In vitro gas production with and without polyethylene glycol (PEG) of the fruits of Crescentia alata and Guazuma ulmifolia was evaluated, the degradation kinetics of lamb diets with added fruit of the tree was determined, and the ration intake and growth rate of lambs fed these diets were measured. Twenty-five entire male lambs of 23.5 ± 0.44 kg body weight were used and distributed in treatments: T0 (control without fruit); T1 and T2, 15 and 30 % of the fruit of C. alata; and T3 and T4, 15 and 30 % of the fruit of G. ulmifolia. Data variables chemical composition, fermentation kinetic, and digestibility in vitro were analyzed by a completely randomized design and data production response factorials design of five treatments by three evaluation periods. The total phenolic content (TP) (23.0 g/kg DM) was higher (P < 0.01) in the fruits of G. ulmifolia. The addition of PEG increased (P < 0.05) in vitro gas production (156.6 mL/g DM) in fruits of G. ulmifolia. In the fermentation kinetics, the total gas volume was higher (P < 0.01) at T0 (b = 293 mL/g DM), and the rate of degradation (c) but Lag time (t lag) was not different. In animal response, total dry matter intake was higher in lambs that received T4 (1.35 kg), and the daily weight gain and feed conversion did not differ (P > 0.05) among lambs receiving the treatments. Thirty percent G. ulmifolia fruit added in the diet increased dry matter intake and improved feed conversion but did not increase weight gain.