Date of this Version
Selection of dairy cows for economic merit requires the use of a model for genetic merit that involves the cross-product of the genetic values for milk and milk fat percentage. A linear and a quadratic index are described as procedures for selection of cows, the linear index being b1pm + b2pt and the quadratic index being b1pm + b2pt + b2pm2 + b4pt2 + b5pmpt where pm and pt are the phenotypic deviations for milk and milk fat percentage, respectively, and the b's are selection index weights. These indices are compared to one another and to other possible selection procedures in terms of the expected genetic progress in economic merit that would result from their use. Predicted gains are maximum for the quadratic index. Relative to these gains, the gains from the linear index are nearly equivalent, while those from a restricted index and a simplified form of the quadratic index are somewhat less, the extent of the decrease depending on the means for milk and milk fat percentage. Gains in economic merit from selection for milk are only slightly less than those from the quadratic index at all but very high mean levels of milk production. Selection for milk thus appears to be a reasonable selection procedure to improve economic merit, except at high mean levels of milk production. Changes in the parameters of milk production and milk fat percentage and in economic values could affect these comparisons.