Maintaining an appropriate balance between energy and protein in feedlot diets is crucial to ensure productive performance while preserving animal health. This study aimed to evaluate the body development of lambs fed whole-corn diets supplemented with extruded protein concentrates containing different protein levels. The project was approved by the Animal Ethics Committee of the Federal University of Uberlândia (protocol no. 092/16). A total of 22 crossbred (Dorper × Santa Inês) lambs, with an initial body weight of 21.46 ± 6.36 kg and aged 3 ± 1 months, were used. The animals were housed in collective pens for 70 days in a randomized block design and assigned to four experimental groups differing in the crude protein content of the extruded supplements (20%, 24%, 28%, and 32%). Feeding was carried out three times daily at 08:00, 12:00, and 16:00. Biometric measurements and body condition scores (BCSs) of the animals were assessed every two weeks. For body growth evaluation, in vivo biometric measurements were taken using a measuring tape by the same evaluator to minimize subjectivity. BCS was assessed by three trained evaluators through palpation of the 12th and 13th lumbar vertebrae using a 1–5 scale. The mean of the three evaluators was used to improve data accuracy. The data were tested for variance assumptions using SAS software and analyzed by contrast tests to evaluate the linear and quadratic effects of protein levels (PLs), considering significance at p≤0.05. There was no effect of PL on biometric measurements (p>0.05). However, a negative quadratic effect was observed for BCS (p=0.05), with the quadratic response indicating a minimum value of 3.42 at a 28% PL. It was concluded that the protein level of the extruded supplement in high-grain diets for lambs affects body condition score without influencing body development measurements.