Effects of forage NDF strategies on growth performance, starch digestion, ingestive behavior, and inflammatory responses in feedlot Angus × Nellore bulls.

This study evaluated the effects of forage neutral detergent fiber (fNDF) level and feeding strategy on growth performance, carcass traits, ingestive behavior, starch utilization, and inflammatory biomarkers in Angus × Nellore bulls fed energy-dense diets. Sixty Angus × Nellore bulls (initial BW = 398 ± 10 kg) were housed in four pens (15 bulls/pen) and individually monitored using GrowSafe and Intergado systems. During d 0 to 50, bulls received diets containing either 12% or 7% fNDF. During d 51 to 100, 30 bulls remained on their original diets, whereas the remaining 30 bulls were switched to the opposite fNDF level (12→7% or 7→12%). Dry matter intake (DMI), average daily gain (ADG), and feed efficiency did not differ during the first period or across the overall feedlot period (P > 0.05). However, during the second period, bulls fed 7% fNDF exhibited greater ADG, improved feed efficiency, greater calculated energy requirements for gain (Eg; Mcal/d), greater performance-predicted dietary net energy for maintenance (NEm; Mcal/kg DM), greater performance-predicted dietary net energy for gain (NEg; Mcal/kg DM), and lower DMI required for maintenance than bulls fed 12% fNDF (P = 0.01). In contrast, bulls fed 12% fNDF allocated a greater proportion of DMI to maintenance requirements (P = 0.01). Feeding 7% fNDF increased fecal starch concentration and reduced estimated starch digestibility (P < 0.05), whereas increasing fNDF from 7% to 12% during the second period improved starch utilization compared with both the continuous 7% fNDF treatment and the 12→7% strategy (P = 0.01). Carcass traits, water intake, and the frequency of visits to the water trough were not affected by treatment (P > 0.05). Diets containing 12% fNDF increased chewing and rumination time during the first period (P < 0.05), whereas reducing fNDF, particularly following the 12→7% transition, decreased rumination time and increased idle time without affecting eating time (P < 0.05). Concentrations of lactate dehydrogenase, lipopolysaccharide (LPS), and haptoglobin were not affected by treatment (P > 0.05), whereas serum amyloid A (SAA) concentrations increased in bulls fed 7% fNDF during the second period (P = 0.01). The 7% fNDF strategy improved late-feedlot growth performance but at the expense of starch utilization, as evidenced by greater fecal starch concentration and lower estimated starch digestibility. In contrast, increasing fNDF during the finishing phase improved starch utilization without compromising animal performance. Although these responses were evident during the second period, they did not result in detectable differences in overall feedlot growth performance.