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MEKARN Conference 2010 |
Rumen fermentation results in the production of a pool of reduced cofactors in the pathways of catabolism of organic matter within rumen microbes. These reduced cofactors are regenerated by 1) synthesis of microbial cells, 2) production of more reduced end products such as propionate and 3) saturation of unsaturated long chain fatty acids but by far the largest proportion of reduced cofactor appear to be regenerated by 4) formation of hydrogen. The concentration of hydrogen in rumen fluid negatively feeds back on the rate of fermentation and microbial growth. The rumen ecosystem has evolved to remove this hydrogen through growth of Archae that obtain energy for their growth by reducing carbon dioxide to methane and water in the rumen.
Ruminal methanogenesis represents a loss of dietary energy to the animal and it is a significant greenhouse gas. Ruminants are credited with a large proportion of the methane accumulating in the world’s atmosphere and a high proportion of the radiative heat forcing of greenhouse gases that have accumulated in the atmosphere. These factors have led to a global search for strategies (including nutrional intervention) to mitigate methane emission from ruminants.
The possibility of nitrate as a fermentable N source for rumen organisms and there for as an alternative hydrogen sink to carbon dioxide, has been only tentatively explored.
J. Sokolowski , a masters student in the University of Illinois, Urbana showed as early as 1959 that nitrate could be used quite safely as a fermentable N source in the diet of growing sheep without any detrimental effects. The use of nitrate as a hydrogen sink has been down played, due to the possible toxic effects of nitrite that under some circumstances is formed as an intermediate during the reduction of nitrate to ammonia in the rumen. A few reports have examined the potential of nitrate in vitro as methane reducing feed additive, which appears to lower methanogenesis consistently.
The rumen ecosystem and the animal need time to adapt to any fermentable N source including urea and nitrate. The requirements for different dietary conditions appear to be important in determining the microbial consortiums that use these fermentable N sources.
The potential of nitrate to act as a hydrogen sink in the rumen was reviewed ( Leng 2008)with a clear indication emerging that it was entirely feasible that nitrate could be used as a fermentable N source by ruminants provided the rumen ecosystem was allowed to adapt over a sufficiently long period and provided certain nutritional conditions were met. In particular the availability of sulphur in the diet appeared to be a crucial issue.
Information derived from other microbial anaerobic ecosystems showed that in the presence of a fermentable organic matter and a source of sulphur and nitrate, nitrate reducing organisms developed that can both reduce nitrate to ammonia and oxidize sulphide to sulphate ( NRSOB) without release of nitrite. At least one prominent rumen organism has this capacity (Wollinella succinogenese) and may be termed an NRSOB.
The report has stimulated a number of schools to re-examine the potential of nitrate as a fermentable N source that mitigates rumen methane. These include studies from Mekarn mostly using ruminants fed low protein agro industrial byproducts. These groups have been the first to show that nitrate can be fed to ruminants safely under practical conditions and maintain or increase production. Two other groups, with the facilities to measure methane production, have concentrated on the extent to which nitrate lowers methane production. The results from these studies indicate
Nitrate can substitute for urea as a fermentable N source when fed following an adaptation period. The discipline oriented work has demonstrated stoichiometric reduction of methane in ruminants fed increasing levels of nitrate. Approximately 1 mole of nitrate in a diet reduces methane production by 10% and there is evidence of an interaction with dietary sulphur levels
Future research on nitrate as a fermentable N source for ruminants requires a discipline oriented research to
Establish the best combination of nutrients to allow the rumen ecosystem to evolve to use nitrate,
Major microbiological studies to establish the characteristics of the ecosystem