In this study, the objective was to evaluate the efficacy of protein supplementation on gastrointestinal nematode control in lambs naturally infected. Sixty weaned Ile de France and Texel crossbred lambs, with 60 days of age, were divided into three groups (n=20) according to the level of protein in the concentrate: low protein (LP; 8.5%), moderate protein (MP; 15%), and high protein (HP; 25%). The daily amount of concentrate offered was based on 3% of live weight, which was continued throughout the experiment of 98 days. Evaluations were conducted biweekly from days 0 (D0) to 98 (D98). There were no significant differences (p > 0.05) in the average weight among the LP, MP and HP groups, which was 22.0, 21.4, and 21.1 kg, respectively, on D0 and 46.4, 48.3, and 48.2 kg, respectively, on D98. The daily weight gain was not significantly different among the groups (LP, 250 g; MP and HP, 276 g; p > 0.05). The average egg count per gram of feces (epg) of the LP group on D98 (6,765; an increase of 5,690 epg from D0) was higher than that of the MP and HP groups (1,617.5, p < 0.05 and 3,435, p > 0.05, respectively), which remained constant throughout the study. The hematocrit values decreased (p < 0.05) from D0 to D98 only in the LP group. Only in the HP group (p < 0.05), the mean serum total protein concentration increased from D0 (5.3 g dL-1) to D98 (5.8 g dL-1). The average albumin level on D98 (2.10 g dL-1) was lower than that on D0 (2.52 g dL-1) in the LP group (p < 0.05). During the study, 10 lambs overall were treated with anthelmintic because the epg values exceeded 10,000; six treated lambs were from the LP group, 3 from the HP group, and only 1 from the MP group. These results indicate that supplementation of diet with 15% protein increases the resistance and resilience of lambs to gastrointestinal parasites.
