Selective dry cow therapy is based on only using intramammary antibiotics at dry-off to treat those cows that have been identified with previous or current intramammary infections. The methods used to identify cows to treat (or not) include 1) reviewing histories for previous episodes of clinical mastitis or high somatic cell counts (SCC), 2) performing individual quarter SCC tests at the time of dry-off, or 3) identifying active intramammary infections through the use of bacteriologic culture or molecular diagnostics such as PCR. However, as Dairy Herd Management said in March of this year, the jury is still out regarding the use of selective dry cow therapy. Well-managed herds may be able to employ a successful selective dry cow therapy program, but there remain reasons for dairies to consider the use of blanket therapy to cure existing and new intramammary infections.
Economic considerations are driving much of the conversation around selective dry cow therapy, highlighted by two recent articles published in the Journal of Dairy Science. Scherpenzeel et al (JDS, Vol. 101, No. 2, 2018) developed a mathematical model to show the economic optimization of selective dry cow therapy on Dutch herds. Predictably, they demonstrated that the optimal percentage of cows to be treated with antimicrobials at dry-off is dependent upon the udder health situation, described through bulk tank SCC and the incidence of clinical mastitis. The primary take-home message was that the economic benefits gained through improved udder health outweigh the cost savings simply due to restricting the use of dry cow antibiotics. Consequently, it was suggested that blanket dry cow therapy might be the choice of dairies necessarily focused on udder health risk avoidance. That said, there was no economic benefit to using more dry cow therapy than necessary, and the prudent use of antibiotics and efforts to improve animal well being make an argument for trying to reduce bulk tank SCC and clinical mastitis incidence overall.
A second article by Vasquez et al (JDS, Vol. 101, No. 6, 2018) utilized a culture-dependent, on farm algorithm to guide the use of selective dry-cow antibiotic therapy on a New York State commercial dairy. Cows were identified as low risk (cows that might not benefit from dry-cow antibiotics) or high risk (cows that would likely benefit). Low-risk cows were those that had all of the following: SCC ≤200,000 cells/mL at last DHIA test, an average SCC ≤200,000 cells/mL over the last 3 tests, no signs of clinical mastitis at dry-off, and no more than 1 clinical mastitis event in the current lactation. The expected dry period had to be <100 days, and each low-risk cow could not have been treated with antibiotics in the last 30 days. Utilizing only on-farm data eliminated the need for microbiologic or cow-side methods that require labor, time, and money.
Low-risk cows were randomly assigned to receive intramammary antibiotics and external teat sealant, or external teat sealant only at dry-off. In this study, not using the antibiotic at dry-off in low-risk cows did not have adverse effects on new intramammary infection risk, milk yield, first test linear score, clinical mastitis, or culling as compared with low-risk cows that were treated with antibiotics at dry-off. However, antibiotic-treated quarters were more likely to be bacteriologically cured. That said, 95% of the non-cures were due to minor pathogens and contributed to only a single case of clinical mastitis in low-risk cows.
Ultimately, the use of selective dry cow protocols has the potential to reduce the risk of bulk tank antibiotic residues and minimize the potential for antimicrobial resistance. It appears that well managed dairies have the potential to reduce dry cow antibiotic use by approximately 60% without negatively impacting intramammary infection risk, or cow-level production and health outcomes. The willingness and ability of producers to adopt a selective treatment strategy at dry-off is reliant upon accurate, easily accessible, and cost-effective methods to identify cows. The challenge is in identifying those dairies that can successfully implement selective dry cow treatment algorithms and reserve antibiotic treatment for cows that will benefit the most.