Toxicity of Leucaena leucocephala. The effect of iodine and mineral supplements on penned steers fed a sole diet of Leucaena.

Steers fed a sole diet of Leucaena leucocephala cv. Peru (leucaena), rapidly developed a hypothyroid condition. Total serum thyroxine (T4) levels declined from c 120 n mol litre-1 to 13 n mol litre-1 within six weeks of full leucaena feeding. Associated with the hypothyroidism was a declined in feed intake, poor weight gain, hair loss, excessive salivation and the development of lesions on the oesophagus of some animals. Light steers (119 kg) were more severely affected than heavy steers (220 kg) and two light steers died. During the eight week period of full leucaena feeding animals gained only 0.18 kg head-1 day-1 compared with 0.73 kg head-1 day -1 for steers fed cowpea (Vigna sinensis) hay. Supplementation with minerals (Fe, Cu, Zn) significantly increased mean daily intake (85.4 v 68.9 g/kg0.75), daily liveweight gain, (334 v 14 g), and serum T3 resin uptake (42.5 v 39.6%); decreased hair loss and skin lesions but did not alleviate the low serum T4 levels. Iodine supplementation did not affect intake, bodyweight gain, serum T4 levels, serum T3 uptake or toxicity scores compared with unsupplemented leucaena fed steers. When the leucaena fed animals were subsequently fed cowpea hay, appetite improved and the serum T4 levels returned to normal within two weeks. For the heavy steers over this period, weight gain was similar to the controls; however, the light steers continued to gain poorly.