The mixture of carbohydrates (CHO) and non-protein nitrogen (NPN) with different ruminal degradation may affect the feeding behavior and feed selection by ruminants. This study was carried out to evaluate the ingestive behavior and diet selection by feedlot lambs fed different sources of CHO and NPN. It was used 20 non-castrated crossbred lambs with 4 mo of age and 26 kg of body weight. The diets were composed by 57% of grass hay (Brachiaria hybrid) and 43% of concentrate. The hay was harvested at late mature stage with high (72.4%) neutral detergent fiber and low (4.9%) crude protein content. Four concentrate feeds were formulated from the combination of 2 CHO (ground corn: starch; whey permeate: lactose) and 2 NPN (fast and slow release urea) sources. Orts samples were taken to determine the proportion of leaves (LVS), stems (STS) and leaf:stem ratio, and activities of eating (EAT), ruminating (RUM) and resting (RES) were recorded. The efficiency of rumination (RE) and dry matter intake (IE) were also calculated. It was used a completely randomized design with 4 diets and 5 replicates. Data were analyzed by ANOVA following a 2 × 2 factorial scheme. LVS (56.2 ± 2.7%), STS (43.8 ± 2.7%), leaf:stem ratio (1.60 ± 0.16), EAT (318 ± 8 min), RUM (537 ± 11 min), RES (585 ± 14 min), RE (1.82 ± 0.06 g DM/min) and IE (3.10 ± 0.11 g DM/min) were not affected (P > 0.05) by diets. The mixture of CHO and NPN with more distinct potential degradation rates (ground corn + fast release urea; dry whey permeate + fast release urea) did not increase (P > 0.05) the selection of LVS, neither the mixture of sources with closest potential degradation rates (ground corn + slow release urea; dry whey permeate + fast release urea) reduced the RUM. It is possible that these results were not effective to increase the ruminal degradation of low quality forage. Such hypothesis is supported by the similarity of RE values between diets. In the present study the association of CHO and NPN with similar or distinct degradation rates does not influence the diet selection and the ingestive behavior.