Understanding productive life.
Dairy Pipeline: October 1996
Extension Dairy Scientist, Genetics and Management
Most producers want to improve longevity in their herds. USDA began calculation of genetic evaluations for productive life a couple of years ago to help farmers do a better job of selecting for longevity. Some farmers are not aware that such evaluations exist. Others don't understand how they are calculated, especially on bulls just out of sampling programs. The trait evaluated by USDA is "months in milk by 84 months of age" with a maximum of 10 months in milk each lactation. The average Holstein cow would have about 27 months in milk in her lifetime. Productive life cannot be measured on cows still living until they reach their seventh birthday. By that time, their sire would be at least nine years old and have many second crop daughters in his milk proof. Since we can't wait that long, we use correlated traits like yield, partial herdlife measures (is she alive at 36 months of age?) and type data to predict productive life on young bulls just out of sampling. Predicted productive life has a very low heritability of 3% or less according to one study. Directly measured productive life also has a very low heritability of less than 9%. The truth is, we can't change productive life very quickly through selection. We never could, but in years past we didn't know it and talked and acted like longevity was the trait most important to us in breeding programs. Some farmers have expressed frustration about the small range in genetic evaluations for productive life. A scan of Holstein bulls on the Virginia Tech AI sire summary (I might have missed one) showed a low of -1.1 months and a high of 4.3 months for productive life. Statistics from the summer bull proofs show that about two-thirds of active AI Holstein bulls are between .5 and 2.3 months for productive life. The small differences between bulls has caused some producers to resort to selection on type traits as a "better" alternative. Type traits aren't "better" at improving productive life than selecting to improve productive life. Type traits are "better" for changing the shape of cows. If your goal is to breed a more profitable herd without concern about cow shape, the USDA Net Merit index is a good selection criteria. It combines productive life with genetic evaluations for MFP$ and SCS. If you wish to change cow shape to meet your personal standards, other indexes or additional selection pressure on type after screening on Net Merit would be a better sire selection practice. Recognize the limitations placed on us in changing longevity of our dairy cows. Most cows eliminate themselves because of reproduction or mastitis and research results clearly show that cow shape has little to do with either of those problems.