Method R estimates of heritability for milk, fat, and protein yields of United States dairy cattle

Heritabilities for milk, fat, and protein yields were estimated from first lactation data used for USDA-Dairy Herd Improvement Association (DHIA) gene tic evaluations. Contemporary group assignments and standard deviations wit hin herd-year were determined with the procedure used for national evaluati ons. Pedigree data were included for animals born since 1970; yield data we re included for cows born since 1980. Lactation records were divided into f our mutually exclusive data sets based on standard deviations. Ranges for s tandard deviations were chosen so that data sets were approximately equal i n size. Method R was used to estimate heritability with 25 different random samples of half of the data for each data set. Because of the large number of Holstein observations, estimates of heritability for Holsteins were bas ed on random subsets of the complete data file; each subset included approx imately 5% of the data. Mean heritability estimates increased with standard deviations, and estimates ranged from 0.18 to 0.51 across breeds. Repeatab ility estimates for milk yield of Holsteins were approximately 0.50 and did not change with standard deviation. These heritability estimates were high er than those previously used in the USDA-DHIA genetic evaluation. Heritabi lity used in the USDA-DHIA genetic evaluation have been increased based on these results.