Tissue Depletion in Dairy Cattle Following Administration of Label or Extralabel Doses of Penicillin G Using Two Different Formulations.

Penicillin G has been one of the main violative residues detected at slaughter for 25+ years. Accordingly, a residue depletion study was initiated to determine possible reasons, using a two-phase approach. Phase 1 involved a 2 × 2 matrix using either the 4- or 14-day withdrawal period product at label dosage (1×) or three times the label dose (3×), with tissues collected by laparoscopy. Phase 2 employed just a 3× dose of both products, with tissues collected at necropsy. Plasma and muscle penicillin G levels were below 50 ng/g (official tolerance level: 0.050 ppm) before the 4 and 14-day withdrawal period when dosed at 1× and 3×. Penicillin G in kidney and liver samples from heifers administered the 14-day withdrawal product were always below 50 ng/g by Day 14 regardless of dose or means of tissue collection. Kidney and liver concentration of Pen G from laparoscopically collected samples were in close agreement with results obtained from whole organ analyses. Administration of the 1× dose for the 4-day withdrawal period product resulted in residues in kidneys above 50 ng/g in 25% of study heifers (2 of 8) and in 50% (4 of 8) or more (3 of 5) following a 3× label dose.