Subchronic toxicity of monensin in broiler chickens.

One hundred ninety-two male broiler chicks were dosed with monensin at concentrations of 0, 121, or 242 mg/kg feed throughout the normal growing period (50 days). Body weight gain and feed efficiency were determined weekly, and cardiac muscle was examined grossly and histologically at the end of the experiment. Livers also were weighed and examined grossly. Feed intake was determined daily, allowing continuous monitoring of drug intake. No depressing effects of the drug on growth rate and efficiency were observed until after four weeks, and then were evident only in the chicks receiving the 242 mg/kg diet. Subepicardial hemorrhage and congestion occurred in 40% of the hearts from the chickens fed the high monensin dose and were nonexistent in the other treatments. There appeared to be an inverse relationship between monensin dose and liver weight. The paralytic effects previously reported from acute dosing experiments were not observed. The results show that the heart and probably the liver are sensitive indicators of monensin toxicity and that the subchronic toxic dose is less than 18 mg/kg body weight per day.