The use of induced mammary infections for evaluating dry cow treatment products. II. Trial of a proposed method to compare three levels of novobiocin.

Infections were induced at the end of lactation in all udder quarters of 19 cows by the infusion of 0.2 mL of a 10(-4) dilution in milk of a six hour milk culture of Staphylococcus aureus, strain 305 (A.T.C.C No. 29740). Two right or two left udder quarters were infected at 15 days and the opposite two five days before the last milking of lactation. Following the last milking all four udder quarters of eight cows were treated with 400 mg novobiocin in 10 mL of 2% aluminum monosterate in peanut oil, gelled. All udder quarters of eight other cows were treated with 50 mg novobiocin in the same vehicle and the udder quarters of three cows were treated with the vehicle only. At calving, eight of 32 quarters treated with 400 mg novobiocin were still infected, as were 18 of 32 treated with 50 mg of novobiocin and all those quarters treated with vehicle only. Results were identical from udder quarters infected 15 and five days before drying off. No significant differences were found between quarters in milk yield on the last day of lactation, nor the length of the dry period. An increasing number of udder quarters were infected at calving with increase in lactation age of the cow, although the small number of cows would not allow a firm conclusion. A significant difference in results was found between front and hind udder quarters, only five of 32 front quarters were infected at calving as compared to 21 to 32 hind quarters. The method proposed was found to give essentially the same results as those from a large field trial using the same antibiotic. It should therefore be useful in evaluation trials of new antibiotic products for dry cow treatment.