Effects of selective and complete dry therapy on prevalence of intramammary infection and on milk yield in the subsequent lactation in dairy ewes.

The study was carried out in a commercial flock on 286 Churra breed ewes (566 half-udders) assigned to three lots depending on the type of antibiotic dry therapy received in the lactation previous to the one studied. One-hundred-and-four ewes were given complete therapy in all udders, 103 received selective therapy of infected half-udders, and 79 received no therapy at all. Half-udders of all animals were sampled for bacteriological study at < or = 72 h (lambing), 60 d, 120 d, and 155 d (drying-off) post partum. Dry therapy, parity number, lactation stage and therapy x parity interaction contributed significantly to variation in intramammary infection prevalence. Antibiotic dry therapy had the most significant effect. Prevalence during the whole of the subsequent lactation was significantly lower in lots receiving complete (18.8%) and selective (15.6%) dry therapy than in the untreated control lot (48.3%). Coagulase-negative staphylococci and streptococci (in particular Streptococcus agalactiae) were the organisms most significantly affected by dry therapy. In untreated ewes, prevalence increased noticeably from the 2nd to the 6th and subsequent lactations, but no significant changes were observed in the treated lots. Milk yield in the dry treated lots was 6.9% higher that in the untreated one. It was concluded that complete and selective treatments of ewes at drying-off were efficient and comparable methods of reducing the intramammary infection prevalence, improving bacteriological quality of milk, and increasing milk yield.