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.

I. To obtain a quantitative model for nitrogen pathways in sheep, a study of ammonia and urea metabolism was made by using isotope dilution techniques with ['SN]ammonium sulphate and [Wlurea and [14C]urea.2. Single injection and continuous infusion techniques of isotope dilution were used for measuring ammonia and urea entry rates.3. Sheep were given 33 g of chaffed lucerne hay every hour; the mean dietary N intake was 23.4 gjd.4. It was estimated that 59 yo of the dietary N was digested in the reticulo-rumen; 29 yo of the digested N was utilized as amino acids by the micro-organisms, and 71 % was degraded to ammonia.5. Of the 14'2 g N/d entering the ruminal ammonia pool, 9.9 g N/d left and did not return to the pool, the difference of 4'3 g N/d represented recycling, largely within the rumen itself (through the pathways : ruminal ammonia + microbial protein + amino acids --z ammonia).6. Urea was synthesized in the body at a rate of 18.4 g N/d from 2.0 g N/d of ammonia absorbed through the rumen wall and 16.4 g N/d apparently arising from deaniination of amino acids and ammonia absorbed from the lower digestive tract. 7.In the 24 h after intraruminal injection of [15N]ammonium salt, 40-50 % of the N entering the plasma urea pool arose from ruminal ammonia; 26% of the lSN injected was excreted in urinary N.8. Although 5-1 g N/d as urea was degraded apparently in the digestive tract, only 1.2 g N/d appeared in ruminal ammonia; it is suggested that the remainder may have been degraded in the lower digestive tract. 9. A large proportion of the urea N entering the digestive tract is apparently degraded and absorbed and the ammonia incorporated in the pools of nitrogenous compounds that turn over only slowly. This may be a mechanism for the continuous supply to the liver of ammonia for these syntheses.10. There was incorporation of 16N into bacterial fractions isolated from rumen contents after intraruminal and intravenous administration of [15N]ammonium salts and [lSN]urea respectively. 11. A model for N pathways in sheep is proposed and, for this diet, many of the pool sizes and turn-over rates have been either deduced or estimated directly.

Nitrogen metabolism is reviewed with emphasis on methods for quantitating various nitrogen-transactions in the rumen of animals on a variety of diets. Ammonia kinetics, microbial cell synthesis, the inputs of endogenous nitrogen, degradation of dietary protein, and availability to the animal of dietary bypass protein are discussed. The efficiency of microbial protein from the rumen is discussed in relation to the ratio of protein to energy in the nutrients available to meet the requirements of the animal. The ratio is determined largely by the maintenance requirements of microbes and the breakdown of microbial materials, which result in the recycling of microbial nitrogen in the rumen. Emphasis is placed on the role of rumen protozoa in decreasing the ratio of protein to energy in absorbed nutrients in ruminants on diets that are marginally deficient in protein. Recent studies of the dynamics of protozoa in the rumen and their contribution to microbial protein outflow are summarized.

The effects of dietary nitrate on DM digestion, rumen volatile fatty acid concentrations, microbial protein outflow, rumen water kinetics, and methane production were studied. Eight rumen-cannulated sheep were acclimated to a diet consisting of chaffed oaten hay supplemented with either 4% KNO3 or 0% KNO3 but made iso-nitrogenous by the addition of urea. Nitrate supplementation did not affect blood methaemoglobin concentration, DM intake, whole tract or ruminal DM digestibility and the sheep appeared healthy at all times throughout the acclimation and experimental periods. Nitrate did cause changes in rumen fermentation consistent with its acting as a high-affinity hydrogen acceptor, i.e. there was a tendency towards a lower molar percentage of propionate in the rumen volatile fatty acids, and higher molar ratio of acetate to propionate. Methane yield (MY, L methane/kg DM intake) was reduced by 23% in KNO3-supplemented sheep (P < 0.05) and these sheep tended to have a shorter mean fluid retention time in the rumen (MRT). There was a significant association between MRT and MY, such that a shorter MRT was associated with a lower MY. The results confirmed that the presence of nitrate in the diet lowers enteric methane production even though there was considerable between-animal variation in gut kinetics and methane production.

I.A study of ammonia and urea metabolism in sheep was made using isotope dilution techniques with (lSNH4),S04, [lSN]urea and [Wlurea in order to determine quantitatively the movements of urea-N and NH,-N throughout the body of normal, feeding sheep.2. Single injections of 15N-labelled compounds were made into the rumen fluid NH,, caecal fluid NH, and the blood urea pools, in order to estimate the rates of flux through, and the transfer of N between, these and other nitrogenous pools in the body. fWr EDTA was injected into the rumen and caecum with (16NH&S0, to allow estimation of fluid volumes and to provide an indication of mixing, and of times of transit of isotopes between different sampling sites in the digestive tract.3. The sheep ate approximately 22 g lucerne chaffjh and the mean dietary N intake was 16.3 g/d.4. The rate of flux of NH, through the rumen NH, pool was 15.0 g/d (i.e. 90y0 of the dietary N ingested; however, this amount also included N from plasma urea (1.1 g/d) and other endogenous sources including NH, derived from caecal NH, (0.4 g/d).5. Only 40% of the N in isolated rumen bacteria was derived from NH,, indicating that a considerable proportion of their N requirements were obtained from compounds other than NH, (e.g. peptides and amino acids).6. There was evidence of recycling of N between nitrogenous pools in the rumen, probably through rumen NH, -+ microbial N --+ NH,. 7.It was estimated that 5-3 g blood urea-N/d entered the digestive tract: 20% of this urea was degraded in the rumen, 25 yo in the caecum and the remainder was apparently degraded elsewhere; there was evidence of urea degradation in the large intestine posterior to the caecum and it is suggested that urea degradation and absorption of the resultant NH, may occur in the ileum. 9.A large proportion of the NH, entering the caecal NH, pool (70% or 3.2 g N/d) was apparently derived from degradation of nitrogenous products, other than urea, including rumen microbial N (1.0 g N/d) passing undigested from the small intestine. 10. Less than half the NH,-N of caecal origin entering the rumen passed through the blood urea pool; the remainder was apparently transported as other nitrogenous compounds in the blood or body fluids.11. The results of the three experiments were combined in a general three-pool, opencompartment model which formally recognizes an unlimited number of other unspecified, interconnected pools together comprising the whole-animal system. Rates of flux through, and transfer of N between these and other nitrogenous pools in the body were calculated by solving this model and the information derived has been applied to whole-animal models with a view to subsequently using these models in computer simulation studies.