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Dairy Pipeline: June 2004
-- Bennet G. Cassell
Extension, Dairy Scientist
Genetics and Management
(540) 231-4762
email: bcassell@vt.edu
Genetic evaluations for a trait called "Productive Life" or PL are available to increase length of herd life. PL is a tough trait to improve through sire selection because it has low heritability (8.5%), and a bull's daughters need up to seven years to fully express the trait. Commercial bull studs usually put progeny tested bulls back in active AI service as their daughters approach second calving. That's three years of age, not seven! So how can producers select for longer herd life when newly proven active AI bulls aren't really "proven" for longevity? USDA uses what are called "direct" evaluations of PL based on survival of daughters, regardless of their age. Dead cows receive credit (maybe discredit is the right term if a cow died in first lactation) for months of production, while living cows less than seven years of age receive credit for predicted additional months. PL for living cows is like a partial milk record that is projected to expected yield through 305 days of lactation. USDA combines "direct" evaluations with "indirect" evaluations based on correlated traits - production, type, fitness - that are expressed in young animals. The strongest genetic correlation between PL and another trait for which genetic evaluations are available is for daughter pregnancy rate, which has a genetic correlation of 0.59 with PL. Cows with desirable genes for fertility live longer than genetically less fertile cows. Somatic cell score has a favorable genetic correlation of -0.35 with PL, while udder composite has a genetic correlation of 0.30. The next strongest genetic relationship with PL is daughter calving ease, 0.24, which makes sense considering the detrimental effects of a difficult first birth on production and fertility of a heifer. She may not even survive the process! Genetic progress in PL from selection on correlated traits depends on more than the genetic correlation, however. The heritability of the correlated trait and selection intensity to change it also come into play. Daughter pregnancy rate has the strongest genetic association with PL, but it has low heritability (4%). Udder composite, on the other hand appeals to many producers and is about as heritable, around 30%, as production traits. For producers, the best way to improve PL is to select bulls based on genetic evaluations for PL, because genetic evaluations for PL make the best use possible of all the data available at any point in time on a bull. I have seen some promotional material advocating sire selection for PL using a formula that only includes type information. I just don't understand the argument that producers should ignore the "direct" data, or the indirect information from cell score, fertility, or dystocia. There are challenges to improving herd life if the breeding program is going to include younger AI proven sires, but ignoring useful information won't make the job any easier.