Estimation of (co) variance functions of test day yields in first and later lactations of United States Holstein cows

(Co) variance components for milk, fat, and protein yields during first and second, representing later, lactations were estimated from data for test days from 23,029 Holstein cows from 37 herds in Pennsylvania and Wisconsin. Four lactation stages of 75 d were defined in each lactation, and the test day nearest the center of each interval was used. A total of 9110 observations were available for the final analysis of lactations with test days in all four lactation stages. Data were preadjusted for lactation curves within lactation stages.