PSXIII-27 Effect of diet physically effective neutral detergent fiber and starch levels on ruminal fermentation of finishing beef cattle.

Abstract This study aimed to evaluate the effect of diets containing different concentrations of physically effective neutral detergent fiber (peNDF) and starch on ruminal fermentation of finishing beef cattle. Eight ruminally cannulated Nellore steers (468.6±87.5 kg) were housed in individual pens (21 m²) and assigned to a replicated 4×4 Latin square design, balanced for carryover effects. The experiment followed a 2×2 factorial arrangement of treatments, with two peNDF levels (low: 8.5% of diet dry matter [DM] vs. high: 10.3% of diet DM) and two starch levels (22.5% vs. 45% of diet DM). Concentrate feedstuffs were ground through a 4-mm sieve before feeding, and all ingredients were accounted for in the peNDF calculation. Each experimental period lasted 21-d, consisting of a 14-d adaptation followed by a 7-d sampling phase. Corn silage was harvested from a single field, chopped to a theoretical chop length of 2.0 (low peNDF) or 9.0 mm (high peNDF), and included at 20% of DM in all diets. Additionally, diets contained finely ground corn grain, soybean hulls, soybean meal, citrus pulp, minerals, urea, and sodium monensin. Mean, minimum, duration, and area pH < 5.5 were not affected by peNDF, or interaction (P≥0.47). There were no peNDF or interaction effects (P≥0.47) for mean and ruminal minimum pH, and duration and area of ruminal pH < 5.5. Cattle-fed high starch diets had lower mean (high: 5.5 vs. low: 5.7; P< 0.01) and minimum ruminal pH (high: 4.9 vs. low: 5.4; P<0.05), and greater duration (high: 792.7 vs. low: 328.9 min/d; P< 0.01) and area of pH < 5.5 (high: 317.4 vs. low: 78.7 pH×min; P< 0.01). Cattle-fed low peNDF and low starch, and high peNDF and high starch had lower (interaction; P=0.02) maximum pH, with those fed high peNDF and low starch being intermediate but not different from cattle-fed low peNDF and high starch. There were no peNDF or peNDF×starch interaction effects (P≥0.47) for molar proportions of ruminal acetate, propionate, and butyrate, and ruminal ammonia concentration. Total short-chain fatty acids concentration tended (P=0.08) to be greater for cattle-fed low peNDF and low starch. Feeding high starch decreased the molar proportion of acetate (high: 64.1 vs. low: 68.9 mol/100 mol; P< 0.01) while increasing the molar proportion of propionate (high: 21.0 vs. low: 17.6 mol/100 mol; P=0.04) and ruminal ammonia concentration (high: 9.7 vs. low: 5.7 mg/dL; P< 0.01). Altering the theoretical chop length, a practical approach to manipulating peNDF while maintaining the same dietary NDF from roughage did not affect the ruminal fermentation. However, increasing starch levels in finishing diets affected ruminal fermentation. The present data suggest that starch level has a greater influence on ruminal fermentation of finishing beef cattle than peNDF, with potential implications for ruminal health.