DISTRIBUTION AND CLEARANCE OF ORALLY CONSUMED LEAD IN LACTATING COWS

Abstract

Nonselective eating habits of cattle often result in exposure and sometimes toxicosis from lead. Five lactating Holstein cows (639- 26 kg) were used in a study to determine the distribution and excretion of lead in milk, urine, and feces. Lead (500 mg, approximately 0.8 mg/kg/d) as acetate was administered daily by bolus for 49 d. Two weeks after the last dose, a pilot study was conducted to determ ine if excretion of lead could be altered by therapy with the chelators m eso-2,3-dim ercaptosuccinic acid (DMS A) and m onoisoamyl m eso-2,3dimercaptosuccinate (Mi-ADMSA). Cows received 10 iv doses of approxim ately 0.3 mg/kg of chelator at 12-h intervals. Neither lead nor chelators produced any clinical signs of toxicity. Concentrations of lead were determined by graphite furnace atomic absorption and inductively coupled plasma spectroscopy. Prior to dosing, lead concentrations in blood, milk, and urine were 42.0- 5.6, 35.1- 5.9, and 81.2- 9.7 ng/g, respectively. Concentrations in feces were <2 ug/g on a dry basis. Steady state was apparent after wk 2 of lead dosing, averaging approximately 2.6, 1.7, 1.6, and 30 times the predose concentrations for blood, milk, urine, and feces, respectively. After dosing, initial lead disappeared from feces with a first order t1/ 2 of 1.0 d, reflective of elimination of unabsorbed materials. Lead concentrations in milk also declined rapidly (t1/ 2 =3.2 d) when dosing was terminated. Disappearances of lead from blood and urine were t 1/ 2 = 19.3 and 29.7 d, respectively. Lead concentrations and total excretion in milk and urine, in response to these am ounts of the chelators, were not significant.