Diets with low forage concentration may depress feed intake and average daily gain, while diets with high forage concentration may limit feed intake due to rumen filling. Adequate forage level may range from 7% to 14% in ration for feedlot lambs, depending on forage source and ingredient composition. Small amounts of good quality forage may improve growth performance of sheep in the first stage of fattening; however, higher energy efficiency can be obtained in feedlot lambs fed diets with low-quality roughage. Forages are required in low percentages in fattening diets for ruminants, mainly as functional ingredients, since they contribute by stimulating rumination and saliva production that reduce acidosis-related digestive disorders. The functionality of forage is influenced by its neutral detergent fiber (NDF) concentration. Feedlot cattle may have different productive performance when fed diets with constant levels of forages of different quality. These differences can be reduced when diets are formulated to contain similar NDF levels from forage. Diets for feedlot cattle formulated at 15% of NDF from forage can moderate ruminal upsets without altering feed intake; however, the optimal level might be between 4% and 8% of NDF from forage. In feedlot lambs there is limited information. Approximately 6% of NDF from forage may be included in the ration for maximum feed efficiency. At these NDF levels from forage, the forage source may have minimal influence on growth performance of feedlot animals. There are few reports on lambs on intensive feeding indicating maximum and minimum levels of NDF from forage in diets for better growth performance. This is an important scientific topic that must be considered in feedlot lambs to improve their productive performance. IntroductionIt is possible to obtain better growth performance in lambs fed finishing diets with moderate to low levels of forage [1]. However, small amounts of forage in the diet are required for better weight gain efficiency in feedlot lambs. The main benefit of forage in growing-finishing diets for feedlot lambs is the reduction of acidosis, because fiber stimulates chewing that increases saliva production, which neutralizes the acids in rumen. It is reported that feedlot cattle receiving diets with low forage level had greater amounts of rapidly fermentable carbohydrates and greater ruminal short-chain fatty acid production, although they had lower ruminal pH and saliva production than cattle fed diets with high forage level [2].Diets with low forage concentration may depress feed intake and average daily gain, while diets with high forage concentration can limit intake due to rumen filling. Considering that the function of the fiber is given by its NDF content, a better approach to formulation of growing-finishing diets for ruminants is based on NDF concentration in forage feed [3]. Diets for feedlot cattle with different forage source (maize silage, sugar cane and sugarcane bagasse) formulated at 15% of NDF from forage can moderate ruminal u...