Two hundred and thirty-two cows were assigned alternately to complete dry cow therapy (infusion in all quarters on the day of drying off) or selective therapy (infusion in all quarters if a history of mastitis, California Mastitis Test score of +2 or +3 in any quarter, or if cell counts from bucket milk samples as determined by the membrane filter-deoxyribonucleic acid procedure were above 500,000 cells/ml). A dry cow product containing 10(6) units of procaine penicillin G and 1 g of dihydrostreptomycin in a slow release base was used. A 1% iodophor teat dip was used throughout the experiment. Infections of Staphylococcus aureus, Streptococcus agalactiae, other streptococci, and gram negative rods were eliminated from 85.4% of the infected quarters with complete therapy and 88.2% of the infected quarters with selective therapy. New infections occurred in 3.1% of quarters with complete therapy and in 6.5% of the quarters with selective therapy. Incidence of mastitis following the dry period was less with complete therapy compared to selective therapy (4.6% vs. 7.8% of the quarters). Selective therapy was as effective as complete therapy in eliminating existing infections. Complete therapy would be the choice in situations where new infections in dry period are of concern.

Mastitis history, California Mastitis Test scores, and filter-deoxyribonucleic acid cell counts from 232 cows were used to project the effectiveness of criteria which could be used to determine which cows to treat in a selective dry cow therapy program. Bacteriological analyses of quarter milk samples were used to identify infected cows. Effects of month, lactation parity, stage of lactation, and interaction of stage of lactation with lactation parity on somatic cell numbers in milk were determined. The various criteria singly or in combinations correctly identified from 50 to over 92% of the cows with one or more infected quarters at drying off. However, from 25 to 80% of the cows free of infection would have been treated as well. The most discriminating criteria were two monthly cell counts exceeding 500,000 cells per milliliter where 53% of the cows with infected quarters were identified correctly and only 25% of the uninfected cows would be treated, or the two monthly cell counts exceeding 500,000 cells per milliliter plus a California Mastitis Test score of +2 or +3 at drying off. Here 89% of the cows with infected quarters would be treated, and 56% of the uninfected cows would be treated.