Effect of Leucaena leucocephala intake on thyroid hormones and energy balance of Brangus bulls.

Mimosine, a non-proteinogenic amino acid found in Leucaena plant, is a toxic compound that induce thyroid disorders. Feeding this legume has been shown to reduce serum triiodothyronine (T3 ) and thyroxine (T4 ) levels in cattle. Since these hormones are involved in regulating basal metabolism in mammals, we hypothesized that decreasing levels of these hormones would impact energy metabolism, affecting heat production (HP) and energy requirements for maintenance. Six non-castrated Brangus bulls with an average body weight (BW) of 311 ± 17 kg and 20 ± 2 months of age were randomly distributed in a crossover design 3x6 with repeated measures (two animals per experimental unit). Treatments were levels of inclusion of Leucaena in to the diet: control (CTL, 0%); low level (L1 , 30%) and high level (L2 , 60%) (dry matter basis). Each experimental period lasted 34 d with 14 d of dietary adaptation in individual metabolic cages followed by 20 d for measurement of response variables. Thyroid hormones (TH) concentration in blood serum, diet digestibility, energy metabolism and gas exchange were measured. Oxygen consumption (O2), carbon dioxide (CO2 ) and methane (CH4 ) production were measured for 2 consecutive d in fed and fasted bulls in two open-circuit respiration chambers. Mimosine intake was 0, 15.10 and 31.33 g/d, respectively. However, no effect on the concentration of T3 and T4 in blood serum was observed (P = 0.72 and P = 0.57, respectively). Similarly, O2 consumption, CO2 and CH4 production were similar among treatments in both fed and fasted states (P = 0.18). The mean net energy requirement for maintenance of Brangus bulls was 407.9 kJ/kg0.75/d and was not influenced by treatment (P = 0.32). Energy lost in feces (MJ/d and as a proportion of total gross energy intake) was greater in the treatment with high level of Leucaena (P = 0.0005 and P = 0.03, respectively) and had a linear effect in both variables. However metabolizable energy intake (MEI) showed no differences among treatments (P = 0.12). Retained energy (RE) ranged from 3.20 to 5.04 MJ/d with no significant differences observed (P = 0.51). These results indicate that the inclusion of Leucaena at the maintenance level of feeding does not affect the concentration of thyroid hormones or energy metabolism of cattle.