Effect of <i>Leucaena leucocephala</i> 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 (<b>T₃</b> ) and thyroxine (<b>T₄</b> ) 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 (<b>HP</b>) and energy requirements for maintenance. Six non-castrated Brangus bulls with an average body weight (<b>BW</b>) 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 (<b>CTL</b>, 0%); low level (<b>L₁</b> , 30%) and high level (<b>L₂</b> , 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 (<b>TH</b>) concentration in blood serum, diet digestibility, energy metabolism and gas exchange were measured. Oxygen consumption (<b>O₂)</b>, carbon dioxide (<b>CO₂</b> ) and methane (<b>CH₄</b> ) 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 (<i>P</i> = 0.72 and <i>P</i> = 0.57, respectively). Similarly, O₂ consumption, CO₂ and CH₄ production were similar among treatments in both fed and fasted states (<i>P</i> = 0.18). The mean net energy requirement for maintenance of Brangus bulls was 407.9 kJ/kg^0.75/d and was not influenced by treatment (<i>P</i> = 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 (<i>P</i> = 0.0005 and <i>P</i> = 0.03, respectively) and had a linear effect in both variables. However metabolizable energy intake (<b>MEI</b>) showed no differences among treatments (<i>P</i> = 0.12). Retained energy (<b>RE</b>) ranged from 3.20 to 5.04 MJ/d with no significant differences observed (<i>P</i> = 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.