As You Keep Cows Longer, Mastitis Increases in Concern

Keeping a higher proportion of older cows means mastitis rates may increase if genetic selection pressure is not increased. ( Farm Journal, Inc. )

As dairy farmers reduce culling rates and milk a larger proportion of their herd as third- and later- lactation animals, rates of clinical mastitis are likely to increase.

New research from Zoetis Animal Health shows that the cost of the disease can increase by as much as 50% with later-lactation cows. The field research involved 11 herds and nearly 3,000 cows.

When just first- and second-lactation cows were examined in the first year of the study, the best quartile of cows had an 8.5% rate of clinical mastitis. The worst quartile had a clinical mastitis rate of 15.9%. When these cows were studied a year later, the best quartile of cows had a 13% clinical mastitis rate while the worst quartile shot up to 25.3%.

This finding has major implications for herds that are lowering their cull rates and keeping a higher proportion of older cows in their herds, says Dan Weigel, Director of Outcome Research for Zoetis and lead researcher of the study.

“The goals [of keeping cows longer] are to lower costs by raising fewer replacement heifers, increase milk production by having more of the herd near maturity, and increase revenue by the use of beef semen on some of the herd,” he says. “Producers who adopt this strategy would have more economic incentive to select for cows that are more resistant to clinical mastitis.”

However, the current Net Merit $ (NM$) formula places very little emphasis on clinical mastitis resistance, with a weighting of 0.77% of the total. (Clinical mastitis is part of the Health sub-index that is now included in NM$.) The genetic progress for clinical mastitis is only about 30¢/year out of the total progress of $63.18 that is expected, says Weigel.

One option for dairy farmers is to customize their sire selection index to include clinical mastitis resistance as a higher proportion of their index. However, before doing so, farmers should consult with their genetic adviser on the proper weighting for their herd.

The study was presented at the 2019 National Mastitis Council meeting Savannah, Ga. in January. For more information, go to