For the last decade, selective dry cow therapy (SDCT) has been a “hot topic” in the world of mastitis prevention and judicious use of antimicrobials. Yet, glaring questions remain. “Which herds should consider or implement SDCT? Which cows should forgo antimicrobial treatment at dry-off? Which quarters should only receive an internal teat sealant?
During the 2021 National Mastitis Council (NMC) Annual Meeting, Sam Rowe, University of Sydney, Camden, New South Wales, Australia, said SDCT reduces antibiotic use by screening cows or quarters at the time of dry-off to identify candidates that will likely not benefit from antibiotic treatment, which is the norm for dry cow therapy. “Clinical trials have found that implementing SDCT can successfully reduce quarter-level antibiotic use by 21 to 58 percent at dry-off – without compromising cow health in the subsequent lactation,” reported Rowe.
“I expect the proportion of herds practicing SDCT in North America and Australia will continue to increase. This has already occurred in Europe – through increased voluntary adoption or government or industry mandates,” said Rowe.
While SDCT sounds promising, it may negatively affect post-calving udder health – if not properly implemented. “This risk highlights the significant need for milk quality advisers, veterinarians and producers to use evidence-based SDCT protocols that are practical, minimize cow health risks and optimize economic returns to producers,” stated Rowe.
For a SDCT program to be broadly successful, it should include these components:
- Oversight from a milk quality expert to ensure appropriate selection and program implementation
- Valid screening test
- Ongoing monitoring of udder health
Milk quality expert oversight
What factors should you consider when determining if it’s logical to implement SDCT? If a dairy does not meet any of these criteria, the dairy should continue with blanket dry cow therapy (BDCT).
- Average bulk tank somatic cell count (SCC) <250,000 cells/ml
- Herd is free of Streptococcus agalactiae intramammary infection (IMI), as identified using culture or polymerase chain reaction (PCR) of bulk tank milk and clinical mastitis cases
- Control Staphylococcus aureus IMI in the herd (i.e., low prevalence with ongoing monitoring and a control program)
- Monitor clinical and subclinical mastitis and/or bulk tank surveillance
- Required for algorithm-guided SDCT
- Consistent detection and recording of clinical mastitis, based on visualization of milk abnormalities
- Regular cow-level SCC testing
- Use systematic dry-off lists
- Excellent hygiene during the dry-off procedure
- Written standard operating procedures and employee training program
- Use teat sealants in all cows at dry-off
Effective SDCT requires consistent and efficient execution of protocols. Give dry-cow antibiotic treatment to the cows (or quarters) identified as needing it and use hygienic and proper infusion methods. SDCT is a bit more complicated than BDCT, so only recommend SDCT if the farm team is up to the task.
Would you like to implement SDCT, but Staphylococcus aureus and Streptococcus agalactiae are present in the dairy herd? Assess the herd for these pathogens by culturing milk samples from cows with clinical mastitis and cows approaching dry-off and/or after calving.
Whether you use SDCT or BDCT, all dairies should use an internal teat sealant in all quarters at dry-off. “Using an internal or external teat sealant appears to be an important ingredient for SDCT on most dairy farms.”
Several studies have evaluated the economic viability of culture- and algorithm-guided SDCT. Two Netherlands studies found that using algorithm-guided SDCT resulted in slightly higher net cash returns than BDCT – despite small increases in clinical and subclinical mastitis in cows that did not receive DCT.
In a U.S. study, researchers found no increase in clinical or subclinical mastitis in cows receiving SDCT. For culture-guided SDCT, producers could expect to save an average of +$2.14 USD per cow-dry-off, compared with BDCT and was profitable in 75.5 percent of herd scenarios. For algorithm-guided SDCT, the mean net cash impact was +$7.85 per cow-dry-off, with the practice being cost effective in 100 percent of the scenarios. In the average herd, algorithm-guided SDCT cost $5.71 less than culture-guided SDCT. To look at the economics for a particular herd, use the partial budget calculator.
Allocating DCT
“Ideally, the screening test you use for SDCT is rapid, cost effective, easy to implement and accurately identifies cows/quarters that require antibiotic treatment and those that do not,” Rowe explained.
Culture-guided SDCT uses aerobic culture methods, either conducted in a laboratory or on a farm. Results help dairy producers allocate antibiotic treatments – for quarters or cows with an IMI. Two Minnesota studies found that combining an internal teat sealant with milk culturing achieved a 21 to 55 percent antibiotic reduction – without impairing dry period IMI and post-calving clinical and subclinical mastitis.
Algorithm-guided SDCT uses health records (typically test-day SCC and clinical mastitis history) to determine a cow’s mastitis risk on the day of dry-off, with “high risk” cows being the exclusive recipients of antibiotic therapy. Studies evaluating SDCT protocols that did not use teat sealants found slightly worse or substantially worse udder health outcomes than BDCT. In contrast, studies using teat sealants found more favorable results, including similar udder health to BDCT or slightly worse udder health.
For a quarter-level perspective, a German study found slightly lower quarter-level, dry period IMI cure risk among cows randomized to algorithm-guided SDCT than BDCT. Similarly, an Irish trial found that low-risk cows that received DCT had lower quarter-level SCC during the first 120 days in milk (about 7,000 cells/ml) of the subsequent lactation. In both studies, Staphylococcus aureus were among the most commonly isolated pathogens at dry-off.
Algorithm use in selection process
“Studies evaluating the ability of algorithms that use SCC and clinical mastitis history have consistently found poor agreement with the reference test (laboratory culture) when all pathogens are considered significant,” commented Rowe. “Thus, it appears that there is no ‘perfect’ SCC threshold that simultaneously maximizes diagnostic sensitivity and specificity. Select thresholds that match the dairy producer’s aspirations for antibiotic reduction and attitude toward risk for negative impacts on udder health.”
Predictive algorithms offer significant advantages over other testing strategies. First, algorithm-guided SDCT efficiently uses data that are already available on many farms. “This allows for selection to be conducted automatically in advance of the dry-off event, which eliminates lag time between starting the dry-off process and administering treatments,” Rowe explained. “However, algorithm-guided SDCT requires regularly testing cow-level SCC and accurately detecting and recording clinical mastitis events into electronic herd records.”
Once a dairy producer decides to implement SDCT and determines milk quality thresholds, more work remains. Continue to monitor:
- Prevalence of IMI caused by Staphylococcus aureus and/or Streptococcus agalactiae
- Clinical mastitis incidence in early lactation
- Subclinical mastitis incidence in early lactation
- Dry-off procedures differing from established standard operating procedures
“To achieve this, I recommend regular bulk tank culturing or PCR, as well as culturing clinical mastitis samples,” said Rowe. In addition, analyze SCC and clinical mastitis records to identify mastitis outbreaks.
“Selective DCT provides an opportunity for the dairy industry to improve antibiotic stewardship,” concluded Rowe. “However, improper implementation may pose risks to cow health and economic returns. Therefore, producers should first consider if their herd is an appropriate candidate for SDCT. If it is, choose an evidence-based SDCT approach. The current scientific evidence indicates that culture- and algorithm-guided SDCT are likely the safest, most cost-effective and practical approaches for most farms. Milk quality advisers should closely monitor udder health after SDCT is implemented to identify shifts in pathogen profiles and disease incidence.”
To review Rowe’s complete 2021 NMC Annual Meeting proceedings paper, log into the NMC Member Center. References available upon request.
