In the past three months, a number of farms have initiated a milk quality investigation with members of our veterinary mastitis investigation team. At first glance, this may not seem surprising, but what is notable is that the majority of these investigations were not “grade busts”. Instead, farms have been initiating milk quality work with our vets because they were dissatisfied with their level of clinical mastitis cases, or BTSCC, or both. They recognised the opportunity for improvement, and additional farm profit, and took action. It is worth taking a moment to discuss the “triggers for action” that spurred these farm teams into an investigation and, ultimately, an action plan.

Any milking herd in the world can have their mastitis situation described in simple terms. An individual cow, or more specifically her four quarters, are either uninfected or infected.

For the uninfected group: 

• milk from all quarters will not grow mastitis causing bacteria 
• the individual cow cell count (ICCC) is low – below 150,000 cells/ml. 

The majority of infected cows will be subclinically infected:

• no visible changes are seen in the milk 
• mastitis bacteria are present 
• the ICCC is elevated. 

The infected group in the herd may also include some clinically infected cows periodically.

There can be opportunities for cows to leave the uninfected group and become infected (new infections). There are also, more limited opportunities for cows to re-enter the uninfected group, via lactating cow or dry cow treatment and hence become a cure.

The herd mastitis dynamics, as just described, can be visually represented in the above diagram. The red “become infected” arrow is larger than the blue “cure” arrow as there are more risk factors to control that potentially lead to new infections.

Measures that are being taken on farm every vat pickup, or every month, can tell you a lot about where your herd is positioned in its own “mastitis dynamics” chart.

For example, the BTSCC, gives us a pretty direct measure of approximately what proportion of the herd is sitting in either the Uninfected or Infected groups. The number of clinical mastitis cases, per 100 cows, per month, tells us a lot about the size of the “become infected” arrow in a herd. During the lactation, beyond the first 14 days in milk, if the case rate is over 1/100 cows/mth this can be a trigger to indicate that the new mastitis infections are greater than desired and investigation is warranted. The number of clinical cases in heifers warrants special attention as, typically, these animals enter the milking herd in the uninfected group.

Finally, for herds that are enrolled in herd testing, the change in ICCC between herd test periods, particularly for first lactation heifers, gives an accurate estimate of the new infection rate in the herd over a period of months. For example, a between herd test, new infection rate of 10% or greater would be a trigger for action.

For all herds, even if you are not herd testing, information is at hand to tell you what your individual herd mastitis dynamics looks like. These are straight forward measures to assess (with the help of your Vet team). If you think improvement is possible, or desirable, take action to reduce the costs of clinical mastitis, subclinical infection and elevated BTSCC.