MSU Extension

While no link has been found between use of antimicrobials for mastitis and any increase in the development of antimicrobial resistance by pathogens, any use of antimicrobials could increase risk of resistance. Therefore, it is important to develop practices that can reduce antimicrobial use while not increasing the risk of cattle heath problems. Researchers have been studying the use of selective dry cow treatment to reduce overall use of antimicrobials in dairy production.

In this second article of a three-part series from Michigan State University Extension on selective dry cow treatment, we will examine the questions that researchers have been asking and the results they are finding.

Researchers have designed studies to answer several questions about selective dry cow treatment:

• What criteria should be used to identify cows for treatment at dry-off?
• How effective is that criteria in diagnosing infected cows?
• What is the infection rate of untreated cows in the next lactation?
• What are the impacts on herd somatic cell count (SCC) and milk production?

Several researchers have looked at selection criteria for selective dry cow therapy. Torres and Rajala-Schultz at The Ohio State University used cow SCC and clinical mastitis history as potential selection criteria. They examined these indices either for the entire lactation or for a period near the end of lactation. These are measures that are easily available to dairy producers.

The investigators used four dairy herds with average SCC of 162,000; 340,000, 288,000 and 305,000. None of the herds used internal teat sealants. Milk samples were taken at dry-off for laboratory culture to determine how well the criteria did in identifying uninfected cows for no dry treatment. They found that an individual cow criteria of lactation SCC < 100,000 and no history of clinical mastitis in that lactation had the highest sensitivity (84 percent) for identifying quarters and cows free of infection compared to higher SCC thresholds.

At freshening, there was no statistical difference in the infection rate (determined by standard culture) of untreated low SCC cows compared to treated low SCC cows, and there was no difference in the percentage of clinical mastitis cases between treated and untreated low SCC cows. Likewise, there were no milk production differences for the subsequent lactation. Treated low SCC cows had 16 percent lower SCC in the subsequent lactation. However, high SCC, even though treated, continued to have higher SCC in the next lactation.

Another study looked at the use of a new method, the Petrifilm-based on-farm culture system to determine infected quarters 24 hours before dry-off as criteria for dry treatment. In the study, cows identified with an infection were infused with an antimicrobial dry treatment and then an internal teat sealant was applied. Cows without infection only received the internal teat sealant. The use of an internal teat sealant has been shown to reduce the risk of new infections during the dry period.

Sixteen herds with SCC < 250,000 were used in this study. The Petrifilm-based on-farm culture system only failed to identify 4 percent of infected quarters. However, only 67 percent of cows that cultured positive with that method had at least one infected quarter according to standard culture methods. This is a new method that could be adopted by producers but which does add an additional step, complexity and expense.

In this study, selecting cows for treatment based on results of the on-farm culture method was just as effective in controlling mastitis as was blanket dry cow treatment as measured by the infection status at calving.

Both of these studies, in agreement with others, show that herds can control mastitis without treating every cow and every quarter at dry-off, and do so without an increased risk of infections or clinical mastitis in the next lactation. The key is in having practical and good selection criteria for cows that will not receive treatment.

Part 3 of this series, “Increasing the likelihood of success without blanket dry cow treatment” will discuss herd criteria and management that can help increase the likelihood of good results with anything less than blanket antimicrobial use. In the meantime, review the culture reports on mastitis in your herd to determine what organisms are causing mastitis in your herd.

Additional articles in this series:

• Is it time to reconsider blanket dry cow treatment programs?
• Increasing the likelihood of success without blanket dry cow treatment