RESUMEN
The partial substitution of soybean meal by Gliricidia sepium or Guazuma ulmifolia leaves in the rations of growing lambs was evaluated at an experimental station in the dry tropics of Mexico. Sixteen weaned crossbred male Pelibuey × Blackbelly lambs (initial weight 19 ± 1.66 kg), distributed in a completely randomized design, were assigned to the following protein sources: (a) 100% soybean meal, (b) 50% soybean meal + 50% G. sepium, (c) 50 % soybean meal + 25% G. ulmifolia + 25% G. sepium, and (d) 50% soybean meal + 50% G. ulmifolia in isoproteic rations. The lambs were housed in individual pens and fed ad libitum. Substitution of protein from soybean meal with G. ulmifolia did not affect gain, intake, or feed conversion; however, the inclusion of G. sepium reduced (linear effect, P < 0.01) gain and intake and impaired feed conversion (linear effect, P < 0.01). Partial substitution up to 50% of soybean meal with 50% G. ulmifolia leaves resulted in similar growth and lamb performance, but inclusion of G. sepium leaves adversely affected lamb growth, presumably due to other factors not related to the protein content.