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Trends in Estimated Transmitting Ability from Sire Evaluations Based on Different Lactations

Virginia Polytechnic Institute and State University, Blacksburg 24061

H. D. Norman

Animal Improvement Programs Laboratory, Agricultural Research Service, US Department of Agriculture, Beltsville, MD 20705

ABSTRACT

Most recently calculated sire evaluations through the January 1984 Modified Contemporary Comparison for first, all, and later lactations were available on 770 Ayrshire, 3188 Guernsey, 29,665 Holstein, 3551 Jersey, and 988 Brown Swiss bulls. A subset of 6006 Holsteins entering artificial insemination service before 49 mo of age was also examined. Averages for evaluations from first, all, and later lactations by year of birth of sire were studied. Regression of evaluation on year of birth showed two distinct slopes for each breed, with slopes changing in 1968 for Jerseys, 1969 for Holsteins, 1971 for artificial insemination Holsteins, 1972 for Ayrshires and Guernseys, and 1974 for Brown Swiss. Annual change was greater in more recent years for each group of lactations and ranged from 25 kg/yr for later lactations in Ayrshires to 55 kg/yr for later lactations in all Holsteins. Linear contrasts showed significantly greater annual change in transmitting ability for later than first lactations in both groups of Holsteins, while first lactation showed greater change in Ayrshires. Differences in slopes for first vs. all, first vs. later, or all vs. later lactations were not significant in other breeds.