Sire Evaluation for Economic Merit

Procedures for selecting sires are compared in terms of making maximum genetic progress in economic lnerit. A quadratic index, a simplified form of the quadratic index and a linear index to evaluate economic merit, and a restricted index to improve milk as much as possible while holding milk fat percentage constant, all result in nearly equal expected genetic progress in economic merit. Selection for milk production results in less expected genetic progress in economic merit, the extent of the decrease depending on the economic vMue of milk production at a given base test and on the test differential. Evaluation of sires for economic merit depends on the economic values and on the means of milk production and test. The simplified form of the quadratic index appears to be a useful index, since sire economic merit index values can be calculated for any mean levels of milk production and test and for any economic values, once index values for milk and test have been found. The economic value of dairy cattle production depends on the amount of milk and the percentage of milk fat under most present pricing systems. Selection for economic merit among cows with one record each has been discussed in an earlier paper (5). The purpose of this paper is to examine sire selection for economic merit by comparing the expected genetic progress in economic merit resulting from the use of different indices, and by examining changes in sire index values at several combinations of levels of milk production and milk fat percentage (test). Experimental Procedures 1. Description of total economic merit. The value of milk production can be expressed as : D = P[v~+ v,(T T~)], Received for publication August 12, 1968. Present address: Department of Animal Science, University of Guelph, Guelph, Ontario, Canada. in which D is dollars received, P is kilograms of milk produced, vl is the value per kilogram of milk at a base test, v2 is the test differential, or the change in the value of each kilogram of milk with each change of 1.0% in test, T is the test of the milk, and Tb is the base test. The genetic value for total economic merit can then be expressed as: M : ( l~ + g~)[a~ + a:(txt + g,)],