The methane emission of grazing ruminants can be estimated using the Emissions from Ruminants Using a Calibrated Tracer (ERUCT) technique by applying sulphur hexafluoride (SF6) in low-release permeation tubes. The low background concentration of SF6 coupled with its low detection limits and solubility similar to methane made SF6 a good tracer. However, discovery of alternative tracer gases is desirable since SF6 itself is a greenhouse gas. The development of high-release permeation tubes has given new opportunities to use tracer gases with higher detection limits than SF6. An extended literature search for alternative tracer gases was undertaken screening approximately 300 potential gases. The suitability of stable isotope tracers was also screened. A large number of candidate gases were eliminated due to potential health risks, differences in their chemical properties relative to methane, being unsuited to delivery by permeation tubes or requiring high purchase or analytical costs. Laboratory evaluations of leading candidate gases (ethane, propane, butane) were conducted. It is concluded that ethane and stable isotopes of methane have the potential as alternative tracers for the ERUCT technique but optimisation of the permeation tube is required before validation in animal studies.

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.

Climate change presents a range of challenges for animal agriculture in Australia. Livestock production will be affected by changes in temperature and water availability through impacts on pasture and forage crop quantity and quality, feed-grain production and price, and disease and pest distributions. This paper provides an overview of these impacts and the broader effects on landscape functionality, with a focus on recent research on effects of increasing temperature, changing rainfall patterns, and increased climate variability on animal health, growth, and reproduction, including through heat stress, and potential adaptation strategies. The rate of adoption of adaptation strategies by livestock producers will depend on perceptions of the uncertainty in projected climate and regional-scale impacts and associated risk. However, management changes adopted by farmers in parts of Australia during recent extended drought and associated heatwaves, trends consistent with long-term predicted climate patterns, provide some insights into the capacity for practical adaptation strategies. Animal production systems will also be significantly affected by climate change policy and national targets to address greenhouse gas emissions, since livestock are estimated to contribute ~10% of Australia's total emissions and 8-11% of global emissions, with additional farm emissions associated with activities such as feed production. More than two-thirds of emissions are attributed to ruminant animals. This paper discusses the challenges and opportunities facing livestock industries in Australia in adapting to and mitigating climate change. It examines the research needed to better define practical options to reduce the emissions intensity of livestock products, enhance adaptation opportunities, and support the continued contribution of animal agriculture to Australia's economy, environment, and regional communities.

Improvements in feed conversion efficiency (FCE) can be applied to individual animals as well as to production from land, as in a farm system. Our focus relates mainly to food production from individual animals within any animal population where there is divergence in the efficiency that individuals use ingested feed for maintenance and production; primarily due to differences in digestion and metabolism. Intake variation from the predicted mean for individuals of a similar size and level of production in a population has been termed residual feed intake (RFI), with low values indicating an efficient animal. Efficient animals require less feed than average and can be expected to produce less CH₄ and N₂O per unit product than the population average at a similar level of production. Selection for this trait will lower CH₄ emissions per animal, unless more animals are kept to eat the feed not required by efficient animals. There are few published evaluations of CH₄ yields from animals with divergent RFI and there is little evidence that efficient animals have a different CH₄ yield expressed as CH₄/kg dry matter (DM) intake. Of equal or greater importance than RFI is the need to select high producing animals, as this will reduce emissions/unit of product, referred to as emissions intensity (Ei). Research should identify productive individuals that have a low RFI to minimise Ei and maintain food production. The extent to which CH₄ can be reduced by selection for RFI will depend on the heritability of efficiency, dispersal of efficient animals through all populations and their resilience in a production system (i.e., robustness). The benefit of RFI to lowering greenhouse gas (GHG) emissions is its application, irrespective of farming system (i.e., confined, intensive, extensive grazing), especially because efficient animals are likely to increase farm profitability. Efficient animals are already in all herds and flocks and research must identify and remove inefficient individuals, while retaining and ensuring efficient ones are fit to purpose. However, the biggest benefits to reducing emissions and increasing production will be associated with good animal management practice (e.g., appropriate genetics, reproductive performance, longevity) with efficient animals superimposed. Good animal systems management will improve profitability, and apply to both intensive and extensive systems to increase food production and lower Ei. One dilemma for agriculturists will be the practice of feeding grains to ruminants, as gains in animal efficiency, especially in reduction of Ei, are likely to be biggest with high energy density rations, but feeding grain to ruminants may become an unsustainable practice if food supplies for humans are limited.

The growth and development of 387 crossbred lamb progeny from 9 Poll Dorset sires representing muscle (M), control (C), and growth (G) sire-types was studied. Sires were selected on the basis of their LAMBPLAN estimated breeding values (EBVs) for post-weaning growth (PWWT) and depth of loin muscle (PEMD). Lambs were provided with either LOW or HIGH levels of available grazing nutrition from 10 days of age onwards. Liveweight gain (LWG) throughout the study was less on LOW nutrition than on HIGH nutrition, leading to a 9.5 kg lower weaning liveweight (LW) and a 14.9 kg lower final LW in LOW lambs. After adjustment for final LW, HIGH lambs had significantly greater fat depth at the C-site (approximately 40 mm from the midline over the 12th rib) and tissue depth at the GR site (110 mm from the midline over the 12th rib) than did LOW lambs. This effect was consistent across sire-types. Depth of fat at the C-site was positively associated with the EBV of the sire for fat depth. The improvement in pre-weaning LWG, weaning weight, and final weight of lambs resulting from use of sires with a greater PWWT EBV was dependent upon the level of nutrition. This interaction was identified as different slopes (coefficients) for the regression between PWWT and trait for the 2 nutrition levels, indicating that the expression of the sire's genetic potential for growth at these times was significantly moderated by nutrition. The additional depth of lamb loin muscle resulting from use of sires of higher PEMD EBV was consistent across both LOW and HIGH nutrition treatments, with 1 mm of PEMD leading to a 0.6-mm increase in loin depth. Other consequences of sires having a high genetic capacity for loin muscle depth were reduced carcass C-fat depth with increasing sire PEMD and a tendency for conformation score to improve with the PEMD of the sire. The wool-growth response to improved nutrition was less in M lambs than in lambs of other sire-types, suggesting a difference in priority for protein partitioning between muscle and wool in lambs differing in genetic propensity for muscle growth.

This study investigated whether the expression profile of GDF8 (myostatin), myogenic regulatory factors (MRFs: MYF5, MYOD1, MYOG (myogenin), and MYF6), and IGF-system (IGF1, IGF2, IGF1R) genes are correlated with anatomical muscle, nutrition level, and estimated breeding values (EBVs) for muscling, growth, and/or fatness. Real-time PCR was employed to quantitatively measure the mRNA levels of these genes in the semimembranosus (SM) and semitendinosus (ST) muscles of growing lambs. The lambs were sired by Poll Dorset rams with differing EBVs for growth, muscling, and fatness, and were fed either high or low quality and availability pasture from birth to ~8 months of age. With the exception of MYOD1, the mRNA levels of all genes examined in this study showed varying degrees of nutritional regulation. All the MRF mRNA levels were higher in the SM muscle than the ST muscle, whereas myostatin mRNA was higher in the ST muscle than the SM muscle. Interactions between muscle type and nutrition were detected for IGF2, MYF6, and myogenin, while positive correlations between IGF2 and IGF1R and between MYOD1 and myogenin mRNA levels were apparent in both muscles. At the genotypic level, subtle differences in mRNA levels suggested interactions between nutrition and sire EBV. The findings of this study confirm that the MRFs, IGFs, and myostatin genes are differentially affected by a variety of factors that include nutrition, muscle type, and sire EBVs. Together, these data suggest that this suite of genes has important roles during postnatal muscle growth, even at quite late stages of growth and development.

A short term enteric methane emission measurement is not identical to a measure of daily methane production (DMP) made in a respiration chamber (RC). While RC curtail most variation except that from quantity and composition of feed supplied, all short-term measurements contain additional sources of variation. The points of difference can include measurement time(s) relative to feeding, feed intake before measurement, animal behaviour in selection of diet and level of activity before measurement. For systems where a short-term emission measurement is made at the same time in the daily feeding cycle (e.g. during twice-daily milking) scaling up of short-term emission rates to estimate DMP is feasible but the scaling coefficient(s) will be diet dependent. For systems such as GreenFeed where direct emission rates are measured on occasion throughout day and night, no scaling up may be required to estimate DMP. For systems where small numbers of emission measures are made, and there is no knowledge of prior feed intake, such as for portable accumulation chambers, scaling to DMP is not currently possible. Even without scaling up to DMP, short-term measured emission rates are adequate for identifying relative emission changes induced by mitigation strategies and could provide the data to support genetic selection of ruminants for reduced enteric emissions.

Extensive grazing of beef cattle is the principal use of the northern Australia land area. While north Australian beef production has traditionally utilised a low-input, low-output system of land management, recent innovations have increased the efficiency with which beef is produced. Investment to raise efficiency of cattle production by improving herd genetics, property infrastructure, the seasonal feed-base and its utilisation, as well as promoting feedlot finishing can all be expected to reduce the number of unproductive animals and reduce age-at-slaughter. Consequently, these innovations can all be expected to contribute to a reduction in the emissions intensity of greenhouse gases (GHG; t GHG/t liveweight gain). The North Australian Pastoral Company (NAPCO) has adopted these technologies to enhance reproductive and growth efficiency of the herd and has coupled them with changes in other aspects of property operation, such as use of solar energy systems, establishment of introduced perennial pastures and minimum tillage, to achieve production and operational gains, which also reduce the emissions intensity of their pastoral properties. Investments to improve production efficiency have been consistent with both financial and, in principle, environmental objectives of NAPCO. While NAPCO supports the development and implementation of new mitigation strategies, the company requires greater knowledge on pastoral emission levels and clarity on the future position of agriculture in a carbon economy. This information would enable confirmation of current emission levels, modelling of mitigation options and evaluation of the efficacy of potential on-farm carbon sinks. This paper presents NAPCO's perspective on GHG emissions in the context of its pastoral enterprise, including current and future research and mitigation objectives.

The efficacy of technologies to reduce enteric CH₄ emissions from ruminants are typically evaluated on individual animals with little consideration of enterprise scale impacts. While impacts of the many rumen manipulations being studied are hard to anticipate, there is adequate information to assess impacts of farm management changes and potential animal genetic changes on whole farm productivity and enteric CH₄ emissions. Seven common sheep production systems grazing an annual pasture in central New South Wales, Australia, were modelled using GrassGro® (version 3.1.2). A range of animal management and animal genetic strategies were examined for their impact on total enteric emissions, emission intensity (i.e., kg CO₂ equivalent/kg live weight (LW) of animal sold) and profit. Within enterprises, mitigation options were compared at their respective sustainable economic optimum stocking rate as it was assumed that farmers would seek to achieve the highest sustainable profit achievable from a finite land resource. Management options considered were choice of lambing time, mating ewes for the first time as lambs rather than yearlings, and feeding lambs to reduce time to slaughter. The potential for using selective animal breeding was also tested, with sheep physiological parameters being altered in GrassGro® to represent genetic improvement in fecundity, LW gain, residual feed intake and CH₄ yield. In general, the management choices delivering lowest emission intensity were also the most profitable within sustainability constraints. Mating maiden ewes as lambs was only effective in reducing enterprise emissions intensity in self-replacing flocks (i.e., no purchased replacements). When stocking rates were at the sustainable economic optimum, choice of enterprise or management had little influence on total enteric emissions from the enterprise. If decisions are guided solely by economics, farmers are likely to continue with similar levels of production (and emissions) until a price on enteric CH₄ emissions makes the sheep enterprise unprofitable, or an alternative more profitable enterprise than sheep grazing emerges. Improving animal genetics for residual feed intake or CH₄ yield offers opportunity to reduce enterprise emissions, but industry progress toward higher genetic merit for these traits is expected to be slow due to relatively low heritability and competing economic imperatives for progress in other traits.

The aim of this study was to assess the effect of low or high whole-of-life nutritional planes on bone growth, maturation, and carcass composition in lambs from sires (n = 9) with high estimated breeding values (EBVs) for post-weaning eye muscle depth (PEMD) or liveweight gain (PWWT), compared with sires of industry average for both traits. Lambs (n = 54) were killed at 8 months of age before measurement of forelimb bones, radiographic scoring, and histological measurement of growth plates, and bone ash mineral analysis. A subset of these (n = 36) had carcass composition serially assessed during growth by CAT-scan. Results reveal that the nutritional restriction imposed in this experiment caused significant restriction of skeletal growth, as reflected by shorter, thinner forelimb bones, altered limb proportions, narrowing (and in some cases permanent closure) of growth plates, and an altered bone mineral profile. CAT-scan analysis showed restriction of bone growth was similar to that of muscle growth. Progeny of high muscling (PEMD) sires showed greater muscle growth, but were possibly more susceptible to some of the skeletal effects of nutritional restriction. Greater sire EBVs for PEMD, PWWT, or fat depth were associated with narrower growth plates, suggestive of slower longitudinal bone growth and shorter adult limb length, although bone mass was not affected according to earlier CAT-scan data. Results also suggest that progeny of high PEMD or PWWT sires are earlier maturing in terms of skeletal (or at least limb) growth, although their bone mineral profile (magnesium content) was more consistent with that of physiologically less mature animals.