Overall Classification as a Selection Criterion for Improving Categorically Scored Components of Type in Holsteins

The efficiency of final classification score as a selective criterion for improving genetically the twelve components of type descriptively classified by HolsteinFriesian Association of America was examined. Comparisons were between selection on final score and direct selection on each descriptive trait, and between selection on final score and selection on several least squares indexes including descriptive traits. Procedures for adjusting phenotypic and genetic correlations between categorically scored descriptive traits for the effects of discontinuity and skewness were determined from theory and were tested by computer simulated data. Final classification score was an efficient selective criterion for improving individual descriptive components of type and several linear combinations of components. Expected correlated responses in descriptive traits to selection on final score ranged from 53 to 124% of the expected direct response. Addit ionally, selection on final score was expected to be 80 to 119% as efficient as index selection for improving genetically seven linear combinations of type traits. INTRODUCTION Recently introduced descriptive or subtrait type classification programs have augmented traditional classifications of final score, general appearance, dairy character, body capacity, and mammary system with categorically coded evaluations for numerous components of type. The Received April 19, 1976. Published with the approval of the Director of the Division of Animal Sciences. 2Director, Program Development and Research, Holstein-Friesian Association of America, Brattleboro, VT 05301. purpose of these programs is to enable dairymen to evaluate specific conformational strengths and weaknesses of their own cows and of daughters sired by bulls through artificial insemination. However, the large number of traits in these programs may increase substantially the difficulties of establishing and maintaining selective criteria consistent with the predetermined goals of a breeding program. The complexity of combining properly numerous evaluations may encourage overemphasis of particularly good or poor performance for a specific trait relative to its heritabili ty, economic value, and relationship to other traits. This is especially critical with regard to the possible erosion of intensity of selection for product ion traits. Addit ionally, opt imum procedures of selection should consider effects of discontinuity on heritabilities of, and genetic and phenotypic correlations among, descriptive type traits. The purpose of this study was to evaluate the efficiency of selecting for overall classification score alone in improving categorically scored components of type after accounting for the discontinuous nature of the latter. MATERIALS AND METHODS Data and Adjustment for Discontinuity Data were from the Descriptive Type Classification Program of the Holstein-Friesian Association of America (6). Cows classified in this program are assigned a descriptive code for each of twelve components of overall type, with the number of codes ranging from three to five. Individuals also are coded as "desirable" or "undesirable" for each component with descriptive codes "1" and "2" being desirable for all components except udder support , for which only code "1" is desirable. Previous work (1) has determined heritabilities of, and genetic and phenotypic correlations among overall classification score (final score) and the twelve descriptive components and has adjusted heri-