Approaches to estimating daily yield from single milk testing schemes and use of a.m.-p.m. records in test-day model genetic evaluation in dairy cattle

Statistical models were presented to estimate daily yields from either morn ing or evening test results. The 64,451 test-day records from 10,392 lactat ions of 8800 cows were available for analysis from experiments that were de signed to investigate the accuracy of an alternate morning and evening four -weekly milk-testing scheme. The experiments were conducted in 152 herds fr om six German states and covered a span from 1994 to 1998. Milk yield, fat, and protein percentage were recorded for all of the morning and evening mi lkings. Seven statistical models were fitted to the data to derive formulas for estimating daily yields from morning or evening yields. In general, us e of evening milkings less accurately estimated yields than did use of morn ing milkings. Among the three yield traits the lowest accuracy of estimatio n of daily yield was found for fat yield. Although the models do not differ much in the correlation between estimated and true daily yields, systemati c under- and overestimation of daily yield at the beginning and end of lact ation were observed in all models with the exception of model 6, which acco unted for heterogeneous variances by parity class, milking interval class, and lactation stage by fitting separate regression formulas within each com bination of the three factors. A study to validate the models showed that m odel 6 is also robust for the analyzed populations. Smoothing model 6 regre ssion formulas across lactation stages caused a systematic pattern of estim ation error, although loss in accuracy was minimal by fitting far Fewer par ameters in the regression formulas. Differences in the accuracy of alternat e milking schemes to predict daily yields were found between traits, betwee n morning and evening milkings, and between parity classes. Compared with t rue daily yields from different lactation stages, variances and correlation s of the estimated yields were reduced, which must be accounted for in gene tic evaluation. The use of estimated daily yields from morning or evening m ilkings has a smaller impact on estimated breeding values of bulls than cow s. As a result of lower heritability and repeatability of estimated daily y ields than true daily yields, the weight on own test-day records for estima ting cows' breeding values is lower when cows are in a.m.-p.m. than convent ional monthly testing schemes. However, the difference in the weights betwe en estimated and true daily yields decreases as lactation progresses. Use o f estimated daily yields is less reliable for estimating breeding value tha n use of true daily yields.