Submitted to: American Dairy Science Association Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: July 26, 2000
Publication Date: N/A
Technical Abstract: Impact on repeatability (R) and heritability was examined for adjustment for effects of herd test day prior to use of best prediction to calculate 305-day lactation records from test-day data. The purpose of adjusting for each herd test day is to remove its effect directly before test-day yields are combined to estimate 305-day yield. If herd test-day adjustments account for environmental effects more effectively, the genetic component of lactation records should contribute to higher R and heritability. Estimates of R and heritability were compared with and without adjustment for herd test day. Test-day yields of milk, fat, and protein from U.S. cows that calved after 1989 were analyzed for Ayrshires, Brown Swiss, Guernseys, Jerseys, and Milking Shorthorns; test-day records for Holsteins were limited to cows that calved in California, Pennsylvania, Texas, and Wisconsin. Because R varies with closeness of parity, separate estimates of R were calculated for each parity pair by calculating regression of yield from later parity on yield from earlier parity within herd and calving year. Similar comparisons were made for impact of herd test-day adjustments on heritability by calculating regression of daughter yield on dam yield. Across breeds for all yield traits, R for second parity based on first parity was .53 to .64 regardless of adjustment for herd test day. For Jerseys and Brown Swiss, R was equal or slightly higher (.01) after adjustment for herd test day; for Holsteins, R was .01 higher for all three yield traits. First-parity heritability were equal or higher (less than or equal to .03) for all breeds and yield traits after adjustment for herd test day. Some improvement in accuracy of genetic evaluations will be achieved when lactations are adjusted for effect of herd test day.