Effects of fixed and changing dietary forage neutral detergent fiber levels on ingestive behavior and blood biomarkers of Nellore bulls in the feedlot.

This study investigated the effects of fixed and variable dietary forage neutral detergent fiber (fNDF) levels during the finishing feedlot period on ingestive behavior and blood biomarkers in Nellore bulls. Fifty-six Nellore bulls (initial BW: 460 ± 38 kg; age: 20 ± 1 months) were randomly allocated to four pens, with 14 animals per pen equipped with an automated feeding system (GrowSafe), the animal was considered the experimental unit. The study followed a completely randomized design. Bulls were fed for 100 d. dietary treatments were evaluated in two, fifty-day periods. In the first period (0-50 days), animals received diets with 12% or 7% fNDF. In the second period (51-100 days), one pen per treatment remained on the same diet, whereas the others switched fNDF levels (12%-7% step-down strategy or 7%-12% fNDF step-up strategy). Corn silage was used as the source of fNDF. Feeding, rumination, and resting behaviors were visually assessed by trained observers over 24 h on days 21, 42, 71, and 92. Blood samples were collected after each observation to determine lactate dehydrogenase (LDH), haptoglobin, and serum amyloid A (SAA) concentrations. Data were analyzed using the GLM procedure of SAS, considering a completely randomized design. Means were compared using Tukey's test (P ≤ 0.05). In the first period, bulls fed 12% fNDF exhibited greater rumination at 21 (354.7 vs. 296.3 min/day; P = 0.02) and 42 days (363.6 vs. 254.2; P < 0.01). In the second period, on day 71, the increase from 7% to 12% fNDF step-up strategy raised rumination, while the decrease from 12% to 7% fNDF step-down strategy reduced rumination time (346.7 vs. 269 min/day; P = 0.02), with bulls maintained on continuous 12% or 7% fNDF exhibiting intermediate rumination times. On day 92, bulls maintained on 12% fNDF exhibited longer rumination times (343.3 vs. 285 min/day; P = 0.03). Idle time was greater in bulls fed 7% fNDF in period 1 (1037.4 vs. 935.2; P < 0.01). In period 2, switching 7%-12% fNDF step-up strategy reduced idle time (993.7), whereas 12%-7% fNDF step-down strategy increased it (1045; P < 0.01). On day 92, 12% fNDF exhibited lower idle time (935 vs. 1075; P < 0.01). Eating time was greater in bulls fed 12% fNDF at day 71 (P < 0.01). Chewing time was greater at 42 days (504.2 vs. 395.7; P < 0.01) and at 71 and 92 days for 12% (477.1 vs. 408; P < 0.01 and 493 vs. 351; P < 0.01). Regarding blood parameters, LDH concentration was lower for 12% fNDF at 71 (1808 vs. 2274 U/L; P < 0.01) and 92 days (P < 0.01). No differences were observed for haptoglobin or serum amyloid A (P > 0.05). During the first period (days 0-50), a significant interaction between rumination time and fNDF level was observed for LDH concentration (P < 0.01); in bulls fed 7% fNDF, LDH decreased linearly with increased rumination (P < 0.01). Haptoglobin was also affected (P < 0.01), decreasing with increased rumination in the 7% group (P = 0.02). Diets containing 12% fNDF promoted more favorable ingestive behavior and were associated with lower LDH concentrations, suggesting differences in physiological responses compared with 7% fNDF diets.