EFFECTS OF ANTIMICROBIAL TREATMENT AT THE END OF LACTATION ON MILK-YIELD, SOMATIC-CELL COUNT, AND INCIDENCE OF CLINICAL MASTITIS DURING THESUBSEQUENT LACTATION IN A DAIRY-HERD WITH A LOW-PREVALENCE OF CONTAGIOUS MASTITIS

Objective-To determine whether treating cows with antimicrobials at th e end of lactation would lower the incidence of clinical mastitis, imp rove milk production, and decrease somatic cell count (SCC) in the sub sequent lactation. Design-Randomized blind field trial. Animals-233 Ho lstein cows from a single herd. All cows were in lactation 2 or greate r. Procedure-Cows were randomly assigned to treatment groups. Treated cows were given procaine penicillin G and novobiocin by intramammary i nfusion. Control cows were not treated. Farm personnel recorded cases of clinical mastitis. Milk yield and SCC were recorded during the subs equent lactation. Results-Treatment did not significantly reduce the i ncidence of clinical mastitis when data for all cows were grouped or w hen data were stratified by lactation groups (lactation 2 vs lactation greater than or equal to 3) or by last SCC (less than or equal to 500 ,000 cells/ml vs > 500,000 cells/ml). Somatic cell counts (first, mean of first 5, maximum of first 5) for treated and control cows were sim ilar, and proportions of treated and control cows with SCC > 500,000 c ells/ml at least once were not significantly different. Treated cows p roduced 179 kg (394 lb) more milk during the first 17 weeks of lactati on than did control cows. Clinical Implications-Treating cows with ant imicrobials at the end of lactation increased 17-week milk production during the subsequent lactation and, at current milk prices, was finan cially preferable to not treating them.