Effect of biochanin A on the rumen microbial community of Holstein steers consuming a high fiber diet and subjected to a subacute acidosis challenge

Authors: Harlow, Brittany; Flythe, Michael; Klotz, James; HARMON, DAVID - UNIVERSITY OF KENTUCKY; AIKEN, GLEN - UNIVERSITY OF FLORIDA

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2021
Publication Date: 7/21/2021
Citation: Harlow, B.E., Flythe, M.D., Klotz, J.L., Harmon, D.L., Aiken, G.E. 2021. Effect of biochanin A on the rumen microbial community of Holstein steers consuming a high fiber diet and subjected to a subacute acidosis challenge. PLoS ONE. 16(7). Article e0253754. https://doi.org/10.1371/journal.pone.0253754.
DOI: https://doi.org/10.1371/journal.pone.0253754

Interpretive Summary: Subacute ruminal acidosis (SARA) is commonly associated with adaptation to high grain diets and is considered to be the most common ruminal disorder in feedlot cattle. When an adapted animal consumes a diet rich in starch, resident starch-utilizing bacteria proliferate and produce fermentation acids including lactic acid. Ruminal lactate-utilizing bacteria then metabolize lactate into propionate which can be absorbed by the animal to meet energy requirements. Because high-starch diets are rapidly fermented in the rumen, fermentation acids can accumulate leading to a decline in environmental pH (SARA). SARA can result in reduced fermentative efficiency of other dietary nutrients (e.g., fiber), damage to the rumen tissue (rumenitis), increased inflammation, liver abscesses, and laminitis. Dietary ionophore antibiotics (e.g., monensin) have been used in the prevention and treatment of SARA in cattle. Due to the increased concern for the spread of antibiotic resistance, antibiotic alternatives are of interest. Biochanin A (BCA) an isoflavone produced by forage legumes has been shown to reverse the impacts of rumen acidosis in the laboratory. The following experiment was conducted to determine the microbiological impacts of BCA in comparison to monensin in steers subjected to a SARA challenge. Rumen fistulated Holstein steers were assigned to 1 of 4 treatments: high fiber control (HF CON), SARA CON, Monensin, or BCA. The study consisted of a 2-wk adaptation (100% basal diet: corn silage and dried distiller’s grains), a 1-wk high fiber period (100% basal diet), and an 8-d SARA step-up challenge (40% corn diet to 70% corn diet). Rumen contents were sampled over the course of the experiment to determine the number and fermentation of different bacterial functional groups (amino acid/peptide, fiber, starch-utilizing bacteria) and pH. The SARA challenge increased starch- and amino acid/peptide utilizing bacteria leading to an increase in fermentation acids. Consequently, rumen pH declined (SARA) and fiber-utilizing bacteria were inhibited. Both BCA and monensin were effective treatments for reversing the impacts of SARA, but in most cases BCA was more effective. In conclusion, BCA may be an effective antibiotic alternative for mitigating SARA in cattle production systems. The impact of this research is a plant-derived antimicrobial that can be applied in cattle consuming high starch diets, like those in feedlots, to reverse SARA and increase animal health and productivity.

Technical Abstract: Subacute ruminal acidosis (SARA) occurs when highly fermentable carbohydrates are introduced into the diet, decreasing pH and disturbing the microbial ecology of the rumen. Rumen amylolytic bacteria rapidly catabolize starch, fermentation acids accumulate in the rumen and reduce environmental pH. Historically, antibiotics (e.g., monensin, MON) have been used in the prevention and treatment of SARA. Biochanin A (BCA), an isoflavone produced by red clover (Trifolium pratense), mitigates changes associated with starch fermentation ex vivo. The objective of the study was to determine the effect of BCA on amylolytic bacteria and rumen pH during a SARA challenge. Twelve rumen fistulated steers were assigned to 1 of 4 treatments: HF CON (high fiber control), SARA CON, MON (200 mg d-1), or BCA (6 g d-1). The basal diet consisted of corn silage and dried distiller's grains ad libitum. The study consisted of a 2-wk adaptation, a 1-wk HF period, and an 8-d SARA challenge (d 1 – 4: 40% corn; d 5 – 8: 70% cracked corn). Samples for pH and enumeration were taken on the last day of each period (4 h). Amylolytic, cellulolytic, and hyper-ammonia producing bacteria (HAB) were enumerated. Enumeration data were normalized by log transformation and data were analyzed by repeated measures ANOVA using the MIXED procedure of SAS. The SARA challenge increased total amylolytics and HAB, but decreased pH, cellulolytics, and in situ DMD of hay (P < 0.05). BCA treatment counteracted the pH, microbiological, and fermentative changes associated with SARA challenge (P < 0.05). Similar results were also observed with MON, but in most cases BCA was more effective (P < 0.05). These results indicate that BCA may be an effective antibiotic alternative for mitigating SARA in cattle production systems.