Inclusion of nitrate (NO₃⁻) in ruminant diets is a means of increasing non-protein nitrogen intake while at the same time reducing emissions of enteric methane (CH₄) and, in Australia, gaining carbon credits. Rumen microorganisms contain intracellular enzymes that use hydrogen (H₂) released during fermentation to reduce NO₃⁻ to nitrite (NO₂⁻), and then reduce the resulting NO₂⁻ to ammonia or gaseous intermediates such as nitrous oxide (N₂O) and nitric oxide (NO). This diversion of H2 reduces CH₄ formation in the rumen. If NO₂⁻ accumulates in the rumen, it may inhibit growth of methanogens and other microorganisms and this may further reduce CH4 production, but also lower feed digestibility. If NO₂⁻ is absorbed and enters red blood cells, methaemoglobin is formed and this lowers the oxygen-carrying capacity of the blood. Nitric oxide produced from absorbed NO₂⁻ reduces blood pressure, which, together with the effects of methaemoglobin, can, at times, lead to extreme hypoxia and death. Nitric oxide, which can be formed in the gut as well as in tissues, has a variety of physiological effects, e.g. it reduces primary rumen contractions and slows passage of digesta, potentially limiting feed intake. It is important to find management strategies that minimise the accumulation of NO₂⁻; these include slowing the rate of presentation of NO₃⁻ to rumen microbes or increasing the rate of removal of NO₂⁻, or both. The rate of reduction of NO₃⁻ to NO₂⁻ depends on the level of NO₃⁻ in feed and its ingestion rate, which is related to the animal's feeding behaviour. After NO₃⁻ is ingested, its peak concentration in the rumen depends on its rate of solubilisation. Once in solution, NO₃⁻ is imported by bacteria and protozoa and quickly reduced to NO₂⁻. One management option is to encapsulate the NO₃⁻ supplement to lower its solubility. Acclimating animals to NO₃⁻ is an established management strategy that appears to limit NO₂⁻ accumulation in the rumen by increasing microbial nitrite reductase activity more than nitrate reductase activity; however, it does not guarantee complete protection from NO₂⁻ poisoning. Adding concentrates into nitrate-containing diets also helps reduce the risk of poisoning and inclusion of microbial cultures with enhanced NO₂⁻ - reducing properties is another potential management option. A further possibility is to inhibit NO₂⁻ absorption. Animals differ in their tolerance to NO₃⁻ supplementation, so there may be opportunities for breeding animals more tolerant of dietary NO₃⁻. Our review aims to integrate current knowledge of microbial processes responsible for accumulation of NO₂⁻ in rumen fluid and to identify management options that could minimise the risks of NO₂⁻ poisoning while reducing methane emissions and maintaining or enhancing livestock production.

One of the major effects of climate change is disruption in normal weather patterns, especially an increase in long-term annual temperature, and more frequent and intense droughts and floods. These changes have impact on the natural resource base that includes plants, animals and biodiversity. Consequently, this diminishes feed and water resources which livestock depend on to survive and therefore impacting negatively on food security and household incomes of smallholder livestock producers and pastoralists, the majority of whom are found in the tropics. Efficient utilization of available feed resource by ruminants, most of it being high in fibre and low in protein content is often constrained by low digestibility and inefficient metabolism of absorbed nutrients at the tissue metabolic level. The low digestibility of high fibre forage in ruminants is mainly attributed to a high level of lignification and a deficiency of essential nutrients, especially nitrogen (N) and sulphur (S) that are required by rumen microbes for optimal growth. Furthermore, the absorbed nutrients also tend to be imbalanced in the ratio of protein to energy and/or acetogenic to glucogenic substrates. As a result the intake of high fibre forages in ruminants is often associated with a significant loss of feed energy as heat increment and methane (CH4) gas production, with the later also contributing significantly to global warming through greenhouse gas emissions. This review gives an overview of the various strategies in the form of treatment and supplementation that have been shown to improve digestion and intake of high fibre forages in ruminants, and also reducing CH4 gas production. The role of rumen degradable nutrients as well as by-pass nutrients in enhancing digestion and absorption of nutrients that are balanced in protein: energy ratio and/or acetogenic: glucogenic substrates is also reviewed and suggested as one way of increasing metabolic efficiency of absorbed nutrients at the tissue level to reduce heat increment. The role of glucogenic substrates such as propionate and protein/amino acids in ensuring an adequate supply of reducing equivalents in the form of reduced nicotinamide adenine dinucleotide phosphate (NADPH) that is required for the conservation of excess acetate as fat in the adipose tissue and also for regeneration of oxaloacetate for efficient VFA energy metabolism in the body tissues is also reviewed. It is concluded that a multipronged approach combining treatment with supplementation with cheap and locally available rumen degradable nutrients (e.g. molasses-urea liquid mixture and the urea-molasses-mineral based multi-nutrient block) and bypass nutrients that are compatible with low input ruminant production systems holds the key to increasing efficiency in the utilization of high fibre-low protein forage in ruminants. This can play a major role in increasing the capacity of smallholder livestock producers and pastoralists in most parts of the tropics to adapt and therefore mitigate the adverse effects of climate change.

This study was designed to investigate whether delaying the age of weaming, or feeding a protein-rich supplement alters the rate at which lambs develop immunity to Haemonchus contortus and whether there is any interaction between nutrition, stress of weaning, and gander and the development of this immunity. Ninety-six Merino lambs were allocated to one of four treatmeat groups: supplemented-unweaned; supplemented-weaned; unsupplemented-unweaned; unsupplemented-weaned. There were approximately similar numbers of male and female lambs in each group. Supplemented lambs received 80 g/head/day of a protein-rich pellet from 16 to 23 weeks of age. Over the same period the lambs were drenched with 300 H. contortus larvae twice per week. Faecal worm egg counts were determined every week, and PCV and liveweight every 2 weeks for each lamb. Neither weaning nor sex had any effect on PCV (P>0.05) but from day 50 after the start of infection, the decline in PCV was more pronounced in unsupplemented than in supplemented lambs. Faecal werm egg counts were higher (P < 0.001) in unsupplemented-weaned than in supplemented-weaned lambs and in females when compared with castrates (P < 0.03). There was a weaning X supplementation X age interaction (P < 0.03) with unsupplemented-weaned lambs developing a higher faecal egg count than supplemented-weaned lambs. There was also a significant (P < 0.02) sex X weaning X age interaction with weaned-female lambs having a higher faecal egg count than weaned-castrate lambs; egg counts increased more rapidly in unweaned-female lambs than in weaned-female lambs. Tegether, these data suggest that protein supplementation of lambs enhanced the development of immunity to haemonchosis, whereas weaning at 4 months of age had no significant effect.

Methane is a potent greenhouse gas. In Australia, only about 11% of national emissions of methane are produced by sheep and cattle, but enteric methane emissions attract media attention and a disproportionate and undeserved amount of blame for global warming is directed at ruminant livestock. Media fascination with 'animal farts' and enteric methane has in turn helped to generate negative social perceptions of the red meat industries. Some strategies for reducing enteric methane emissions and arguments that could help redress these negative perceptions are included in an Appendix to this report. After 2015, calculations of methane emissions from ruminants may be included in carbon accounting schemes and, if this occurs, will place financial imposts on the grazing industry. Proposed methodologies that could be used to measure methane emissions from livestock in Australia are considered along with inventory systems in overseas countries. Current knowledge of the microbiology and biochemistry of the rumen and the reasons why methane is produced by ruminants are reviewed, along with proven and potential ways to reduce methane outputs at an individual animal, or farm, or national level. Currently available strategies that reduce methane emissions are described, along with potential strategies that need further research and development.

In Australia, agriculture is responsible for ~17% of total greenhouse gas emissions with ruminants being the largest single source. However, agriculture is likely to be shielded from the full impact of any future price on carbon. In this review, strategies for reducing ruminant methane output are considered in relation to rumen ecology and biochemistry, animal breeding and management options at an animal, farm, or national level. Nutritional management strategies have the greatest short-term impact. Methanogenic microorganisms remove H₂ produced during fermentation of organic matter in the rumen and hind gut. Cost-effective ways to change the microbial ecology to reduce H₂ production, to re-partition H₂ into products other than methane, or to promote methanotrophic microbes with the ability to oxidise methane still need to be found. Methods of inhibiting methanogens include: use of antibiotics; promoting viruses/bacteriophages; use of feed additives such as fats and oils, or nitrate salts, or dicarboxylic acids; defaunation; and vaccination against methanogens. Methods of enhancing alternative H₂ using microbial species include: inoculating with acetogenic species; feeding highly digestible feed components favouring 'propionate fermentations'; and modifying rumen conditions. Conditions that sustain acetogen populations in kangaroos and termites, for example, are poorly understood but might be extended to ruminants. Mitigation strategies are not in common use in extensive grazing systems but dietary management or use of growth promotants can reduce methane output per unit of product. New, natural compounds that reduce rumen methane output may yet be found. Smaller but more permanent benefits are possible using genetic approaches. The indirect selection criterion, residual feed intake, when measured on ad libitum grain diets, has limited relevance for grazing cattle. There are few published estimates of genetic parameters for feed intake and methane production. Methane-related single nucleotide polymorphisms have yet to be used commercially. As a breeding objective, the use of methane/kg product rather than methane/head is recommended. Indirect selection via feed intake may be more cost-effective than via direct measurement of methane emissions. Life cycle analyses indicate that intensification is likely to reduce total greenhouse gas output but emissions and sequestration from vegetation and soil need to be addressed. Bio-economic modelling suggests most mitigation options are currently not cost-effective.

This study aimed to determine the dose-dependent relationship between oral doses of fenbendazole (FBZ) and the plasma concentration of its metabolites, oxfendazole (OFZ) and fenbendazole-sulphone (SUL). Twenty five, two year-old, Merino wethers were equally allocated to treatment groups of different oral dose rates of FBZ (n = 5) and housed in individual pens. Treatment groups were designed to provide daily oral doses of 0.25, 0.5, 1.0, 2.0 and 4.0 mg/kg live weight of FBZ, suspended in water, for six days. Blood samples were collected from each animal at 48, 96, and 144 hours after administration of FBZ. Plasma was equally combined within each animal and analysed to determine concentrations of FBZ, OFZ and SUL. There was a positive linear relationship between FBZ dose rate and FBZ metabolite plasma concentration (R² = 0.991, P <0.001). Mean separation of plasma concentrations indicated significant differences (P <0.05) between treatments designed to provide 1.0, 2.0 or 4.0 mg/kg/day FBZ. Plasma concentrations of animals which received 0.25 or 0.50 mg/kg/day FBZ were significantly lower than other treatments (P <0.05). The results from this experiment provide preliminary support for the investigation of FBZ as a useful marker to estimate supplement intake of grazing animals.

Nitrate (NO₃⁻) supplementation is a promising methane mitigation strategy for ruminants, but can cause nitrite (NO₂⁻) poisoning. Because some nitrite reductases are NADH-dependent, we hypothesised that replacing glucose with glycerol would increase the NADH yield and so enhance nitrite reductase activity and reduce ruminal NO₂⁻ accumulation and toxicity risk. We also hypothesised that adapting sheep to dietary NO₃⁻ would limit the accumulation of NO₂⁻ when NO₃⁻ was added to rumen fluid. Changes in NO₃⁻ and NO₂⁻ catabolism and CH₄ production, resulting from supplementation with glycerol to enhance NADH supply, were studied in vitro. In Experiment 1, rumen fluid from sheep adapted to dietary NO₃⁻ (2% of DM intake) or urea (1.1% of DM intake) was incubated with NO₃⁻ or urea, respectively. Additionally, ground oaten hay was added to incubations alone (control), or with glucose or glycerol. In Experiement 2, sheep were adapted for 9 weeks to dietary NO₃⁻ or urea. Nitrate (2% NO₃⁻ of substrate DM) was added to incubated digesta from NO₃⁻ - or urea-supplemented sheep, while urea (1.1% of substrate DM) was added to digesta from urea-supplemented sheep. In both studies, triplicate incubations were terminated at nine time points up to 24 h. Methane emissions were lower in all NO₃⁻ treatments (P < 0.05). Contrary to our hypotheses, both glycerol supplementation (Experiment 1) and prior adaptation to NO₃⁻ (Experiment 2) increased NO₂⁻ accumulation. In Experiment 1, there was no difference in ruminal NO₂⁻ concentration between the unsupplemented control and added glucose treatments. Nitrous oxide accumulated in NO₃⁻ treatments only with rumen fluid from sheep adapted to dietary urea (P < 0.05). In summary, NO₂⁻ accumulation in vitro was not reduced by adaptation to NO₃⁻ or by glucose or glycerol supplementation, disproving the hypotheses regarding the role of NADH availability and of NO₂⁻ adaptation in reducing ruminal NO₂⁻ accumulation and toxicity risk.

In a previous study, we showed that two new cultivars of triticale, Bogong and Canobolas, developed by a crop-breeding group at the University of New England, Australia, were superior to wheat in nutritive value for broiler chickens. In this follow-up study we investigated the benefits of further supplementation of diets containing these cultivars with xylanase and phytase (Econase XT and Quantum XT, AB Vista, UK), individually or in combination. The diets contained 65 % triticale and were fed from hatch to 21 days of age. Data were analysed by the General Linear Model of Minitab. Feed intake to day 7 and 21d on supplemented Bogong diets (combined xylanase and phytase or Phytase alone) was higher (P<0.05) than on the control Bogong diet, while for the diets with Canobolas, xylanase and phytase ensured higher (P<0.05) feed intake than on the other Canobolas diets. Live weight at7 and 21d on the diets was improved (P<0.05) by xylanase and phytase in combination or by phytase only. Feed conversion ratio was similar between the diets. The digestibility of crude protein, gross energy, starch, calcium and phosphorus on all enzyme-supplemented diets was improved by enzyme supplementation. There were no major differences between the cultivars. The results indicate a positive response to the tested enzyme supplements. Further studies will be conducted into the effects of these treatments on intestinal microbial profiles and digestive enzyme activities.

To investigate the impact of nutritional and environmental factors on bacteriophage activity in the rumen, it is first valuable to determine the extent of natural variations and fluctuations in phage populations from different animal species, and from animals located together and separately, and variation in animals over time. Differences in phage populations between sheep on different diets, between sheep and gnats, and within the rumen over time were investigated by using pulsed-field gel electrophoresis and comparing total phage DNA in ruminal fluid. It was found that no two individuals had similar DNA banding patterns, even when similarly fed and penned together, indicating there is considerable individual diversity in phage populations between animals. Despite these individual differences, the quantities, but not the banding patterns, of phage DNA were similar for animals within groups but varied between groups, suggesting that nutritional factors may influence overall phage activity in the rumen. In sheep fed once daily, a distinct diurnal variation in the phage population was observed. Two hours postfeeding, total phage DNA dropped to its lowest level. The phage population then increased, reaching a maximal level 8 to 10 h postfeeding before declining over the next 4 h to reach a stable concentration for the rest of the cycle. The general trend in phage DNA concentration appeared similar to previously recorded diurnal fluctuations in ruminal bacterial populations in cattle fed once daily.

Inclusion of nitrate in the diet of ruminants has been effective in reducing enteric emission of the greenhouse gas methane, but increases the risk of nitrate toxicity in the animal. An experiment was conducted to investigate if coating nitrate salts with lipid would reduce risks of nitrite toxicity in sheep without compromising the methane mitigating effect achieved using uncoated nitrate. Three forms of nitrate (uncoated nitrate; coated with palm oil or coated with paraffin wax) were administered intraruminally to sheep, with nitrate toxicity risk being evaluated by determining blood methaemoglobin (MetHb) levels. Nitrate and nitrite concentrations in plasma and rumen fluid, as well as methane and nitrous oxide production were also evaluated. Sheep supplemented with isonitrogenous amounts of urea were used as negative controls. There was no significant effect of palm oil coating on MetHb but coating with paraffin wax lowered MetHb levels, rumen and plasma nitrate concentrations (P < 0.05) relative to concentrations in urea-supplemented sheep. Total VFA concentrations in rumen fluid were unaffected by coating nitrate, but acetate proportion increased while butyrate and propionate proportions declined over time in all treatments after intraruminal nitrate administration (P < 0.05). It is suggested that these changes were caused by the strong capacity of nitrate to act as an electron acceptor. There was substantial variation between animals in ruminal nitrate and nitrite concentrations and in blood MetHb when the same mass of nitrate was administered directly into the rumen, showing that individuals differ in their ability to metabolize nitrate. Whereas methane production over the 22 h period of measurement was unaffected by the treatments, methane production during the first 3 h of measurement post-feeding was reduced similarly by both coated and uncoated nitrate supplements compared to urea. The small amount of supplemented nitrate introduced and the rapidity of nitrate reduction may both explain why methane mitigation was only observed for a short period after administering the treatments. Over 22 h in respiration chambers, nitrous oxide emissions were significantly increased by uncoated nitrate supplements compared to urea (P < 0.05). Nitrous oxide emissions by sheep fed coated nitrate did not differ from those of sheep fed urea. It is concluded that coating dietary nitrate can protect sheep against nitrite toxicity without adversely affecting methane mitigation.