The objectives of this study were to quantify the apparent digestibility of the diet components and the performance of lambs consuming increasing levels of the balanced (weight) mixture of Opuntia ficus indica bran and Mimosa tenuiflora leaves and fine branches hay substituting 0, 33 and 67% of Pennisetum purpureum hay, in a 60% roughage fodder to 40% concentrate diet formulated to a daily live body weight gain of 150 g/animal. Experiment was carried out at UFCG facilities, in Patos-PB, using 18 non-emasculated lambs with (20.4±1.16) kg (mean weight±SE). Dry matter, OM, CP, EE and total carbohydrates CD were affected by the levels of the mixture (P<0.05) according to a second degree model, with minimum CD values between 34 and 52% of P. purpureum hay replacement. Neutral detergent fiber and ADF CD decreased linearly (P<0.05), and those for CE and NFC were not affected (P>0.05). The inclusion of the mixture affected linear and positively the daily mean body weight gain (120, 149 and 170 g/animal, respectively for 0, 33 and 67% P. purpureum hay replacement levels), and DM, OM, CP, TCH and NFC consumptions. The balanced mixture of Opuntia ficus indica bran and Mimosa tenuiflora hay can replace up to 67% of the roughage fodder fraction (P. purpureum) of Santa Inês lamb diet and improve the daily body weight gain by 50 g/animal.