Dye marked antibiotics in dry cow mastitis therapy

Eight dry cow dye/antibiotic preparations were used to determine dye and antibiotic excretion endpoints. Formulations contained 400 mg of novobiocin with 25 or 250 mg Food Drug and Cosmetic Blue No. 1 per infusion or 400 mg novobiocin plus 200,000 IU penicillin with 25, 125, or 250 mg Blue No. 1 or 2 dye per infusion. Fifty-two cows entering their second or later lactations were treated in two or four quarters. Milk was sampled postpartum and tested for residues. Dye content was determined visually and subvisually. Antibiotic content was analyzed by the cylinder plate, or Delvotest-P, or by both methods. One sample contained penicillin residues. Three samples had visual dye content of Blue No. 1. Blue No. 2 was not detected. Subvisual dye was detected for one to four milkings in most cows treated with preparations containing Blue No. 1. Cows were treated within 1 to 7 d prepartum with novobiocin plus penicillin containing 250 mg Blue No. 1. Visual dye persisted from 3 to 15 milkings postpartum and subvisual dye for one to two milkings longer. Penicillin was present in postpartum milk in five of seven cows. Novobiocin persisted in three of seven cows.