Thermodynamics of Rumen VFA Studied
Acetate, propionate, and butyrate make up about 95% of the volatile fatty acids (VFA) in the rumen. In turn, they provide about 75% of total metabolizable energy (ME) for ruminants.
Researchers at Virginia Tech University have investigated what effects, if any, infusing extra acetate or propionate, or lowering the pH, would have on thermodynamics of VFA in the rumen. Their studied, was published recently in the Journal of Dairy Science.
The team wanted to explore rumen VFA thermodynamics to determine whether or not ruminal efficiency could be manipulated and improved. Their work was based on previous research indicating that the stoichiometry of production of the three major VFA determines the amounts of methane (CH4) and carbon dioxide (CO2) that are generated.
That research also noted that the profile of VFA formed in the rumen reflects different fermentation pathways and partially determines the efficiency of energy utilization. They also cited previous work indicating that thermodynamics control which pathway branches are available during ruminal fermentation, and thus the final VFA produced.
Their in vitro analysis started using ruminal fluid as a base substrate. Using dual-flow continuous culture, included 4 study groups:
- Control – pH buffered to an average of 6.75
- Control plus 20 mmol/day of infused acetate
- Control plus 7mmol/day of infused propionate; and
- A 0.5-unit decline in pH, created by adjustment of the buffer
All four fermenters were fed 40 grams a daily “diet” of a 50/50 mix of whole alfalfa pellets and concentrate mix pellets.
After 7 days of this feeding routine, sequential, continuous infusions were fed to all four batches, containing a consistent mix of sodium acetate (3.5 mmol/day); sodium propionate (2.9 mmol/day); and sodium butyrate (0.22 mmol/day).
Filtered, liquid effluent was extracted and evaluated from all 4 fermentation chambers 8 times per day. A larger filtered sample was drawn to quantify aqueous concentrations of hydrogen and methane. Headspace hydrogen and methane also were monitored continuously.
Significant results included:
- The digestibility of neutral detergent fiber (NDF), acid detergent fiber (ADF), and starch in continuous cultures was not affected by low pH, acetone infusions, or propionate infusions.
- Microbial carbon sequestered from VFA was not affected by any treatment.
- Reduction in pH increased net propionate production and decreased emissions of hydrogen and methane.
- VFA interconversions were not affected by any treatment.
Ultimately, the Virginia Tech researchers concluded that thermodynamics might not be a primary influencer of metabolic pathways used for VFA formation.
