Selective dry cow therapy (SDCT) is in the news these days. One reason is cost. Treating every cow at dry up takes time and money. Also, some argue that SDCT is more responsible antibiotic use than blanket dry cow therapy (BDCT). All around the world, societies are becoming more concerned about the negative effects of antibiotic use, including possible development of drug resistance to antibiotics used in humans. So how do you decide if SDCT is for your herd?
    At a recent symposium at the annual meeting of the National Mastitis Council, experts from a number of countries presented findings and opinions regarding SDCT. Remember there is little question that BDCT has been and still is an effective way to improve udder health in most dairy herds. SDCT is not promoted as a method to improve udder health. At best, SDCT will have little or no impact on udder health in a dairy herd. At worst, udder health and cow health will decline.
    According to Nydam (Nydam, et. al. 2018), identification of cows or quarters that would benefit from treatment is the cornerstone of SDCT. The rub is how one identifies them. Dry cow therapy has two purposes: to cure existing infections and to prevent new infections. Fortunately for us, internal teat sealants (ITS) have shown to be effective at preventing new infections in the dry period, so if when one considers the purpose of dry cow therapy in herds using ITS, it is mostly treating existing infections.
    Nydam’s recent paper reviewed a variety of trials using SDCT in combination with internal teat sealants. Cows were chosen for SDCT based on a variety of measures, including bulk tank SCC, composite SCC at dry off, bacterial culture at dry off, cow records (SCC or mastitis events) only, cow side tests, e.g. CMT only, and combinations. Overall it was concluded that there needs to be more research, but in their own trial they did find an economic benefit of $6.87 per cow when 35 percent of cows were treated with dry cow antibiotics. For that study, low risk cows were defined as having no more than one clinical mastitis event, a mean of the last test days less than or equal to 200,000 cells/ml, a last test of less than or equal to 200,000, and a projected dry period of less than 100 days. For this group of cows using ITS alone versus ITS and a dry tube there was no difference in most measures of milk quality. The authors concluded that SDCT is an opportunity to practice good drug stewardship, and in many cases may offer economic benefits. However, they also stated that research indicates that the success of SDCT is farm specific.
    Many producers in New Zealand have been using SDCT since the early 1990s (McDougall, 2018). The current recommendations are that any cow with a maximum SCC at any DHIA test of greater than or equal to 150,000 or 120,000 for multiparous and first lactation cows respectively, and/or a history of clinical mastitis in the current lactation be treated with dry cow therapy. Others are treated only with ITS. McDougall cautions that using ITS alone requires a higher level of skill to ensure hygiene is maintained, and that sporadic reports of deaths have been reported in cows treated with ITS only, most likely due to poor hygiene. Still, it was concluded that with good training and planning, SDCT is a logical step for dairy industries internationally.
    Farmers in the Netherlands have been under a ban of preventative use of antimicrobials since 2012, so farmers have been forced to adopt SDCT (van Werven, 2018). Multiparous cows with a SCC greater than 250,000 and first calf heifers with a SCC greater than 150,000 at last test are allowed to be treated with antimicrobials. The national mean of percent new and percent cured infections during the dry period have not been negatively affected by adoption of SDCT. However, veterinarians and farmers adjusted their focus to other management practices, such as hygiene, to ensure optimal udder health during this time.
    A group of European experts developed a consensus statement regarding SDCT (Bradley, 2018). They propose that farms be designated high risk or low risk. High risk farms have a bulk tank SCC over 250,000, or a problem with Strep agalactia. Both high risk and low risk farms can implement SDCT, but in high risk farms, more attention should be placed on other methods to improve udder health. In low risk farms it is suggested that SDCT should be used in cows with greater than 200,000 cells/ml on at least one of the last three test days, and cows with clinical mastitis within the last three test days.
    A U.S. mastitis researcher reached a different conclusion (Fox, 2018). After reviewing much of the scientific literature, he said, “ … results suggest an advantage to BDCT. Milk production increases, fewer cases of infections and clinical mastitis are noted and lower SCC post calving, have all been associated with BDCT versus SDCT.” He also said there is no evidence to suggest that BDCT has led to selection of antibiotic resistant mastitis pathogens, and that adoption of SDCT in Denmark has resulted in increased incidence of Strep ag mastitis. He said, “ … it does not appear at this time there is any compelling reason to recommend that all, most, or even many herds should choose SDCT over BDCT.”
    Clearly the choice of SDCT or BDCT needs to be herd specific. There are a whole variety of factors to consider when making this choice, and while more research would be useful, decisions will be made before that happens. Be careful. Deciding whether your farm is high risk or low risk is a logical first step. High risk farms may find that only a small number of cows would be eligible for treatment with ITS alone, while low risk farms will more likely find significant economic benefits of adoption of SDCT. Determining what information to use to determine whether a cow is treated with antimicrobial or not is challenging, and the optimal program is likely different between farms. Ask your herd veterinarian for guidance if you think SDCT might be for you.
    Jim Bennett is a dairy veterinarian at Northern Valley Dairy Production Medicine Center in Plainview, Minn. He and his wife, Pam, have four children. Jim can be reached at bennettnvac@gmail.com with comments or questions.