Genetic evaluation of US dairy cattle in the presence of preferential treatment

Abstract

Data were simulated according to the USD A animal model to detennine potential biases in female PTA caused by preferential treatment Ten scenarios for preferential treatment were investigatedScenarios were defined according to three Actors; number of records for the cows receiving preferential treatment, whether all or rally secaid and later records received preferential treatment, and whether preferential treatment was added only to records of cows selected for preferential treatment or to selected cows and their relatives (dam or Ham and maternal sibs). Three levels of preferential treatment were studied. Each level was studied separately to allow straightforward determination of whether bias inaeased linearly as level of preferential treatment increased. Within each scenario, bias increased linearly as level of preferential treatment increased, but magnitude of bias varied across scenarios. As a prqxitiCHi of preferential treatment effect, bias m PTA ranged from .06 to .39. Affording preferential treatment to relatives increased bias more than inaeasing the number of lactations with preferential treatment INTRODUCnON Preferential treatment (FT) can be described as any management practice that increases production and is applied to one or several cows, but not to their contemporaries. Some of these practices might be separate housing, better or more feed, greater number of days open relieve to ccmtempararies, or longer milking intervals on test day for the cow receiving preferential treatment Of course, some PT may occur madvertently. A standard management practice, for example, is to teed cows according to their level of producdon. Presumably, the primary modvadon for intentional PT, however, would be to enhance the likelihocd that a cow will be chosen as a bull-dam or sold at an increased price. Pqjular (pinion is that intendonal PT occurs among U.S. dairy catde. Histoically, a primary argument tor the existence of PT has been that the cow index of bull-dams Mled to predia SOTS' PD as accurately as theory dictated (12). Several studies (6,10,13) found that the cow mdex of bull-dams predicted soi PD better when the cow index was based on first records only than when it was based on second and later records or when it was computed from all records. The typical conclusion from this result was that PT was practiced in second and later parities and was prompted by an outstanding first record. Journal Paper Number J-15545 of the Iowa Agriculture and Home Economics Experiment Station, Ames. Prqjea Number 3141.