Deriving Lactation Yields from Test-Day Yields Adjusted for Lactation Stage, Age, Pregnancy, and Herd Test Date

Lactation records for milk, fat, and protein yields were calculated from test-day data adjusted for the effects of lactation stage, age, previous days open, days pregnant, and test-day class (herd, test date, and milking frequency). Those lactation records reflect the improved accounting of environmental effects from a test-day model and can be combined with historical lactation records. Test-day data were adjusted with existing lactation multiplicative adjustments to maintain variance characteristics. Then additive adjustments for lactation stage, age, previous days open, and days pregnant were applied. The current multiplicative adjustments for previous days open were not applied because its effect was expected to differ by lactation stage. To remove genetic differences, the estimated breeding value from the previous evaluation divided by 305 was subtracted. Effects of test-day class, and permanent environment within and across parities were estimated within herd. The effect of test-day class was subtracted from adjusted test-day yield, and the breeding value restored. Those deviations then were combined with the best prediction procedure into a lactation measure. Heritabilities and repeatabilities of lactation records that were adjusted for test-day class were higher than for current lactation records. The adjusted records should improve the accuracy of evaluations and allow the use of test-day data as well as provide for the continued use of historical data when test-day data are not available. (