Comparing antibiotic treatment at dry-off on one quarter versus all quarters in cows with only one quarter affected with a high somatic cell count or an intramammary infection

The aim of this randomized controlled study was to evaluate, on cows with only one quarter affected at dry-off with an IMI, a high SCC (>200,000 cells/mL), or both, the impact of treating with an antibiotic plus internal teat sealant only the quarter affected (treating unaffected quarters with teat sealant alone; SelectAB) versus all quarters (AllAB) on subsequent lactation SCC (analyzed as log₁₀ transformed SCC) and IMI (odds of cure [i.e., affected quarter at dry-off but not at calving]; and odds of healthy [i.e., not affected at dry-off and calving]). Ninety-six multiparous cows from 3 research herds were randomly assigned to SelectAB or AllAB. Linear and logistic regression mixed models were used for data analysis. Cows assigned to AllAB had a 0.15 units lower log₁₀ SCC (95% CI = -0.19 to -0.11 log points) through the full subsequent lactation compared with SelectAB cows. The odds of cure for an affected quarter were similar for AllAB and SelectAB cows (odds ratio = 0.77; 95% CI = 0.27-2.20), but cows under AllAB had higher odds of having a healthy quarter shortly after calving compared with SelectAB cows (odds ratio = 2.86; 95% CI = 1.21-6.73). In conclusion, a reduced rate of healthy quarters (i.e., increased new infections any time from dry-off to shortly after calving), especially in quarters treated with internal teat sealant alone within an udder with one quarter affected with IMI, high SCC, or both at dry-off, may be a major barrier for the implementation of a quarter-based dry-off treatment approach.