Submitted to: Journal of Dairy Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/9/2006
Publication Date: 8/1/2006
Citation: Van Raden, P.M., Dematawewa, C., Pearson, R.E., Tooker, M.E. 2006. Productive life including all lactations and longer lactations with diminishing credits. Journal of Dairy Science. 89(8):3213-3220. Interpretive Summary: The length of life a cow remains producing milk has a direct impact on dairy herd profit. However, the national average productive life of a cow has been shortening gradually. The present national genetic improvement procedure for productive life uses the lifetime total days in milk, not counting days beyond 305 per lactation – a disadvantage to cows with long lactations. This study redefines productive life by including long lactations giving lesser credit for days beyond 305, based on their diminishing yield. The diminishing credit approach should produce higher genetic improvement than assigning equal credits to all days.
Technical Abstract: Alternative measures of productive life (PL) were compared and life expectancy factors were updated to replace estimates from 1993. Alternatives were proposed with extra credits for lactations longer than 10 mo and beyond 84 mo of age, and for each calf produced so that an extremely long lactation would not receive more credit than multiple shorter lactations with dry periods between. Maximum credits per lactation of 10 mo (original PL), 12 mo, and unlimited were compared. Unlimited credits option either included or excluded a calf value equal to 2 mo of production, and had credits given for all days either uniformly or based on lactation curves (diminishing credits). Standard lactation curves (first, second, and greater lactations) were estimated based on the test-day yields of Holstein cows remaining in lactation, from a set of 903,579 lactation records. For the diminishing credits alternative, credit for a given day of a parity was derived using the predicted yield of the day proportional to the average daily yield of the first 305 d of second parity, daily yields being deviations from a baseline (=13.62 kg). Heritabilities and genetic correlations were estimated by multi-trait REML for alternative measurers of PL, longevity censored at various ages, and for yield traits and somatic cell score in first parity. Data for REML analysis included records from 1,098,329 Holsteins born from 1994 through 1997 from 5,109 sires, and a relationship matrix among sires was included in the model. Lactations beyond 84 mo added little information. Heritability of PL was 0.073 with 10 mo, 0.069 with 12 mo, 0.068 and 0.067 with unlimited (uniform) lactation credits (with and without calf credits, respectively), and 0.070 with unlimited diminishing credits. Corresponding correlations among predicted transmitting abilities for PL and protein yield were 0.07, 0.06, 0.12, 0.23, and 0.09, all much lower than the 0.46 estimated in 1993. Heritability of PL with diminishing credits improved from 0.017 to 0.070 when censoring age increased from 36 mo to 96 mo. There was no further increase in heritability beyond 96 mo. Genetic correlation with the final PL was 0.87 when PL was censored at 36 mo, but the estimate increased steadily with the censoring age. The PL with diminishing credits, favorable in both economic and genetic aspects, was desirable in crediting cows for complete lactations.