Randomized noninferiority study evaluating the efficacy of 2 commercial dry cow mastitis formulations

The study objective was to compare the efficacy of 2 commercial dry cow mastitis formulations containing cloxacillin benzathine or ceftiofur hydrochloride. Quarter-level outcomes included prevalence of intramammary infection (IMI) postcalving, risk for cure of preexisting infections, risk for acquiring a new IMI during the dry period, and risk for clinical mastitis between dry off and 100 d in milk (DIM). Cow-level outcomes included the risk for clinical mastitis and the risk for removal from the herd between dry off and 100 DIM, as well as Dairy Herd Improvement Association (DHIA) test-day milk component and production measures between calving and 100 DIM. A total of 799 cows from 4 Wisconsin dairy herds were enrolled at dry off and randomized to 1 of the 2 commercial dry cow therapy (DCT) treatments: cloxacillin benzathine (DC; n = 401) or ceftiofur hydrochloride (SM; n = 398). Aseptic quarter milk samples were collected for routine bacteriological culture before DCT at dry off and again at 0 to 10 DIM. Data describing clinical mastitis cases and DHIA test-day results were retrieved from on-farm electronic records. The overall crude quarter-level prevalence of IMI at dry off was 34.7% and was not different between treatment groups. Ninety-six percent of infections at dry off were of gram-positive organisms, with coagulase-negative Staphylococcus and Aerococcus spp. isolated most frequently. Mixed logistic regression analysis showed no difference between treatments as to the risk for presence of IMI at 0 to 10 DIM (DC = 22.4%, SM = 19.9%) or on the risk for acquiring a new IMI between dry off and 0 to 10 DIM (DC = 16.6%, SM = 14.1%). Noninferiority analysis and mixed logistic regression analysis both showed no treatment difference in risk for a cure between dry off and 0 to 10 DIM (DC = 84.8%, SM = 85.7%). Cox proportional hazards regression showed no difference between treatments in quarter-level risk for clinical mastitis (DC = 1.99%, SM = 2.96%), cow-level risk for clinical mastitis (DC = 17.0%, SM = 15.3%), or on risk for removal from the herd (DC = 10.7%, SM = 10.3%) between dry off and 100 DIM. Finally, multivariable linear regression with repeated measures showed no overall no difference between treatments in DHIA test-day somatic cell count linear score (DC = 2.19, SM = 2.22), butterfat test (DC = 3.84%, SM = 3.86%), protein test (DC = 3.02%, SM = 3.02%), or 305-d mature-equivalent milk production (DC = 11,817 kg, SM = 11,932 kg) between calving and 100 DIM. In conclusion, DC was noninferior to SM in effecting a cure, and there was no difference in efficacy between these 2 DCT formulations as related to all other udder health or cow performance measures evaluated between dry off and 100 DIM.