Levels of physically effective neutral detergent fiber from forage and narasin inclusion: performance, carcass traits, meat quality, gastrointestinal fermentation, and rumen histology in feedlot lambs.

This experiment aimed to evaluate the impact of different levels of physically effective neutral detergent fiber (peNDF) from forage (peNDF-F) and narasin inclusion on growth performance, carcass traits, meat quality, gastrointestinal fermentation, and ruminal papillae development in feedlot lambs fed high-concentrated diets. Forty-four Dorper × Santa Inês male lambs [initial body weight (BW) = 24.2 ± 1.0 kg and age = 82.1 ± 1.0 d] were assigned to a randomized complete block design. Treatments were arranged in a 2 × 2 factorial design, in which factor 1 was 52 (52peNDF-F) or 104 g/kg of peNDF-F (104peNDF-F), and factor 2 was diets with (N13) or without (N0) inclusion of 13 mg of narasin/kg of dry matter (DM) (Zimprova; Elanco Animal Health, São Paulo, SP, Brazil). The experimental period lasted 112 d, and lambs were slaughtered on day 116. The data was analyzed using the MIXED Procedure of SAS, using lambs as experimental units. No peNDF-F × narasin inclusion interactions were detected (P ≥ 0.12) for any performance and carcass parameters evaluated herein. Lambs fed 104peNDF-F diets had greater (P ≤ 0.05) DM, organic matter, crude protein, NDF, ADF, and ether extract intake compared with lambs fed 52peNDF-F diets. This improvement in intake did not result in changes (P ≥ 0.51) in average daily gain and final BW between peNDF-F levels. Lambs fed 52peNDF-F had greater (P < 0.01) feed efficiency compared with lambs fed 104peNDF-F diets. Lambs fed 52peNDF-F diets had greater (P ≤ 0.03) body wall thickness and perirenal fat than lambs fed 104peNDF-F. Narasin inclusion did not impact (P ≥ 0.20) any performance parameters or intake. The inclusion of N13 in 52peNDF-F or 104peNDF-F diets tended to reduce (P < 0.01) the molar proportion of ruminal acetate and the acetate:propionate (Ac:Pr) ratio compared to N0, while increasing (P < 0.01) ruminal propionate. Lambs fed N13 diets had greater (P = 0.02) cecal propionate and lower (P ≤ 0.05) isobutyrate and Ac:Pr ratio compared to N0. Lambs fed 104peNDF-F diets exhibited an increased (P = 0.04) number of papillae per cm² and reduced (P = 0.05) thickness of the keratinized layer of ruminal papillae compared to those fed 52peNDF-F diets. Including N13 increased (P ≤ 0.04) papillae height, surface area, and absorptive surface area per cm² compared to lambs fed N0 diets. Overall, reducing peNDF-F altered digestive dynamics, while narasin promoted a shift in short-chain fatty acids profiles, and rumen papillae development in feedlot lambs fed high-concentrate diets.