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Ruminal Fermentation of Anti-Methanogenic Nitrate- and Nitro-Containing Forages In Vitro

Robin C. Anderson, Laura H. Ripley, Jan G. P. Bowman, Todd R. Callaway, Kenneth J. Genovese, Ross C. Beier, Roger B. Harvey, David J. Nisbet
Frontiers in Veterinary Science 2016 v.3 no. pp. 62
Astragalus, Denitrobacterium detoxificans, acetates, alfalfa, amino acids, ammonia, butyrates, diet, emissions, fermentation, forage, formates, hydrogen, isobutyric acid, methane, methane production, methanogens, nitrates, rumen bacteria, rumen fermentation, ruminants, volatile fatty acids
Nitrate, 3-nitro-1-propionic acid (NPA), and 3-nitro-1-propanol (NPOH) can accumulate in forages and be poisonous to animals if fed at high enough amounts. These chemicals are also recognized as potent anti-methanogenic compounds, but plants naturally containing these chemicals have been studied little in this regard. Presently, we found that nitrate-, NPA-, or NPOH- containing forages effectively decreased methane production, by 34 to 98%, during in vitro fermentation by mixed populations of ruminal microbes compared to fermentation by populations incubated similarly with alfalfa. Methane production was further decreased, although not necessarily significantly, during incubation of populations inoculated with Denitrobacterium detoxificans, a ruminal bacterium known to reductively metabolize nitrate, NPA, and NPOH. Inhibition of methanogen populations was greatest with the NPA- and NPOH-containing forages. Hydrogen accumulated in all ruminal populations incubated with forages containing nitrate, NPA, or NPOH, but was dramatically higher, exceeding 40 µmol hydrogen/mL, in non D. detoxificans-inoculated populations incubated with NPA-containing milkvetch. This possibly reflects the inhibition of hydrogenase-catalyzed uptake of hydrogen produced via conversion of the added 50 µmol formate/mL to hydrogen. Accumulations of volatile fatty acids revealed compensatory changes in fermentation in the nitrate-containing and NPA- and NPOH-containing forages as evidenced by lower accumulations of acetate, and in some cases higher accumulations of butyrate. Accumulations of ammonia, iso-buytrate, and in some cases iso-valerate, were also lower in incubations with nitrate-, NPA-, and NPOH-containing forages than with alfalfa, thus indicating a potential alteration of amino acid fermentation. Results reveal that nitrate, NPA, and NPOH that accumulate naturally in forages can be made available with ruminal incubations to inhibit methanogenesis. Further research is needed to determine if diets can be formulated with nitrate-, NPA-, and NPOH-containing forages to achieve efficacious mitigation in ruminant methane emissions without adversely affecting fermentative efficiency.