Influence of Dietary Forage Neutral Detergent Fiber on Ruminal Fermentation, Chewing Activity, Nutrient Digestion, and Ruminal Microbiota of Hu Sheep

As the key components of dietary carbohydrates, ensuring a balance between forage-neutral detergent fiber (FNDF) and rumen-degradable starch (RDS) is essential for ruminant health. Eight male Hu sheep equipped with rumen cannulas were randomly divided into four groups based on dietary FNDF content: low FNDF (L-FNDF, 6.08%), middle low FNDF (ML-FNDF, 9.47%), middle high FNDF (MH-FNDF, 12.48%), and high FNDF (H-FNDF, 15.68%), while the RDS levels (15.65% of DM on average) were similar among the four groups. A replicated 4 × 4 Latin square design was employed in this study. The results indicated that mean and minimum ruminal pH increased linearly with increasing dietary FNDF content, while the duration and area of pH under 5.8 and 5.6, along with the acidosis index, reduced linearly (<i>p</i> ≤ 0.002). There were no differences between the MH-FNDF group and the H-FNDF group in these indicators (<i>p</i> > 0.05). The molar proportions of acetate, butyrate, isobutyrate, and isovalerate, as well as the acetate-to-propionate ratio, increased linearly, while propionate and valerate molar proportions and lactate concentration displayed a linear decrease with increasing FNDF content in the diet (<i>p</i> < 0.001). Increasing dietary FNDF content extended ruminating and chewing time while enhancing ruminal microbial diversity, promoting the proliferation of Fibrobacterota and <i>Butyrivibrio</i> in the rumen, and improving fiber degradability (<i>p</i> < 0.05). When the dietary FNDF content exceeded 12.48%, no effects of FNDF on acetate to propionate ratio and fiber utilization were observed (<i>p</i> > 0.05). The results suggest that augmenting FNDF content in the PTMR can reshape ruminal fermentation towards acetate production and promote rumination to enhance ruminal pH, thereby alleviating the risk of ruminal acidosis. When the RDS content in the PTMR was 15.57%, an FNDF content of 12.48% was optimal for maintaining stable ruminal function in sheep, and the recommended ratio of FNDF to RDS was 0.8.