Abstract
Nitrate, 3-nitro-1-propionic acid (NPA) and 3-nitro-1-propanol (NPOH) can accumulate in forages and be poisonous to animals if consumed in 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 35–87%, during in vitro fermentation by mixed cultures of ruminal microbes compared to fermentation by cultures incubated similarly with alfalfa. Methane production was further decreased during the incubation of mixed cultures also inoculated with Denitrobacterium detoxificans, a ruminal bacterium known to metabolize nitrate, NPA, and NPOH. Inhibition of methanogens within the mixed cultures was greatest with the NPA- and NPOH-containing forages. Hydrogen accumulated in all the mixed cultures incubated with forages containing nitrate, NPA or NPOH and was dramatically higher, exceeding 40 μmol hydrogen/mL, in mixed cultures incubated with NPA-containing forage but not inoculated with D. detoxificans. This possibly reflects the inhibition of hydrogenase-catalyzed uptake of hydrogen produced via conversion of 50 μmol added formate per milliliter to hydrogen. Accumulations of volatile fatty acids revealed compensatory changes in fermentation in mixed cultures incubated with the nitrate-, NPA-, and NPOH-containing forages as evidenced by lower accumulations of acetate, and in some cases, higher accumulations of butyrate and lower accumulations of ammonia, iso-buytrate, and iso-valerate compared to cultures incubated with alfalfa. Results reveal that nitrate, NPA, and NPOH that accumulate naturally in forages can be made available within ruminal incubations to inhibit methanogenesis. Further research is warranted 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 or risking toxicity to animals.
Highlights
Nitrate and the naturally occurring nitrocompounds, 3-nitro-1-propionic acid (NPA) and 3-nitro1-propanol (NPOH), are oxidized nitrogen compounds that can accumulate to toxic levels in certain forages and feedstuffs
NPA, and NPOH are recognized as potent inhibitors of ruminal methane production, a microbiological process that can result in the loss of 4–12% of the gross energy consumed by concentrate- or forage-fed cattle, respectively [3]
Rates of nitrate reduction could far exceed the rates of nitrite reduction as the mixed cultures adapted, potentially allowing nitrite to accumulate to inhibitory concentrations in the mixed cultures in experiment 1, for the mixed cultures incubated with the high nitrate-containing barley where higher nitrite accumulations could have persisted for a longer duration than in mixed cultures incubated with the other forages
Summary
Nitrate and the naturally occurring nitrocompounds, 3-nitro-1-propionic acid (NPA) and 3-nitro1-propanol (NPOH), are oxidized nitrogen compounds that can accumulate to toxic levels in certain forages and feedstuffs. Poisoning by NPA and NPOH occurs differently, via inactivation of cellular succinate dehydrogenase activity, which inhibits cellular respiration [2]. NPA, and NPOH are recognized as potent inhibitors of ruminal methane production, a microbiological process that can result in the loss of 4–12% of the gross energy consumed by concentrate- or forage-fed cattle, respectively [3]. Production of methane from ruminant sources contributes nearly 20% of the total U.S emissions of methane, which is a potent greenhouse gas [4], and strategies are sought to reduce the economic and environmental impact of this digestive inefficiency
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