A field trial was set up to compare depletion of Brilliant Blue F.C.F. and penicillin following three infusions of Vetspen cerate 100 into six healthly lactating cows. The presence of dye was noted by milk color at the time of milking, and by the ion exchange resin method in the laboratory. Penicillin residues were determined by a diffusion agar plate microbiological method and by Delvotest-P. Dye extinction times were found to be significantly longer than penicillin extinction times (0.05 level of significance). The proportion of cows requiring more milkings for penicillin extinction than for dye extinction was estimated to be 0.17.

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