It normally takes a couple of days for the teat end to create a natural seal; in the meantime, cows may leak milk, allowing potential pathogens to enter the mammary system. Additionally, cows can maintain pre-existing mastitis across the dry period, since milk is not being flushed out of the udder.
To combat these concerns the National Mastitis Council (NMC) has long recommended blanket dry cow therapy (BDCT), where upon dry-off, all cows are treated with intramammary antimicrobials in all four quarters. This is widely thought to improve udder health, milk quality, milk production, and overall animal longevity.
The rise of drug-resistant bacteria and public concern over antibiotic usage has the dairy industry looking to decrease antibiotic usage without compromising the health and welfare of cattle. One study (Pol and Ruegg, 2007; JDS 90(1):249-261) surveyed dairies in Wisconsin to quantify the use of antimicrobial drugs. They observed that on conventional dairies, 66% of antimicrobials were given via intramammary administration and BDCT accounted for 29% of all antimicrobial usage. For farms hoping to decrease preventative microbial usage while minimizing mastitis risk, selective dry cow therapy (SDCT) seems to be a reasonable alternative.
With SDCT, only some cows (those deemed to be ‘high risk’ by different algorithms or criteria) are treated with antimicrobials at dry-off. Several studies have observed that SDCT and BDCT yield similar milk quality and mastitis rates (Rowe et al., 2020; Rowe et al., 2023; Lipkens et al., 2023). Current recommendations state that teat sealant should be used with SDCT to minimize the risk of contracting new mastitis cases during the dry period. This is a logical recommendation but likely deters farms that don’t use teat sealant from utilizing SDCT.
A recent study by Paiva et al., 2024 from Texas Tech [JDS 107(10):8259-8270] compared BDCT with two methods of SDCT on two Texas farms that don’t use teat sealant. They sought to compare the effect of dry cow therapies on udder health, milk production, and culling rates.
This study was conducted over a nine-month period. Farm A housed 6,900 cows in freestalls with dry manure solids and milked twice daily in a rotary parlor. Farm B housed 3,500 cows in dry-lot pens and milked twice daily in a double-parallel parlor. In the year prior to the study, clinical mastitis rates observed in the first 60 DIM were 9.9% and 5.5% for Farms A and B respectively. At the start of the study, the bulk tank SCC was 187,000 and 135,000 cells/mL for Farms A and B respectively. Cows at both farms were dried off weekly using Orbenin-DC (Merck Animal Health). Neither farm used teat sealant. Cows were randomly assigned to one of three treatments:
CON: Blanket dry cow therapy in all four quarters, where 435 of 435 cows received Orbenin-DC.
ALG1: Only cows with SCC>200,000 cells/mL at any test date or 2+ cases of clinical mastitis during the most recent lactation received Orbenin-DC. This accounted for 222 of 455 cows.
ALG2: Only cows with SCC>200,000 cells/mL at the most recent test date or any case of clinical mastitis during the most recent lactation received Orbenin-DC. This accounted for 107 of 458 cows.
The results are summarized in the table above. Somatic Cell Score and milk yield were recorded up to 6 months of lactation while the cumulative clinical mastitis rate and risk of death or culling were observed for the first 180 DIM.
The results show that SDCT reduced preventative antimicrobial usage by 50-75% depending on which algorithm was used. The clinical mastitis incidence and the risk of death or culling for all cows in each treatment was not statistically different. Average somatic cell score and milk yield were numerically slightly higher for CON cows but were not statistically different across treatments. Overall, treatment did not significantly impact udder health, milk quality and quantity, or animal longevity. This study suggests that SDCT may be a suitable alternative to BDCT even when teat sealant is not used.
It's important to note that success with SDCT protocols is dependent on multiple farm-specific management factors. If you would like to transition to SDCT, it‘s recommended to first consult your veterinarian to discuss if this is a reasonable alternative for your herd.
— Alexandria Bartlett