Association of milk yield and infection status at dry-off with intramammary infections at subsequent calving.

The dry period plays an important role in maintenance of udder health. Cows are most susceptible to intramammary infections (IMI) after dry-off and near parturition and drying-off procedures may affect the likelihood of IMI at calving. The objective of this study was to evaluate the association of milk yield and infection status at dry-off with the likelihood of IMI at calving by examining different drying-off methods. Cows (n=112) at the Ohio State University Waterman Dairy Teaching and Research Herd were randomly assigned to either an intermittent or a standard, twice-daily milking group 1 week prior to dry-off. All quarters of all cows in the herd were treated with an antibiotic dry-cow product after the last milking. Milk samples were collected 1 week prior to dry-off (pre-dry), on the day of dry-off, and within 3 d of parturition to determine infection status of the quarters. Association between IMI at calving and cumulative milk yield for the final week of lactation and drying-off method was examined using generalized estimation equations with logic link, accounting for potential confounders, such as pre-dry and dry-off infection status, and for the correlated data structure due to quarters clustered within cows. Intermittent milking significantly reduced milk yield at the end of lactation. Increasing cumulative milk yield during the last week of lactation was significantly associated with a greater probability of IMI at calving for quarters that were uninfected prior to dry-off: uninfected quarters of cows producing more than 115 kg during the last week of lactation were 7.1-times more likely to be infected at calving (P=0.0081) than uninfected quarters of cows producing less than 75 kg. Even though the overall cure rate over the dry period was relatively high at 84%, the odds of a quarter being infected at calving was 7.6- and 3.3-times higher if it was infected at dry-off with major pathogens (P<0.0001) or minor pathogens (P=0.028), respectively, compared with an uninfected quarter at dry-off. The results suggest that decreasing milk yield prior to dry-off may serve as an effective means to maintain good udder health in a herd.