The effect of supplementing high-quality fresh forage, mainly based on alfalfa, to growing lambs fed with decreasing levels of total mixed ration (TMR) was studied on intake, digestion and ruminal environment. In total, 24 catheterized lambs (25.2±3.67 kg) housed in individual metabolism cages were assigned to one of four treatment diets: ‘TMR100’: TMR offered ad libitum; ‘TMR75’ and ‘TMR50’: TMR at a level of 0.75 and 0.50 of potential intake, respectively, complemented with fresh forage without restriction; ‘TMR0’: only fresh forage ad libitum. The feeding behavior, nutrient intake and digestibility, kinetics of passage and rumen environment were evaluated. As the level of TMR in the diet decreased, lambs increased the forage intake and spent more time eating and ruminating, less time resting and demonstrated a higher rate of intake. Those changes resulted in a higher nutrient intake of dry matter, organic matter, nitrogen, NDF and ADF, but a slightly lower organic matter digestibility, while no differences were detected in the output rate of particles. As a consequence, with the decrease of TMR and increase of forage intake, the ingested energy increased. Higher ruminal pH and NH3–N concentrations were observed for lower levels of TMR in the diet. The total volatile fatty acids, acetate and propionate concentrations presented a quadratic response. Total volatile fatty acids and acetate concentrations were higher and propionate concentration was lower in lambs consuming mixed diets (TMR50 and TMR75). We concluded that the inclusion of high-quality fresh forage in a combined diet with TMR in lambs had positive effects on nutrient intake without negative consequences on digestion and rumen environment.