This study, conducted at the São Paulo State University feedlot, Dracena Campus, Brazil, was designed to evaluate the length of the adaptation period to high concentrate diets on rumen morphometrics and rumenitis of Nellore (NE) and 1/2 Angus × Nellore (AN) cattle. The experiment was designed as a completely randomized block with 2 × 2 factorial arrangement, replicated 6 times (3 animals/pen), in which seventy-two 22-mo-old yearling bulls [36 NE (319.2 ± 18.5 kg), and 36 AN (307.9 ± 29.5 kg)] were fed in 24 pens for 89 d according to the treatments: NE adapted for 9-d; NE adapted for 14-d, AN adapted for 9-d, and AN adapted for 14-d. The adaptation program consisted of ad libitum feeding of 3 diets over adaptation periods with concentrate level increasing from 62% to 86% of diet DM. Each of the adaptation diets containing 62%, 70%, and 78% concentrate was fed for 3-d to cattle adapted for 9-d. For cattle adapted for 14-d, the adaptation diets containing 62%, 70%, and 78% concentrate were fed for 5-d, 4-d and 5-d, respectively. The finishing diet contained: 66.5% cracked corn grain, 14.0% sugarcane bagasse, 16.0% cottonseed meal, 1.5% supplement, 1.2% urea, and 0.8% limestone (DM basis). At harvest, rumenitis incidence (RUM) was determined, on the entire washed rumen, using a scale of 0 (no lesions noted) to 10 (severe ulcerative RUM). Likewise, a 1-cm² fragment of each rumen was collect from cranial sac. The number of papillae per cm² of rumen wall (NOP) was determined, as well as the mean papillae area (MPA). The rumen wall absorptive surface area in cm² was calculated as follows: 1 + (NOP × MPA) – (NOP × 0.002). No significant (P > 0.10) biotypes and adaptation length main effects were observed for any of the rumen morphometrics variables evaluated. However, a significant (P < 0.01) biotype main effect was observed for RUM, where NE yearling bulls had greater RUM scores than AN yearling bulls (2.05 vs. 1.35). No significant (P = 0.15) adaptation length main effect was observed for RUM. Thus, NE yearling bulls are more sensitive to rumen epithelium damage, regardless of length of the adaptation period adopted.

Grant provided São Paulo State Foundation (FAPESP), São Paulo, Brazil.