The effect of different proportions of Leucaena leucocephala in the diet of cattle on growth, feed intake, thyroid function and urinary excretion of 3-hydroxy-4(1H)-pyridone

Abstract

Steers were fed in pens on diets containing 0, 10, 20, 40, 67 and 100% Leucaena for 112 days. The remainder of the diet was a good quality sorghum hay. Liveweight change, feed intake, mimosine intake, urinary output of 3-hydroxy-4(1H)-pyridone (DHP), serum thyroxine (T4), serum triiodothyronine (T3) and effective thyroxine ratio (ETR) were measured. Steers on the 67 and 100% Leucaena diets had low feed intakes (40-50 g (kg LW)-a 75), lost weight and were severely hypothyroid. Steers on the 0, 10 and 20% Leucaena had intakes of 80-90 g (kg LW)-a 75, gained liveweight at 0.3-0.5 kg day-' and showed normal thyroid function. Steers on the 40% Leucaena diet had high feed intakes and grew well initially, but subsequently their feed intakes and rates of liveweighr gain declined. Reduced feed intake was associated with serum T3 levels below 1.0 nmol l-1. DHP excretion in the urine was linearly related to Leucaena and mimosine intake (P < 0.01). Recoveries of mimosine eaten, as DHP in the urine, varied from 33% with the 10% Leucaena diet to 55% with full Leucaena feeding. Feed intake declined as the proportion of Leucaena in the diet increased, but recovery of appetite was rapid when steers were transferred to an all sorghum hay ration. It is concluded that diets containing less than 1 % mimosine on a DM basis have little adverse effect on thyroid function or feed intake, whereas above this level hypothyroidism and low feed intakes may occur. Under Australian conditions Leucaena can only safely be used as a supplement (<30%) to roughage diets, rather than as a major dietary component, until some solution to the toxicity problem is found.