Effects of fixed and changing forage NDF levels on intake, growth performance, and carcass traits of Nellore bulls in the feedlot

Abstract Greater concentrate and reduced fiber intake may induce metabolic disorders, such as acidosis, laminitis, and gastrointestinal inflammation, thereby impairing growth performance. This study evaluated the effects of fixed and changing fNDF inclusion on growth performance, intake, digestion, and carcass traits of Nellore bulls during the feedlot. After a 21-day adaptation, 56 intact males (initial body weight (BW) = 460 ± 38 kg; 20 months) were fed for 100 days in a completely randomized design. Data were analyzed across three periods. During the first period (0 to 50 days), animals received diets containing either 12% or 7% fNDF. In the second period (51 to 100 days), one pen per treatment remained on the same diet, whereas the others switched fNDF levels (12→7% or 7→12% fNDF). Over the 100-day trial, animals were evaluated individually. Corn silage was the sole forage source, and data were analyzed using SAS. During the first 50 days, bulls fed 12% fNDF exhibited greater ADG (1.71 vs 1.43 kg/d; P = 0.01), DMI (10.97 vs 10.00 kg/d; P = 0.01), feed efficiency (0.154 vs 0.138; P = 0.04), Net energy for gain (NEg) (9.96 vs 7.93 Mcal/d; P = 0.01), and dry matter intake for gain (DMIg) (7.12 vs 6.01 kg/d; P = 0.01) compared with those fed 7% fNDF. From 51 to 100 days, DMI was greater for 12% fNDF than for 7% fNDF, whereas diets that switched fNDF levels did not differ from each other. Across the 100-day period, 12% and 12→7% fNDF resulted in greater DMI and DMIg than 7% and 7→12% fNDF (P = 0.01). Carcass traits and water intake were unaffected by treatment (P > 0.05). Fecal pH was greater in animals fed 12% fNDF during the first period, whereas after dietary changes, fecal pH of the 12→7% fNDF treatment was similar to that of 7% and 7→12% fNDF. Over the entire trial, fecal starch concentration and starch digestibility differed between the 12→7% and 7→12% fNDF treatments (P < 0.01), with greater fecal starch concentration and reduced starch digestibility observed in the 12→7% fNDF treatment. The hypothesis that increased dietary fiber would improve growth performance over the entire feedlot was not confirmed. Overall, diets containing 12% fNDF enhanced ADG, DMI, and feed efficiency during the first 50 days of the feedlot, but these advantages were not maintained throughout the 100-day trial.