BLACKSBURG, Va. – Genetic evaluations for a trait called “productive life” or PL are available to increase length of herd life.
Productive life is a tough trait to improve through sire selection because it has low heritability (8.5 percent), 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 artificial insemination (AI) service as their daughters approach second calving. That’s 3 years of age, not 7!
Direct evaluations. 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 productive life 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 7 receive credit for predicted additional months.
Productive life for living cows is like a partial milk record that is projected to expected yield through 305 days of lactation.
Combination. 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 productive life and another trait for which genetic evaluations are available is for daughter pregnancy rate, which has a genetic correlation of 0.59 with productive life.
Genetic correlation. 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 productive life, while udder composite has a genetic correlation of 0.30.
Calving ease. The next strongest genetic relationship with productive life 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 productive life from selection on correlated traits depends on more than the genetic correlation, however.
Heritability. 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 productive life, but it has low heritability (4 percent).
Udder composite, on the other hand appeals to many producers and is about as heritable, around 30 percent, as production traits.
For producers, the best way to improve productive life is to select bulls based on genetic evaluations for productive life, because genetic evaluations for productive life 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 productive life using a formula that only includes type information.
Ignoring info. 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.
(The author is a dairy scientist in genetics and management with Virginia Polytechnic Institute and State University.)
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