Effects of sulfur and cyanide-utilizing bacteria in fermented total mixed ration containing fresh cassava root on rumen fermentation in Thai beef cattle

This study investigated the effects of sulfur supplementation and cyanide-utilizing ruminal bacteria (CUB) on nutrient digestibility, rumen fermentation, and blood urea-nitrogen (BUN) in Thai native beef cattle fed fermented total mixed rations (FTMR) containing fresh cassava root. Two CUB strains, <italic>Enterococcus gallinarum</italic> KKU-BC10 and <italic>Enterococcus faecium</italic> KKU-BF7, were isolated from the rumen based on their hydrogen cyanide (HCN) degrading capacity. Four dietary treatments were prepared by formulating FTMR with 40% rice straw and 40% fresh cassava root (dry matter (DM) basis), supplemented with other concentrate ingredients, and differing by additive: (1) 1% sulfur, (2) 2% sulfur, (3) <italic>E. gallinarum</italic> KKU-BC10, or (4) <italic>E. faecium</italic> KKU-BF7. Each ration was fermented anaerobically for seven days before feeding. Four male Thai native beef cattle (2.5 years old; 222 ± 12.0 kg initial body weight) were allocated to the four treatments in a 4 × 4 Latin square design, with each animal receiving all four treatments across the four experimental periods. All FTMR treatments effectively reduced HCN to safe levels for ruminant feeding. Specifically, HCN content in the FTMR dropped from ~60–84 mg/kg DM before fermentation to ~41–52 mg/kg DM after 7 days, remaining well below the 100 mg/kg DM safety threshold for cattle. No significant differences were observed in dry matter intake, nutrient digestibility, or total volatile fatty acid concentrations (<italic>p</italic> > 0.05). However, fiber intake was enhanced by microbial inoculation. Cattle receiving <italic>E. gallinarum</italic> KKU-BC10 showed the highest neutral detergent fiber intake (<italic>p </italic>< 0.05). In contrast, <italic>E. faecium</italic> KKU-BF7 supplementation resulted in the highest acid detergent fiber intake (<italic>p</italic> < 0.01). Notably, cattle fed <italic>E. faecium</italic> KKU-BF7 also had significantly lower blood urea-nitrogen concentrations at 4 hours post feeding (<italic>p</italic> = 0.04), indicating improved nitrogen utilization. Rumen pH and ammonia–nitrogen levels remained within physiological ranges across all treatments. These findings confirm that sulfur and CUB supplementation in FTMR supports the safe use of fresh cassava root in ruminant diets. Moreover, <italic>E. faecium</italic> KKU-BF7 shows potential to enhance nitrogen efficiency beyond detoxification.