Selection of Dairy Cows for Economic Merit

Selection of dairy cows for economie merit requires the use of a model for genetic merit that involves the cross-product of the genetic values for milk and milk fa t percentage. A linear and a quadratic index are described as procedures for selection of cows, the linear index being blp.~ + bzp, and the quadratic index being blp,~ + b~_pt + b~p,n 2 + b~p, ~ + b~p.,pt where p~ and pt are the phenotypie 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 fa t 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. Selection index procedures have long been recognized as valuable in the selection of dairy cattle. The objective of selection has not been, and is not presently, as clearly recognized. The relative importance of type and production has often been discussed in general terms, but specific economic values for type characteristics have not yet been obtained. The value of production itself is not clear-cut, since practically all payments for production depend upon the percentage of one of the constituents, usually milk fat percentage (test). Three courses of action seem feasible in selection for economic merit or dollar value: a) to select for milk Received for publication April 9, 1968. production alone, assuming that this will increase economic merit considerably, b) to select for milk production holding the percentage of some constituent, say fat, constant as suggested by Butcher et al. (1), or c) to select for production and percentage of some constituent according to their contributioa to economic merit. The failure of selection for milk production alone to make maximum gains in economic merit has been suggested in a recent empirical study by Spahr (5), but the relative genetic progress in the economic merit to be expected from these three procedures has not previously been examined. The purposes of this paper are a) to describe the use of an explicit procedure for selection based on milk production and test according to their contribution in determining economic merit, and b) to compare expected genetic progress in economic merit by selection by the explicit procedure, by selection for milk holding test constant, and by selection for milk yield or for milk fa t yield. Experimental Procedures Description of total economic merit. Selection for economic merit depends on the pricing structure for the product sold. In most fluid milk sales, the value of product is given by the expression : D = P[vl -~ v~(T -Tb)]