Genetic Improvement in Production Attributable to Sires Used in Artificial Insemination in North Carolina

The genetic advantage attributable to the sires used in artificial insemination in North Carelina was evaluated. The effect of season of calving on production was pronounced. Cows freshening during the months of December through May produced 21 lb. of fat and 595 lb. of milk more than cows freshening in June through November. The performance of artificially sired animals was evaluated by using the first-lactation contemporary comparison, and also by using all lactations of the naturally sired progeny as contemporaries. The 305-day, 2 ~ M.E. lactations of the artificially sired progeny were 15.7 lb. of fat and 366 lb. of milk above those of the naturally sired progeny. The use of the records of all contemporaries imposed a negative bias on the comparisons, presunmbly due to the selection of the older animals on the basis of previous performance.