ANALYSIS OF TEST DAY YIELD DATA OF COSTA-RICAN DAIRY-CATTLE

Estimates of variance components for test day records in an animal mod el that considered multiple traits over multiple lactations were calcu lated using REML methodology. Test day records were classified into II periods within first and later lactations. Missing ancestors in the r elationship matrix were classified in genetic groups. Data were collec ted from Costa Rican dairy farms. Estimates of components for total an d additive genetic variance were clearly heterogeneous during the lact ation. Heritabilities for traits in later parities were slightly highe r than those for traits in first parity. Heritabilities were highest f or records of midlactation. Phenotypic and genetic correlations for ad jacent test days were close to 1. Phenotypic correlations were lower t han genetic correlations. Heterogeneity of variances during the lactat ion suggests the adequacy of the multiple-trait test day model to desc ribe milk yield during the lactation. When missing ancestors were allo cated to a single base population instead of genetic groups, the estim ates df residual variance were lower, and the estimates of genetic var iance and genetic correlations were higher. When standardized records were used instead of actual test day records, the estimates of residua l and total variance were lower, and the estimates of genetic variance were higher. Consequently, estimates of heritability and genetic corr elations were also higher. Use of standardized data obtained by interp olation procedures is not advised for estimation of genetic variance c omponents in a test day model.