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.