Differences in morning and evening sample milkings and adjustment to daily weights and percents

Dairy Herd Improvement Association sample-day data were analyzed 1) to explain variation of month-to-month differences of single milking yields and fat percents and 2) to develop adjustment factors for estimating daily yield and fat tests from a single milking. Prior to analyses, single sample data were adjusted for month-to-month differences determined from standard two sample data. Guernsey and Holstein data involving 1,632 and 16,784 cows were available in the single sample files. Analyses were separate for Guernseys and Holsteins and for yield and test. Regression equations involving from 13 to 16 independent variables accounted for proportions of variance of the dependent variable ranging from .31 to .48 for the four combinations of breed and trait. Regression equations for adjustment of Dairy Herd Improvement Association single sample data were developed. Each equation included only two independent variables for adjustment purposes. Those equations developed from Holstein data have been implemented by the Pennsylvania Dairy Herd Improvement Association to adjust single sample data for all breeds.