MSU Extension

On-farm milk culturing allows a dairy producer to make a treatment decision for a cow with clinical mastitis generally within 24 hours. A previous article, “Untreated mastitis – is that wise?” discussed the results of a study by Lago et al. that compared cows with clinical mastitis that were either all treated with a course of antibiotic or were only treated if an on-farm milk culture showed Gram-positive (staphylococci or streptococci) bacterial growth, alone or mixed culture.

The research showed that there was no difference in various measures of infection in the near-term. That is, cows in both treatment groups had no significant differences in bacterial infection measures, culling or subsequent treatment at 14 or 21 days after the treatment decision was carried out. In this report, we examine the results of longer-term impacts of the treatment decisions as reported by Dr. Alfonso Lago et. al. in the September 2011 issue of the Journal of Dairy Science (94:4457-4467)

Obviously, if dairy producers are going to implement a program of on-farm milk culturing of cows with mild or moderate mastitis and treatment only of the Gram-positive cases, then they want to know that overall, untreated (no-growth cultures and Gram-negative growth) cows will recover as quickly and do as well long-term as if they had treated them.

Lago’s work was with eight dairy herds in Wisconsin, Minnesota and Ontario. Researchers looked at the effect of treating all clinical cows or just treating cows likely to respond to antibiotic based on on-farm culturing. There were a total of 7360 cows on these farms with 422 cases of mild or moderate mastitis during the study period.

Lago and associates measured the impact of treatment decisions based on milk-culture for the remainder of the lactation or the next 365 days, whichever came first. Those comparisons between treatment groups included: 1) risk and days to a recurrence of clinical mastitis event in the same quarter, 2) cow survival in the herd, 3) somatic cell count (SCC), and 4) milk production.

The question was not whether cows get mastitis again, or whether they get culled or die, it was whether there was any difference in the percentage of those and the timing of those between cows who only had antibiotic treatment for mild or moderate clinical mastitis if the culture was Gram-positive or whether they all were treated. Let’s look at each measure separately:

•    Risk and days to recurrence of mastitis in the same quarter - Mastitis does recur sometimes, but when the treatment group in which all clinical cows were treated was compared to the one where treatment was only given if culture showed Gram-negative bacterial growth or mixed cultures, there was no significant difference between the groups.

•    Cow survival - In a study by Grohn et al. in 2005, (Prev. Vet. Med. 71:105-125), the culling hazard for clinical mastitis cows was more than twice that of non-clinical mastitis cows. However, in the Lago study there was no significant difference in herd survival between the all-treated and culture-based treated cows.

•    Somatic cell count - In previous research, it has been shown that quarter milk SCC has been greater after clinical mastitis. Deciding to treat cows based on culture results did not affect long term SCC. There was no significant difference between treatment groups.

•    Milk production - Production losses after clinical mastitis are estimated to be over 1100 lbs for the entire lactation. When the impact of the treatment was evaluated for these two treatment groups, there was no significant difference.

Use of milk cultures on-farm to make treatment decisions offers no guarantees. In fact, in examining the results in this study, sometimes the case was split between herds. For example, in the case of SCC impact, this measure was numerically lower for culture-based treatment cows in three herds, the same in two herds and numerically higher in three herds. Statistically, there was no significant difference. That tells us that we cannot predict any difference, but slight differences may occur.

There is also a learning process that producers go through as they culture milk samples. They become better at “reading” the culture plates and better at the decisions they make based on their experience. In the process, they achieve better results more consistently.

Of course, preventing mastitis is always the goal that producers should strive for, but when it occurs, culturing clinical cows helps develop the knowledge base that improves prevention practices. Combining directed prevention with targeted treatment will enable producers to achieve healthy cows and high quality milk. And that is the long-term result we all want.