354 Increasing biochanin A inclusion influences in vitro ruminal fermentation and microbial populations of tall fescue hay.

Abstract Alterations in tryptophan metabolism and serotonergic processes have recently been implicated in the onset of tall fescue toxicosis in cattle, with ergot alkaloid ingestion resulting in decreased serum serotonin concentration. Biochanin A (BCA), an isoflavone produced in legumes, has been documented to influence ruminal fermentation by inhibiting hyper-ammonia producing bacteria. Therefore, the hypothesis of this experiment was that biochanin A would inhibit ruminal tryptophan-utilizing bacteria in vitro. The objectives of this study were to evaluate the effects of increasing BCA levels on digestibility, ruminal fermentation, and microbial populations associated with N digestion in fescue-based diets using an in vitro fermentation model. Ruminal contents were obtained from Angus × Holstein steers (body weight = 390 ± 4.49 kg; n=2) fed a diet of 90% tall fescue hay and 10% cracked corn. Treatments included: basal substrate (negative control; 0 mg/g DM BCA), basal substrate + monensin (positive control; 1.5 or 15 mg/g DM), and basal substrate + BCA (2.5, 5.0, 10, 20 mg/g DM). The basal substrate was a mixture of 80% tall fescue hay and 20% ground corn and the fermentation vessels were incubated for 48 h. Data were analyzed using the GLM procedure of SAS and contrasts were used to assess treatment effects. The apparent dry matter (DM) digestibility, true DM digestibility, and neutral detergent fiber digestibility were lesser (P< 0.03) for monensin-containing diets compared with negative control or BCA-containing diets. Biochanin A inclusion decreased (P< 0.04) the rate of gas production and linearly decreased cumulative gas production. Total volatile fatty acid concentration decreased linearly (P=0.02) with increasing BCA inclusion. Monensin increased (P< 0.002) the molar propionate proportion compared with negative control and BCA treatments. Increasing BCA inclusion linearly increased (P< 0.02) molar branched-chain fatty acid proportions (valerate, isovalerate, isobutyrate) in the fermentation media. Monensin inclusion decreased (P< 0.001) cellulolytic, peptide- and amino acid-utilizing, hyper-ammonia producing, and tryptophan-utilizing bacterial concentration in the fermentation media compared with negative control. Cellulolytic bacterial and peptide and amino acid-utilizing bacterial concentrations responded quadratically (P< 0.001) to increasing BCA inclusion, with nadirs observed at 10 mg BCA/g DM. Hyper-ammonia producing bacteria and tryptophan-utilizing bacteria responded quadratically (P< 0.001) to increasing BCA inclusion, decreasing up to 2.5 mg BCA/g DM and plateauing thereafter. This experiment demonstrated that dietary BCA inclusion altered microbial populations associated with ruminal fiber and protein fermentation similarly to monensin. Decreased viable tryptophan-utilizing bacterial concentration with monensin or BCA inclusion might suggest that these antimicrobial compounds could potentially increase ruminal tryptophan availability in cattle consuming tall fescue-based diets. More research is needed to investigate the effects of BCA inclusion on ruminal tryptophan utilization in vivo to better understand potential implications for reducing the negative effects of fescue toxicosis on ruminant livestock production.