PS13-6. Physically Effective Neutral Detergent Fiber Inclusion and Roughage Level in Finishing Diets Fed to Feedlot Cattle on Ruminal pH and Fermentation.

Abstract This study evaluated the effects of variation in physically effective neutral detergent fiber (peNDF) inclusion and roughage level in finishing diets on ruminal pH and fermentation. Eight rumen-cannulated Nellore steers (588.2 ± 35.4 kg) were assigned to a replicated 4 × 4 Latin square design, balanced for carryover effects, with a 2 × 2 factorial arrangement of treatments (21 d periods). Treatments consisted of two peNDF inclusions (5.48% vs. 6.86% of dietary dry matter [DM]) and two roughage levels (10% vs. 15% of dietary DM). The peNDF content was manipulated by adjusting the theoretical chop length of the roughage source. Experimental diets consisted of corn silage, corn grain fine grind, soybean hulls pellets, soybean meal, citrus pulp pellets, mineral supplement, urea, and sodium monensin. Internal ruminal pH data loggers (T7-1 LRCpH, Dascor, Escondido, CA, USA) were placed in the ventral sac of the rumen on d 17 and removed on d 21, programmed to record pH every 5 min intervals, allowing for 96 h of continuous data collection. The peNDF inclusion did not affect (P ≥ 0.12) mean ruminal pH or the area under the ruminal pH curve below 5.5. However, feeding diets containing 5.48% peNDF increased the duration that ruminal pH was below 5.5 compared with diets containing 6.86% peNDF (P = 0.01). In addition, a tendency (P = 0.07) toward a lower maximum ruminal pH was observed in cattle fed 5.48% peNDF. Cattle fed diets containing 10% roughage exhibited a lower mean ruminal pH (P < 0.01) and a lower maximum ruminal pH (P = 0.02) than those fed 15% roughage. These animals also spent a longer time with ruminal pH below 5.5 (P = 0.05) and had a greater area where ruminal pH was below 5.5 (P < 0.01). No peNDF × roughage interaction was detected (P ≥ 0.10) for mean or maximum ruminal pH, nor for the duration or area in which ruminal pH remained below 5.5. Total short-chain fatty acids (SCFA) concentration, the molar proportions of individual SCFA, and ruminal ammonia concentration were not affected (P ≥ 0.12) by peNDF inclusion or by the peNDF × roughage interaction. Similarly, roughage level did not influence (P ≥ 0.14) the molar proportions of acetate, propionate, butyrate, valerate, isovalerate, isobutyrate, or the acetate:propionate ratio. However, cattle fed diets containing 15% roughage had greater total SCFA concentration (P = 0.04) and higher ruminal ammonia concentration (P = 0.02) compared with cattle fed 10% roughage. Increasing peNDF inclusion by altering the theoretical chop length was not sufficient to counterbalance the reduction in roughage level from 15 to 10% of dietary DM. In contrast, increasing roughage inclusion to 15% had a better effect to reduce the risk of ruminal acidosis.