Genetic and Environmental Factors in Dairy Sire Evaluation. I. Effects of Herds, Months, and Year-Seasons on Variance Among Lactation Records; Repeatability and Heritability

More than 39,000 Iowa Dairy Herd Improvement Association (DHIA) and Dairy Herd Improvement Registry (DHIR) Holstein lactation records provided the basic data for checking some of the parameters and assumptions currently used in dairy sire evaluation procedures in the United States. The components of' variance for herds were about 32% of the total variance with both milk and milk fat records but only 17% with test. The components for months were 1.4 and 1.7% of the total variance with milk and milk fat records and only slightly higher for year-seasons. However, the interactions of herds with months and year-seasons contributed 3 to 5% of the total variance.