Supplementing ruminants with dietary nitrate (NO₃) is an effective methane mitigation strategy if it can be managed so as to not expose ruminants to any risk of clinical nitrite (NO₂) toxicity. The objective of this thesis was firstly to deepen the understanding for NO₃ metabolism in sheep and secondly to develop practical strategies to reducing risk of NO₂ toxicity in sheep supplemented with dietary NO₃.

It has been previously established, that in the rumen NO₃ is reduced to NO₂ and then to NH₃, and that supplementing with excessive amounts of NO₃ can expose ruminants to NO₂ toxicity due to the absorption of NO₂. This thesis reports a series of five investigations of NO₃ metabolism by sheep and identifies:

Nitrate, like urea, is ‘recycled’ within the ruminant. Transfer of ruminal ¹⁵NO₃^--N into the blood and transfer of blood NO₂-N into the rumen being quantified. Only 20% of rumen NO₃^-and 30% of blood NO₂^- were recovered in urine.

That in hourly fed sheep approximately 90% of dietary NO₃^- was rapidly converted to NH₃ in the rumen, with the remainder leaving the rumen by absorption into the bloodstream or passage to the lower gastro-intestinal tract.

Within the rumen, the conversion of NO₃^-to NH₃ is neither simple nor complete. In vitro and in-vivo studies showed NO₃^-is reduced to gaseous nitrous oxide (N₂O) and N₂O may be further metabolised to N₂ gas by the rumen microbiota. Approximately 0.04% and 3.0% of dosed NO₃^--N was recovered over 10 h from sheep as N₂O and N₂ respectively, and this was not affected by whether sheep had prior adaption to NO₃^- or not, identifying denitrification as a reaction not previously reported from the rumen.

From this understanding and a review of the literature on ruminant NO₃ metabolism, eight critical control points for reducing the risk of nitrite toxicity (methaemoglobinaemia), were identified and the potential for manipulating five of these evaluated.

Reducing the rate at which NO₃ became available to the rumen biota by coating calcium nitrate with paraffin wax significantly reduced blood methaemoglobin level (MetHb; an indicator of NO₂ toxicity) in sheep supplemented with NO₃.

The extent of methaemoglobinaemia could also be reduced by the daily ration being consumed at shorter intervals rather than in a single bout, and this established that feed management is pivotal to safe feeding of NO₃^-containing diets.

Enhancing the rumen’s capacity to reduce potentially toxic NO₂ ^-by supplying Propionibactericum acidicpropionici as a direct fed microbial was ineffective in reducing blood MetHb or NO₂^-concentration of sheep fed NO₃^- supplemented diets.

Attempts to increase the rate of removal of NO₂^-from the rumen by providing a substrate (glycerol) to stimulate NADH availability in the rumen, and accelerate the nitrite reductase enzyme system did not reduce the concentration of NO₂ in incubations of rumen contents supplemented with NO₃^-.

We found no evidence that adapting sheep to dietary NO₃^- protected them against NO₂^- toxicity. Indeed, in vitro more NO₂^- accumulated in incubation when donors where adapted to dietary NO₃^-. Also, no signs of reduced MetHb were noticed after several weeks of NO₃^-supplementation in vivo.

Other critical control points such as regulating microbial uptake of NO₃ and ruminal absorption of NO₃ and NO₂ were unable to be assessed in this thesis.

The studies reported here also confirmed the practical impacts of NO₃ as an effective supplement for reducing enteric methane emissions and increasing wool growth of sheep. As well as providing a better understanding of NO₃^-metabolism, studies also showed that the greenhouse gas (GHG) abatement impact of methane mitigation may be partly offset by an associated production of the potent GHG, N₂O. Discovery of the production of N₂O and N₂ from NO₃^-in the rumen and identification of recycling of blood NO₂^- to the rumen has expanded our understanding of NO₃^-metabolism. Coating NO₃^-to decrease the rapidity of NO₃^- release in the rumen as a strategy to reduce NO₂ toxicity was effective but needs further investigation. The applicability of feed grade NO₃^-as a commercially available feed additive will also depend on the cost of NO₃ and the additional cost of the technology to ensure its safe feeding, compared to the cheaper alternative non-protein nitrogen source, urea.

Odour emissions have been identified as a potential threat for the sustainable development of the broiler industry. Few of the methods examined to reduce odour emissions from poultry operations have proved effective. The methods including biofilters, litter treatments, neutralising agents, air scrubbers, ozone treatment are either impractical to apply or too costly to use in commercial farms. If diets could be formulated to more closely meet nutrient requirements, there would be reduced excretion of undigested components and lower level of substrates available for microbes to metabolize them to odorous compounds. Diet affects water intake, water to feed intake ratio, litter moisture, litter pH and litter water activity all of which may have an impact on the emission of odorants from the litter. This thesis details the role of diet composition, necrotic enteritis and litter condition on odour emission from broiler production. This thesis includes a comprehensive review on key odorants from broiler production, their origin, analytical techniques for odour measurements and nutritional factors affecting odour emissions. The five chapters that follow investigate the role of phytase enzyme (chapter 2), protein sources (chapter 3), protein levels (chapter 4), probiotic and saponin (chapter 4), litter condition (chapter 4 and 5), necrotic enteritis and high sodium diet (chapter 5) and the effect of Clostridium perfringens culture (chapter 6) on odour emissions. Nutritional strategies such as increasing meat meal in the diet, decreasing dietary protein levels and the addition of a Bacillus subtilis based probiotic and saponin blend may lower odour emission from broiler production. The results also indicate that odour is related to litter condition and if litter moisture content and water activity can be reduced and necrotic enteritis prevented, odour problems can be alleviated in broiler sheds.