OPINION: CMT better at finding infected quarters, studies say

The California Mastitis Test (CMT) has been around for a long time — so long that many people don’t think that it is very useful anymore. The basis of the test is quite straightforward. The CMT liquid breaks up any somatic cells in a milk sample and then turns the genetic material from inside those cells into a gel. The more gel that forms in a CMT paddle well after the CMT reagent is added, the more somatic cells are in that milk. You may recall that I wrote about the CMT test earlier this year.

When I talk with farmers about the CMT, they tell me that it is not accurate enough compared to some of the newer tests. They also tell me that it isn’t as convenient as some tests too, especially tests that measure somatic cells automatically.

There are a couple of aspects of CMT that are difficult to argue about. The CMT is relatively easy to use and it provides very useful information about each quarter in real time. That could be pretty useful if you wondered if you should dry treat a cow. A big knock on the CMT has been whether it is accurate enough to be very useful.

Over the past couple of years, there have been two studies comparing CMT to DHI test day SCC readings and some other methods of evaluating milk samples. In both the studies, the intention was to determine if CMT performed very well in identifying which cows to treat at dry off.

The first study was done in New Zealand. The impetus to do the study was that the DHI SCC information that was available at dry off might be several weeks old. It was based on composite milk samples rather than on samples taken from individual quarters. The study also looked at the value of measuring electrical conductivity on samples from each quarter. The real measure of whether a quarter was infected was culture of milk taken from each quarter.

They looked at 153 cows and heifers from three farms. On culture, they found that 16 % of quarters were infected and 40 % of the cows had at least one infected quarter. None of the three tests was perfect at picking out infected quarters but the CMT and DHI SCC worked just about the same. Conductivity was the poorest test.

That study was done in New Zealand and experts argue that many things about dairy production there are much different from dairy production in North America.

Another study was recently completed at dairies in the U.S. Again, milk cultures were the standard for saying whether a quarter was infected. Samples from 324 cows at four dairies were tested using CMT, the most recent DHI SCC test and a decision making tool that used several pieces of information including DHI SCC and clinical mastitis records. The cows were producing an average of 27 kg at dry off.

Well, it turned out again that none of the tests was perfect at picking out infected quarters. CMT, though, was slightly better than DHI SCC and the decision making tool was slightly better than CMT.

So CMT stood up pretty well in both research studies. CMT certainly cannot be considered as too old or too inaccurate to be useful now. It is important to note that neither CMT nor any of the other methods that were tested in the research studies found all of the quarters that were positive on milk culture.