Submitted to: California Dairy
Publication Type: Popular Publication
Publication Acceptance Date: December 15, 2006
Publication Date: December 31, 2006
Citation: Van Raden, P.M., Dematwewa, M., Pearson, R.E., Tooker, M.E. 2006. Productive Life Credits for Cows Revised. California Dairy. 15(9):18.
An economic definition of productive life (PL) was introduced to replace the previous definition used since 1994. Cows now get credit for continuing in milk after 305 days of lactation and after 84 months of age. Credits now are based on standard lactation curves, with highest credits at the peak of lactation and diminishing credits across the remainder of lactation. The standard is set such that a second-lactation cow with 305 days in milk gets credit for 10 months. First lactations get less credit and later lactations slightly more credit in proportion to average production. The standard deviation of predicted transmitting ability (PTA) for revised PL is 1.4 times as large as the standard deviation for previous PTA PL because of the credits for longer lactations and additional life after 84 months of age. A conversion formula can be obtained by adjusting for changes in correlations of PL with daughter pregnancy rate (DPR) and somatic cell score (SCC): revised PTA PL = 1.4(previous PTA PL) - 0.11(PTA DPR) - 0.29(PTA SCC - 3.00). With the new definition of PL, some emphasis is shifted away from fertility toward SCC because longer lactations require more mastitis resistance and the correlations of PL with yield traits are slightly higher. Inbreeding adjustments used for yield, DPR, and SCC evaluations since February 2005 are now used for PL evaluations. As compared with previous PTA PL, revised PTA PL decreases by 0.12 for each percentage of expected future inbreeding and increases by 0.05 for each percentage of past daughter inbreeding. Estimated genetic trend decreased by 25% as compared with the trend without inbreeding adjustment. However, trend with the new model is higher because of the larger units. The factors used to predict future longevity for cows still alive also were updated. Heritability of PL was decreased slightly to 8%. The reliability of PTA PL also will decline slightly for recent bulls because the end point for predictions is later. Correlation of revised with previous PTA PL is 0.99 for high-reliability bulls, 0.97 for recent artificial-insemination bulls, and 0.96 for recent cows. Average increase in genetic correlation with the other 18 countries that evaluate Holstein longevity was 0.03 (an increase from 0.73 to 0.76). The United States has the highest average genetic correlation across countries for both Holstein and Jersey breeds, which indicates that the new PL definition is an accurate summary of U.S. data and an excellent predictor of longevity in other environments.