Mastitis incidence is a management challenge on every dairy. While environment and milking management play key roles in maintaining low levels of mastitis, genetic selection for improved mastitis resistance can play a valuable complementary role in a successful mastitis control strategy.
The mastitis challenge is not new, but several recent trends suggest genetic improvement of mastitis resistance should receive greater priority. First, the increased frequency of using recycled solids for bedding rather than sand can create a more challenging environment for udder health. Second, there is a genetic antagonism between selecting for improved production and selecting for improved mastitis resistance. The genetic correlation between milk yield and mastitis resistance is -0.21. Currently, we are making unprecedented progress on improving yield traits, which means we need to put more emphasis on mastitis resistance to maintain current genetic levels of mastitis resistance. Finally, the risk of mastitis increases as cows get older. As heifer inventories decline, there is increasing pressure for cows to remain healthy and productive for more lactations.
A recent review paper published in the Journal of Dairy Science quantified the risk of a variety of health disorders as cows advanced to later lactations. The results for mastitis are summarized in Table 1. Across multiple studies, it was found that cows in the fifth or greater lactation were 2.46 times more likely to get clinical mastitis than cows in their first lactation.
The good news is: There are several tools available to help select for genetic improvement in mastitis resistance. Historically, our primary tool has been predicted transmitting ability (PTA) values for somatic cell score (SCS). Genetic evaluations for SCS have proven to be quite reliable and have been an effective tool. But SCS is just an indicator of mastitis. Its accuracy in selecting for improved mastitis resistance has limitations because SCS is not a perfect predictor of clinical mastitis.
In the past 10 years, direct genetic evaluations for mastitis resistance have become available from Zoetis (Holsteins and Jerseys) and CDCB (Holsteins, Jerseys and Brown Swiss). These evaluations use actual mastitis events in cows to identify those more resistant to cases of clinical mastitis. They provide a valuable supplement to SCS PTA in selecting for improved mastitis resistance.
The mastitis resistance genetic evaluations have the same trait definition, but the evaluation results are expressed differently. Zoetis uses a standardized scale to present the evaluations where the mean is 100 and the standard deviation is 5. Evaluations typically range from 85 to 115 with higher values indicating animals that transmit improved mastitis resistance. CDCB mastitis resistance evaluations are expressed as PTA values where the units are percentage points of mastitis incidence. They have a mean of 0 and range from +5.0 to -5.0. Higher values indicate improved mastitis resistance or lower incidence rates of clinical mastitis.
Studying evaluations for some routinely available A.I. sires (Tables 2 and 3) highlights the value and power of these evaluations.
These two example bulls show that it’s possible for animals with similar SCS PTA to have very different transmitting abilities for mastitis resistance. It demonstrates that using mastitis resistance evaluations in addition to SCS PTA will be more effective at improving udder health.
These example bulls also illustrate how much direct selection for mastitis resistance can impact barn performance. The CDCB Mastitis Resistance PTA values are in incidence rate percentage points and directly indicate that mastitis incidence will be 5.4 percentage points lower in Bull A daughters than in Bull B daughters. PTA values are scaled using the breed-average cow. Based on Table 1, we can expect the difference in mastitis incidence rates between Bull A and Bull B daughters to be a little smaller in first-lactation daughters and higher in daughters during lactations three or greater. Table 4 combines the odds ratios and incidence rates from Table 1 with the PTA values for Bull A and Bull B.
These clearly are meaningful differences in mastitis incidence rates and will be noticed in the barn through reduced disruption, less discarded milk and lower treatment costs. A 1,000-cow herd sired by bulls at +3.4 Mastitis Resistance PTA would have 70 fewer cases of clinical mastitis each year compared to a 1,000-cow herd sired by bulls at -2.0.
Upgrading mastitis selection practices to include both SCS PTA and mastitis resistance genetic evaluations can lead to fewer mastitis events and improved barn performance.
References are omitted but available upon request by sending an email to the editor.
