic gains were predicted for a conventional breeding scheme using stati stical parameters from the literature. Selection on yield and log SCC, with or without mastitis culling rate, increased genetic gains for th e overall profitability of .7 or .9%, respectively, compared with sele ction on yield only. Increases of log SCC and mastitis problems becaus e of selection were substantially reduced (40 to 60%). Consequences fr om constraint of the genetic trend for mastitis liability to zero depe nded on the method used to assess mastitis problems. Use of log SCC ha d a significant and variable negative impact (-8.9 to -36% according t o parameters) on overall efficiency compared with the relevant unconst rained selection. Simultaneous use of log SCC and culling rate had a m oderate effect (-4.9 to -7.4%) on overall efficiency compared with tha t from unconstrained selection.