Impact of different numbers of milk test-day records during lactation on the reliability of estimated breeding values

Abstract The objective of this study was to evaluate the impact of the reduction in the number of test-day records per lactation on genetic parameters of test-day milk yield (TDMY) and the reliability of estimated breeding values for 305-day milk yield in Holstein cattle. Through the random elimination of milk test-days over the lactation period, seven subpopulations were formed, which differed in number of test-day records/lactation/cow, so that all cows in each subpopulation necessarily contained four to ten records per lactation. Estimates of genetic parameters and breeding values were performed using the animal model of random regression and adjustment of the Legendre polynomial (fourth order). When comparing sires with the same number of daughters, greater reliability was found in the subpopulations with the highest number of milk test-day records/lactation/cow. It was also found that the elimination of at least one test-day affected the reliability of estimated breeding value for 305-day milk yield in the sires, regardless of the class of number of daughters/sires. When selecting the 5% best sires and 20% best cows, the lowest order correlations were observed between the population with 10 test-day records/lactation (complete lactation) and the other subpopulations with incomplete lactations (4, 5, 6, 7, 8, 9 test-day records/lactation). The reduction in the number of milk test-day records per lactation interferes in the reliability of the estimated breeding value for sires and cows, negatively impacting the precision of selecting genetically superior animals.