Distribution of radiocarbon after intramammary, intrauterine, or ocular treatment of lactating cows with carbon-14 nitrofurazone.

Three lactating Holstein cows (634 to 698 kg) were dosed, respectively, with 65.6 mg (44.5 microCi/mg), 131.2 mg (20.1 microCi/mg), or 8.4 mg (141.3 microCi/mg) of [14C]nitrofurazone by intramammary, intrauterine, or topical ocular administration. Intramammary and intrauterine treatments were single doses; ocular treatment was daily for 4 consecutive d (2.1 mg/d). Cows were slaughtered after 72-h withdrawal periods. Excreta and milk were quantitatively collected from each cow after dosing. Seventy-two hours after treatment, urine, feces, and milk contained 62.9, 17.6, and 2.3%, respectively, of the radiocarbon administered intramammarily to the cow. Radioactive residues in milk collected from the dosed quarter were 150 ppb (nitrofurazone equivalents) and were 39 ppb in milk collected from the undosed quarters at 12 h after dosing. Urine, feces, and milk from the cow that received the intrauterine dose contained 12.24, 5.17, and 0.13% of the administered dose, respectively, at 72 h after treatment. Concentrations of total radioactive residues in milk were 9.3 ppb at 12 h after dosing. For the cow that was dosed ocularly, the cumulative excretion of radiocarbon in urine, feces, and milk was 17.6, 28.5, and 0.5% of the dose, respectively. Milk residues from the cow that was dosed ocularly were never > 1 ppb of nitrofurazone equivalents. Livers and kidneys contained the greatest amounts of residues relative to other edible tissues. Parent nitrofurazone was not suitable as a marker compound to determine total residues in milk using HPLC analysis. Radioactive residues were available systemically and were excreted in milk after intramammary, intrauterine, or ocular application of [14C]nitrofurazone. Illegal residues in milk and edible tissues would result from the administration of nitrofurazone to lactating cows.