The carbon footprint of UK sheep production: current knowledge and opportunities for reduction in temperate zones

Summary

Livestock production is a significant source of methane (CH₄) and nitrous oxide (N₂O) emissions globally. In any sheep-producing nation, an effective agricultural greenhouse gas (GHG) mitigation strategy must include sheep-targeted interventions. The most prominent interventions suited to sheep systems are reviewed in the current paper, with a focus on farm-level enteric CH₄ and soil N₂O emissions. A small number of currently available interventions emerge which have broad consensus on their mitigation potential. These include breeding to increase lambing percentages and diet formulation to minimize nitrogen excretion. The majority of interventions still require significant research and development before deployment. Research into the efficacy of interventions such as incorporation of biochar is in its infancy, while for others such as dietary supplements, successes in isolated studies now need to be replicated in long-term field trials under a range of conditions. Enhancing understanding of underlying biological processes will allow capitalization of interventions such as vaccination against rumen methanogenesis and pasture drainage. Many interventions cannot be recommended at a regional or national scale because, either, their mitigation potential is inextricably linked to soil and weather conditions in the locality of use, or their use is restricted to more intensive, closely managed systems. Distilling the long list of interventions to produce an effective farm-level mitigation strategy must involve: accounting for all GHG fluxes and interactions, identifying complimentary sets of additive interventions, and accounting for baseline emissions and current practice. Tools such as whole farm GHG models and marginal abatement cost curves are crucial in the development of tailored, practical sheep farm GHG mitigation strategies.

INTRODUCTION

The growing demand for food products and an increasing awareness of the impact of unsustainable production methods are of increasing concern to society. Global food requirements are expected to be 70% higher in 2050 than in 2009 (FAO 2009), placing unprecedented demand on agricultural land and supply chains. Pressures such as soil erosion, reduced numbers of pollinating insects and water stress are of particular concern because they can generate negative feedbacks that may compromise future food production. The contribution of agriculture to global warming through the release of greenhouse gases (GHGs) is another such feedback. Agriculture contributes up to 0·32 of anthropogenic GHGs when land use change is included (Bellarby et al. 2008). Projected consequences for agriculture in the 21st century include increased crop productivity at mid to high latitudes, decreased crop productivity at lower latitudes, decreased water resources in semi-arid areas and changes in precipitation patterns (IPCC 2007).

Up to 0·18 of global GHG emissions are attributed to livestock production when land use change is included (Steinfeld et al. 2006). Of particular concern are the potent GHGs methane (CH₄) and nitrous oxide (N₂O), which have warming potentials of 25 and 298 times that of CO₂ per kg over a 100 year period (Forster et al. 2007). The production of CH₄ as a by-product of feed fermentation in the rumen means that red meat has far greater emission intensity than an equivalent quantity of white meat produced from monogastric animals (Bellarby et al. 2008; Gill et al. 2010; Stott et al. 2010). Red meat produced from pasture-based systems can be a significant source of N₂O emissions, particularly direct emissions from soil as a result of fertilizer applications (Schils et al. 2005; Edwards-Jones et al. 2009). This recognition of agriculture's contribution to climate change is manifest in intra and international GHG policy and emission reduction targets for agriculture and the red meat sector. For example, the UK Climate Change Act requires that all emissions be reduced by 34% (from 1990 levels) by 2020 and 80% by 2050. This has shaped sector-specific targets under the low carbon transition plan, including a 10% reduction for the agriculture industry by 2020 (DECC 2009). A GHG action plan subsequently identified nutrient and livestock management as categories for action, resulting in a red meat GHG reduction strategy (EBLEX 2012). Literature on mitigating GHG emissions from red meat production at a farm-scale level typically focuses on cattle to the exclusion of sheep. The current paper presents an overview of the most prominent mitigation options suited to sheep farm systems, and focuses primarily on options aimed at reducing enteric CH₄ and soil N₂O emissions, as the dominant forms of sheep farm emissions.

SHEEP FARM EMISSIONS

On-farm emissions dominate the sheep supply chain carbon footprint up to the point of sale (EBLEX 2012) and even after-export and consumer-stage emissions such as cooking are accounted for (Ledgard et al. 2010). Enteric fermentation CH₄ emissions constitute the largest component of on-farm emissions from sheep production (e.g. 0·57–0·58), followed by N₂O arising directly from soils in response to nitrogen application as fertilizer or animal waste (e.g. 0·15) (Ledgard et al. 2010; Taylor et al. 2010).

Emissions associated with sheep meat production are linked strongly to farm type. In the UK, for example, sheep produced in lowland systems typically have lower emissions than their upland and hill counterparts (Wiltshire et al. 2009; EBLEX 2012). Better pasture and subsequent silage quality and a milder climate favour faster growth rates and quicker sales in lowland environments. Recent data place the average carbon footprint of lowland lamb produced in England at 10·98 kg CO₂e/kg live-weight (LW) and at 14·42 kg CO₂e/kg LW for hill production (EBLEX 2012). Substantially lower emissions have been reported elsewhere, e.g. 7·2–8·3 kg CO₂e/kg of hot carcase produced in Western Australia (Peters et al. 2010). However, differences in calculation and reporting methods make comparisons problematic (Schils et al. 2007; Edwards-Jones et al. 2009). Carbon footprinting practitioners advocate that carbon footprints should be used as a starting point to steer the process of emission reduction and not to identify poor performers. Some production systems will inevitably have a higher footprint than others, for example, those with a significant area of organic soil may have high N₂O emissions as highlighted by Edwards-Jones et al. (2009). Mitigation options tailored to the requirements of specific systems are therefore required.

Much of the scope for reducing GHG emissions from sheep farms lies in improved productivity and system efficiencies. Enhancing productivity maximizes output per unit of input, reducing emissions per kg of product. Tackling system inefficiencies reduces waste such as feed energy lost as CH₄ and fertilizer nitrogen lost directly or indirectly as N₂O. Other mitigation options target emissions that cannot be avoided directly through system optimization, for example, vaccination against methanogens and addition of nitrification inhibitors (NIs) to pastures. There have been a number of reviews of livestock-related mitigation options (EC Agri DG 2002; Weiske 2005; Johnson et al. 2007; Moorby et al. 2007; Smith et al. 2008; Eckard et al. 2010; Gill et al. 2010; Shibata & Terada 2010). The sheep farm-relevant mitigation options reviewed in the current paper are outlined in Fig. 1 under the headings of enhancing productivity, animal management, and soil and pasture management.

Fig. 1. Schematic representation of the opportunities for reducing CH₄ and/or N₂O emissions on sheep farms. The headings ‘enhancing productivity’, ‘animal management’ and ‘soil & pasture management’ corresponded to subsections within the text.

For a number of the mitigation options, research on mitigation potential originated in cattle only studies. If there were no equivalent sheep system studies available it was necessary to supplement the sheep system-related literature with examples from cattle-based systems, with the understanding that the mitigation options are generic across ruminant systems. It should also be noted that a proportion of the studies were published as industry or project reports, and therefore not all the literature cited has been subject to rigorous peer-review.

ENHANCING PRODUCTIVITY

Despite conflicting results in the scientific literature regarding the efficacy of many mitigation options, there is a general consensus amongst scientists and in the industry that increased productivity is a priority mitigation option (EBLEX 2010; Gill et al. 2010; Shibata & Terada 2010). The underpinning notion is that maximized lamb production from the flock's maintenance feed provision will lead to a reduction in emissions per kg of produce (Smith et al. 2008; Buddle et al. 2011). The productivity of sheep systems can be boosted through a range of strategies targeting growth, fertility, longevity and feed efficiency of the animals (Gill et al. 2010; Hegarty et al. 2010). Relevant strategies include increases in lamb growth rate to reduce time on farm; increases in lamb muscle depth and carcase weight to increase saleable product; increases in lamb births and survivals to increase product output; lambing as yearlings to maximize the ewe's lifetime production capability which in turn decreases the proportion of unproductive time on farm; increases in ewe culling age to increase lifetime lamb output and reduce the need for replacements; reductions in incidences of disease and reducing residual feed intake (RFI) or improving feed conversion efficiency (Genesis Faraday 2008; Hegarty 2009; Hegarty & McEwan 2010; Hegarty et al. 2010; Alcock & Hegarty 2011; P. Amer et al. unpublished).

These strategies can be delivered through genetic improvement, i.e. livestock selection and breeding, and improved animal husbandry, i.e. animal feeding and health management (Gill et al. 2010; Hegarty et al. 2010). Desirable productivity traits can also be attained through changing breeds stocked (Allard 2009; IBERS et al. 2011a). Some sectors of the UK livestock industry have achieved significant GHG reductions as a by-product of genetic selection for productivity, for example, emissions per kg of product from the pig and dairy industries decreased by 0·8% per annum in the 20 years prior to 2008 (Genesis Faraday 2008). Breeding improvements in the UK sheep industry lag behind those made for other livestock (Moorby et al. 2007; Gill et al. 2010), and as a result emissions per kg of product have decreased by just 0·5% in total over the same 20-year period. Studies in other countries suggest that breeding for improved productivity in the sheep industry may further reduce emissions. For example, P. Amer et al. (unpublished) estimated that a 10% increase in ewe-litter size in New Zealand between 1994 and 2006 resulted in a 6% reduction in emissions per kg of lamb carcase produced. The Institute of Biological, Environmental and Rural Studies (IBERS) et al. (2011a) suggested that genetic improvement for productivity based on existing breeding indices could decrease annual CH₄ emissions by 0·03% per tonne of carcase produced in Wales.

There is a growing body of research using emissions modelling to estimate the mitigation potential of productivity improvements in defined flocks. The results of a number of recent studies are summarized in Table 1 and are discussed in the sections that follow.

Table 1. Summary of GHG reductions achieved through improvements in productivity. Data were taken from studies modelling GHG mitigation potential in defined flocks. The greatest reductions modelled in each study are highlighted in bold text

* New Zealand-based study that modelled the emission reductions possible through individual management strategies against a baseline flock of 1000 ewes. Baseline emissions were 15·99 kg CH₄ per lamb sold. Percentage reductions are a percentage change from the base flock in terms of CH₄ emissions per net lamb sold.

† An Australian study which modelled management options to reduce CH₄ output on a range of simulated sheep enterprises. Three common Australian production systems were characterized: (1) merino ewe flock – all replacements from progeny and surplus sold as weaners or hoggets; (2) dual purpose merinos – merino ewes mated to Poll Dorset and all progeny sold as stores or to slaughter; (3) prime lamb enterprise where Border Leicester × Merino ewes are mated with Poll Dorset rams and all progeny sold as stores or slaughter. Percentage reductions are in emissions intensity reported as kg CO₂e/kg LW sold.

‡ A Welsh study that modelled the enteric CH₄ emission reductions possible through selection for single genetic traits to improve productivity in hill, upland and lowland flocks. Reductions are a percentage change in CH₄ emissions over 10 years and per tonne of carcase produced.

§ An English study that calculated the GHG emissions reductions possible per kg of carcase meat produced from a lowland spring lambing flock that breeds its own replacements or buys in ewe lambs.

Animal fertility and longevity

In a self-replacing New Zealand flock of 1000 ewes, Cruickshank et al. (2008) found that lambing replacements as yearlings (hoggets) instead of waiting to lamb them later (as two-tooth ewes) had the greatest potential for reducing enteric CH₄ emissions (Table 1). This strategy maximized lamb output from the maintenance costs of the existing ewes. Similar findings in the direction and magnitude of change were modelled in a study by ADAS (2010), suggesting that lambing at 12 months rather than 2 years could reduce CH₄ and N₂O emissions by 9·4 kg CO₂e/kg of carcase meat. In the self-replacing Australian flocks modelled by Alcock & Hegarty (2011), mating replacements at 7 months was estimated to reduce enteric CH₄ emissions by 12% per kg of LW lamb produced. However, in their second and third sheep enterprise types, replacements were not home-reared but brought in 2 weeks before mating. Consequently, mating at 7 months increased enteric CH₄ emissions between 3 and 9% per kg LW lamb produced. In these scenarios there was no unproductive young stock on farm and mating at an earlier age only served to reduce lambing percentages and growth rates.

The Institute of Biological, Environmental and Rural Studies et al. (2011b) found selection for ewe-litter size to be the genetic trait with the greatest stand-alone potential for emission reduction in Welsh flocks over a 10-year period. Similar strategies of increasing scanning percentage and the number of lambs weaned per ewe resulted in substantial enteric CH₄ savings of 3–4 and 7·8%, respectively (Cruickshank et al. 2008; Alcock & Hegarty 2011). Increasing ewe longevity and decreasing lamb mortality also have potential to reduce lamb production emissions.

Animal growth rates and feed

In each of their three modelled enterprise types (Table 1) Alcock & Hegarty (2011) found that production and creep feeding to finish lambs earlier had the greatest potential to reduce enteric CH₄ per kg of LW lamb produced; however, their study only considered enteric CH₄ emissions and did not consider the emissions burden of grain production. The effect on emissions of genetic selection for faster growth rate in lambs is dependent on whether or not this also results in a correlated increase in ewe mature weight. The Institute of Biological, Environmental and Rural Studies et al. (2011b) estimated that selection for lamb growth over 10 years in Welsh hill flocks would decrease enteric CH₄ emissions by 1·3% with no change in ewe weight, and increase them by 0·4% if ewe weight increased in synchrony. It is reported that improvements in lamb growth rates were behind most of the genetic-related reduction in GHG emissions in the UK sheep industry in the last 20 years (Genesis Faraday 2008). However, the net benefit was constrained by the increased emissions associated with the higher mature weights of the ewes. Net N₂O emissions demonstrated a marginal increase over time as a result of faster lamb growth rates, underlining the importance of incorporating all GHGs in any emissions calculation.

While the efficiency of feed use is widely used for selective breeding in other livestock species, limited use has been made of traits such as RFI in the ruminant industry (Genesis Faraday 2008; Wall et al. 2010). Studies have demonstrated that cattle with lower RFI have reduced dry matter intake (DMI) and may also have lower daily rates of CH₄ production (Nkrumah et al. 2006; Hegarty et al. 2007). The modelled sheep flock scenarios of Alcock & Hegarty (2011) found selection of sheep for lower RFI to be the most promising genetic improvement option for reducing enteric CH₄ emissions. If achieved, low RFI animals will provide a mitigation option suited to both intensive and extensive systems (Waghorn & Hegarty 2011).

There is increasing interest in breeding directly for CH₄-reducing traits and feed nitrogen conversion efficiency (Wall et al. 2008; Keogh & Cottle 2009; Hegarty & McEwan 2010). Inter-sheep variation was estimated to be responsible for 70–80% of the differences in CH₄ emissions per unit of feed intake recorded from livestock fed the same diet in large-scale experiments (O'Hara et al. 2003). Persistent variation in CH₄ emissions between sheep has been recorded under grazing conditions (Pinares-Patino et al. 2003). Making use of this variation in breeding schemes is contingent upon the heritability of CH₄ traits, and the repeatability of this variation for different age classes and diets (Hegarty & McEwan 2010).

Animal health

Improvements to animal health present opportunities to improve productivity and fertility by reducing culling rates and the subsequent number of replacements needed to maintain maternal flock size (Wall et al. 2010). Stott et al. (2010) estimated that prophylactic disease treatment in a hypothetical extensive sheep farm would reduce overall CH₄ emissions by 28%.

ANIMAL MANAGEMENT

Mitigation measures that target direct emissions from livestock and their excreta dominate the ruminant GHG mitigation debate. These measures fall into two principal categories: nutritional management and dietary and ruminal manipulation. Unlike cattle, there is little scope for reducing sheep farm emissions through manure management because the majority is excreted in the field (Smith et al. 2008).

Animal nutrition

Nutritional strategies for reducing emissions from sheep target the inefficient use of dietary nitrogen and the loss of feed energy as CH₄. Between 0·75 and 0·95 of ingested nitrogen is excreted (Eckard et al. 2010), and gross feed energy intake lost as CH₄ ranges from 0·02 to 0·15 (Weiske 2005; Lassey 2007; Hopkins & Lobley 2009; Eckard et al. 2010).

Enteric methanogenesis

The volume of CH₄ produced during digestion depends on intake levels, diet composition and the rate and extent of digestion by microflora (Weiske 2005; PGgRc 2007). Typically, forages of high fibre or low digestibility that have a long residence time in the rumen will tend to produce high levels of CH₄ (PGgRc 2007). Models suggest that as sheep DMI increases LW gain (LWG) and daily CH₄ also increase, the overall result of which is a decrease in CH₄ production per kg LWG (Fig. 2) (Hegarty et al. 2010). As diet digestibility increases, CH₄/kg LWG decreases because of an underlying increase in LWG (Fig. 2) (Hegarty et al. 2010).

Fig. 2. The modelled relationship between DMI and CH₄ production per kg of LWG at three different levels of diet digestibility (▼65%, ○75%, ●85%) for a 30 kg Border Leicester × Merino wether offered ad libitum access to roughage (adapted from Hegarty et al. 2010).

Increasing feed intake and digestibility can be achieved through replacing structural carbohydrates (cellulose and hemicelluloses) in the diet with non-structural carbohydrates (starch and sugars) (O'Mara et al. 2008), or through altering forage type. Feeding higher starch, such as grain-based diets, not only increases diet digestibility and feed intake but also favours propionate production in the rumen providing an alternative pathway to methanogenesis for hydrogen use (Eckard et al. 2010; Martin et al. 2010). Benchaar et al. (2001) estimated that increasing the proportion of concentrates in the diet from 0 to 0·20 would reduce CH₄ production in ruminants as a proportion of gross energy intake (GEI) by 3%. However, in a meta-analysis of 87 studies, Sauvant & Giger-Reverdin (2007) found CH₄ losses as a proportion of GEI to be relatively constant for diets containing 0·30–0·40 concentrate, suggesting higher proportions of concentrates are needed to gain any mitigation benefit. Dragosits et al. (2008) suggested that feeding a high starch diet nationally to sheep flocks would only reduce CH₄ emissions by 1%. Production emissions associated with the grain and the baseline productivity and emissions of the farming system will determine the net GHG impacts of increasing the quantity of grain fed. The applicability of feeding high-concentrates diets is restricted to more intensive production systems.

In other research areas, the breeding of grasses and legumes with high water-soluble carbohydrate (WSC) content may potentially reduce direct CH₄ emissions from both intensive and extensive farming systems. For instance, IBERS (2010) found that lambs reared on a mix of three high WSC grasses produced up to 25% less CH₄/kg LWG compared with the control diet of conventional (normal WSC) grass. This was possibly due to increased ruminal bacterial numbers in lambs on the high WSC diet, leading to greater capture of metabolic hydrogen and reducing availability for methanogenic archaea. Other forage-based options include grazing animals on less mature herbages (Deighton et al. 2010) and feeding ensiled forages (Lima et al. 2011). Results from studies investigating the emission reduction benefits of feeding or grazing leguminous forages and pastures have been inconclusive. It is thought that legumes have a faster rate of ruminal breakdown than grasses and consequently a higher voluntary intake, lowering CH₄ yields/kg of DMI (Rochon et al. 2004; Hammond et al. 2011). Waghorn et al. (2002) found significant promise for mitigating emissions through changing forage type with a doubling of CH₄ emissions/kg DMI over a range of fresh forage diets, ranging from 11·5 g CH₄/kg on a ryegrass and white clover pasture to 25·7 g CH₄/kg on a diet of lotus forage. Knight et al. (2007) also found significant differences in CH₄ yield/kg DMI through varying legume species and proportion in the diet. In contrast, two separate feeding trials concluded that CH₄ yield is not influenced by forage species or maturity and that ‘there are no simple relationships between chemical components of fresh forages and CH₄ yield’ (Hammond et al. 2011; Sun et al. 2012).

Nitrogen conversion efficiency

Low efficiency of dietary nitrogen use in ruminants and subsequent high urea nitrogen losses are primarily attributed to imbalances in dietary protein and energy (non-structural carbohydrates), and feeding regimes that contain nitrogen in excess of dietary requirements (O'Hara et al. 2003; Moorby et al. 2007; Prosser et al. 2008). Decreasing the quantity of nitrogen excreted would be expected to reduce N₂O losses, both directly from soils and indirectly when leached nitrate is converted to N₂O in water bodies or when volatilized ammonia is deposited on the land.

Increasing the efficiency of nutrient use entails correctly formulating animal diets, matching feed provision more closely to animal nutrient requirement, which requires characterization of feed composition and nutritional advice (Moorby et al. 2007; Prosser et al. 2008). This can be achieved by avoiding excess nitrogen diets and by increasing the proportion of dietary nitrogen utilized through feeding a diet balanced in energy and protein. Pastures and fresh forages typically contain high levels of protein, in excess of available energy, resulting in the excretion of ammonia (Abberton et al. 2008; Eckard et al. 2010; Luo et al. 2010). Lowering the crude protein content of the diet is known to reduce dietary nitrogen losses (Schils et al. 2011), although careful management is required to ensure maintenance of yield (Nielsen et al. 2003). For example, Seip et al. (2011) showed that supplementing grass and legume silage of adult sheep with barley reduced urinary nitrogen excretion in an unfertilized grassland system. Numerous examples exist of the efficacy of this strategy in dairy systems (Luo et al. 2010; Schils et al. 2011). Increasing the carbohydrate content of the diet is the alternative option for balancing energy and protein, e.g. balancing high protein forages with high energy supplements (O'Hara et al. 2003; Eckard et al. 2010) or through feeding high WSC grasses (Merry et al. 2006). Feeding trials have shown that high WSC grasses can reduce nitrogen excretion by up to 24% whilst also increasing DMI and improving LWG (IGER 2005).

Feed additives and ruminal manipulation

Many studies have tested the effects of a range of dietary additives and alternative methods of rumen manipulation on enteric CH₄ and dietary nitrogen losses (Table 2). The rumen-based CH₄ mitigation strategies listed in Table 2 have several different modes of action. Feed additives such as condensed tannins and bacteriocins directly inhibit methanogenesis (Kreuzer et al. 1986; O'Mara et al. 2008). Others, such as organic acids and probiotics, provide an alternative sink or pathway for H₂ use in the rumen, displacing CH₄ production (O'Mara et al. 2008; Martin et al. 2010); while plant saponins and ionophores eliminate rumen protozoa that are thought to have a symbiotic relationship with some methanogenic archaea (Kreuzer et al. 1986; Kumar et al. 2009; Eckard et al. 2010). A number of the strategies act to reduce emissions in multiple ways. For example, ionophores are known to improve feed conversion efficiency (Grainger & Beauchemin 2011). Fat supplementation may reduce nitrogen losses and CH₄ emissions concomitantly (Machmüller et al. 2006). Oil supplementation may improve digestibility and energy use efficiency (Klevenhusen et al. 2011).

Table 2. Dietary and ruminal manipulation strategies for emissions mitigation

Research interest appears to be focusing on the use of natural feed additives such as tannins, essential oils and lipids and on the novel approaches of vaccination and defaunation. Supplementation with lipids is one strategy at the forefront of dietary mitigation research. Martin et al. (2010) recently reviewed the results of 67 dietary supplementation experiments from the literature, concluding that overall, for sheep and cattle combined, with every 1% addition of fat, mean CH₄ emissions decreased by 3·8%. Martin et al. (2010) also found that medium chain fatty acids (most frequently coconut oil) showed the greatest mitigation potential. In a similar study, a meta-analysis of studies limiting supplementation within the practical range of feeding, Grainger & Beauchemin (2011) found a slightly greater decrease in cattle CH₄ emissions/g of fat added to the diet. In contrast to Martin et al. (2010), Grainger & Beauchemin (2011) found that fatty acid type had no effect on CH₄ yield. Nor did the form of fat added (oil v. oilseed) or fat source (e.g. coconut v. sunflower). Grainger & Beauchemin (2011) suggested that their results were more robust than those of Martin et al. (2010) because they were based on a covariance analysis of CH₄ yield data as opposed to average data, and also because the dataset used by Grainger & Beauchemin (2011) was restricted to practical dietary fat levels. Grainger & Beauchemin (2011) also highlighted a significant difference in the relationship between dietary fat and CH₄ yield among beef, dairy and sheep, finding that more data are needed to give an accurate assessment of the effect of fat supplementation in sheep. In a recent study in Wales, IBERS (2010) measured CH₄ production and nitrogen retention in store lambs fed diets supplemented with linseed oil or a novel high fat naked oat. Linseed oil supplementation reduced CH₄ emissions by 22% and the naked oats by 33% compared with the control diet. Neither supplements affected nitrogen retention significantly.

Lipid supplementation research highlights the uncertainties that persist in the application of many dietary mitigation strategies, e.g. optimal lipid source, dosage level, dependence on diet type, transfer to animal products and possible human health impacts and limited sheep specific data (Hook et al. 2010; Martin et al. 2010). Despite these uncertainties, implementation is beginning to be considered including using drinking water to administer supplements in extensive grazing systems and the identification of high fatty acid content grasses (Grainger & Beauchemin 2011).

SOIL AND PASTURE MANAGEMENT

Soil and pasture-based mitigation options aim to limit direct and indirect N₂O emissions. Nitrogen enters the soil through animal excretion in the field, manure and fertilizer application, crop residues, fixation by leguminous crops and atmospheric deposition (Schils et al. 2011). Losses from the system can occur directly as gas (dinitrogen (N₂) or N₂O) or indirectly through leaching (nitrate (NO₃⁻); dissolved organic N), runoff (NO₃⁻ and ammonium (NH₄⁺)) or volatilization (ammonia (NH₃)). Skiba et al. (1998) estimated that 0·017 of the nitrogen input from mineral fertilizer and animal excreta applied to a sheep-grazed pasture in Scotland, was emitted as N₂O.

There are multiple pathways through which N₂O is produced in soils (Fig. 3), not all of which have been fully characterized. Denitrification (the anaerobic reduction of NO₃⁻ or nitrite (NO₂⁻) to N₂) is thought to be the primary source of N₂O in soils. However, nitrification (the oxidation of ammonia (NH₃) →NO₂⁻) is now known to be a significant source of N₂O in some situations (Baggs & Philippot 2010). The importance of other N₂O production pathways such as nitrifier denitrification and aerobic denitrification are also now being recognized (Wrage et al. 2001; Baggs & Philippot 2010). Soil conditions regulate the activity and relative importance of microbial pathways. Understanding the conditions favoured by each is crucial when targeting mitigation strategies to ensure net N₂O reductions (Richardson et al. 2009; Baggs & Philippot 2010).

Fig. 3. Soil microbial pathways of N₂O production within sheep pasture systems (adapted from Baggs 2008).

Soil moisture

Several studies have demonstrated that N₂O emissions and overall nitrogen losses are accentuated in high moisture conditions. For example, Chambers et al. (2000) showed that NO₃⁻ leaching from the application of organic manure to grassland sites was greatest when applied in the autumn and winter. Cardenas et al. (2010) reported far higher N₂O emissions from fertilized grazed grasslands in the West of the UK compared with the East, which they attributed to the wetter conditions in the West. Frequently, N₂O emissions positively correlate with soil water-filled pore space (WFPS), with maximum emissions occurring at 0·60–80 m³ water/m³ pore space (Fig. 4) (Clayton et al. 1997; Jones et al. 2007; Rafique et al. 2011). In poorly aerated soils (WFPS >0·60 m³/m³) denitrification becomes dominant, and >0·80 m³/m³ N₂ becomes the dominant product of denitrification (Dalal et al. 2003). Flechard et al. (2007) found that N₂O emission factors from European grassland sites were highest for soils where WFPS mostly remained in what they called the ‘optimum range for N₂O emissions of 60–90%’.

Fig. 4. Relationship between WFPS in soil and the relative fluxes of N₂O (●) and N₂ (○) from both nitrification and denitrification within sheep pasture systems (adapted from Dalal et al. 2003).

Water table management

In many northern European countries, water table manipulation through soil drainage presents a practical option for controlling WFPS in sheep-grazed grasslands (Dobbie & Smith 2006). A small number of studies have investigated the relationship between water table level and N₂O emissions in the field (Table 3). Dobbie & Smith (2006) and Kammann et al. (1998) demonstrated a significant decrease in N₂O emissions as water table depth below the soil surface increased. As the water table falls, WFPS and soil moisture decrease, leading to an increase in aeration in the upper soil, which in turn reduces the presence of anaerobic zones for denitrification and enhances root growth leading to better fertilizer N use efficiency. Dobbie & Smith (2006) concluded that draining grasslands to keep the water table more than 0.35 m below the surface when nitrogen is available for denitrification could cut N₂O emissions by 50% during the growing season. However, mitigation through water table management is complex (Fig. 4). If for example, soil is drained below saturation but WFPS remains above 0·40 m³/m³, N₂O emissions could potentially increase (Eckard et al. 2010). The WFPS values at which nitrification and denitrification dominate N₂O production are site- and soil-specific (Müller & Sherlock 2004). Although drainage can effectively reduce CH₄ and N₂O emissions from mineral soils, the case for GHG is more complicated for organic (peat) soils. Draining peat soils may reduce CH₄ and N₂O; however, this can be negatively offset by increased CO₂ emissions as the increased oxygenation stimulates aerobic mineralization of soil organic matter (e.g. van Beek et al. 2010; Table 3). The overall GHG balance of improved drainage is also uncertain due to the increased potential for nitrate leaching (and increased indirect N₂O emissions) (Smith et al. 2008; Eckard et al. 2010).

Table 3. The influence of water table depth on N₂O emissions from grassland soils in Western Europe

Soil compaction

The deposition of excreta on waterlogged soils increases nitrogen supply for denitrification and subsequent emissions may be exacerbated by soil compaction through animal trafficking. The likelihood and severity of compaction increases at elevated soil moisture content creating anaerobic sites in the soil (Rafique et al. 2011). In separate field experiments, Sitaula et al. (2000), Van Groenigen et al. (2005) and Bhandral et al. (2007) demonstrated that soil compaction increased average N₂O emissions from agricultural soils receiving urine and/or fertilizer by a factor of 1·7, 2·2 and 7, respectively, compared with no compaction. On an intra-farm scale, Matthews et al. (2010) showed that poached land surrounding water troughs on beef and sheep farms can have significantly higher N₂O emissions rates than surrounding managed pasture. Information on the impact of sheep grazing on soil compaction and subsequent N₂O emissions is scarce (Saggar et al. 2007). While the hoof pressures of sheep are lower than those of cows (83 kPa compared with 192 kPa), there is evidence that infiltration in soil decreases with increased sheep-stocking rate (Willatt & Pullar 1984). Decreased infiltration indicates that soil is compacted. Betteridge et al. (1999) found that the effect of a severe short-term treading event on wet hill soils was greater for cattle than sheep stocked at the same metabolic LW/ha, but they also indicated that at soil water contents above the critical water content for compaction the ratio of soil compaction to deformation may be greater for sheep than for cattle. Many opportunities to reduce soil compaction on pastures are already well established as best practice for limiting poaching, water pollution and safeguarding animal welfare when out-wintering stock. These include sale of barren ewes to reduce stocking rates in winter, and the use of electric fences to control access to forage crops and boggy areas.

There has been little follow-through research on the impact of these measures on N₂O emissions. Restricted grazing on wet pastures (e.g. through housing animals) may reduce N₂O emissions provided that collected excreta is spread uniformly (Hopkins & Lobley 2009). The extent to which this mitigation measure is relevant to sheep farms will depend on the stocking rate and current winter housing and grazing practice. Schils et al. (2005) modeled the GHG budget of reducing grazing time on a case study dairy farm. Reduced N₂O emissions from excreta were offset by an increase in CH₄ emissions from manure storage, suggesting that restricted grazing may not offer mitigation potential at a whole farm level. Luo et al. (2010) suggested that for grazed winter forage crops, the method of tillage used to establish the crop will impact on the subsequent soil compaction by grazing animals and therefore N₂O emission. Direct drilling to establish forage crops was suggested as a means of emissions reduction.

Reducing stocking rates also holds potential for emissions reduction. Howden et al. (1996) found that CO₂e emissions/ha grassland increased linearly with stocking rate at low to moderate stocking rates (from 2 to 8 or 9 ewes/ha), but remained constant at higher stocking rates from 10 to 14 ewes/ha, although the causality of this relationship was not explored. Rafique et al. (2011) found that intensively grazed grasslands produced N₂O fluxes up to three times higher per hour than their extensive counterparts, which they attributed to greater urine and dung excretion and soil compaction on intensive sites.

Fertilizer and nutrient management

Soil moisture should also be taken into account when planning fertilizer applications. High WFPS, low oxygen conditions promote denitrification when carbon and NO₃⁻ supplies are non-limiting, indicating that fertilizer applications should be avoided in late autumn and winter and early spring. In conditions where denitrification predominates, such as during cool, wet months, N₂O emissions may be lower from the application of a urea-based fertilizer than a NO₃⁻ -based fertilizer. Conversely, emissions may be expected to be higher from ammonium rather than NO₃⁻ -based fertilizers in drier soil conditions favouring nitrification (Eckard et al. 2006).

Other fertilizer management opportunities for emissions reduction limit the supply of nitrogen feedstock for N₂O producing soil microbes. When fertilizer applications exceed pasture or forage requirements the nitrogen surplus can be immobilized, becoming part of the organic nitrogen pool or lost through the pathways previously defined. As nitrogen supply exceeds the requirements of the pasture the efficiency of use for growth declines (Eckard et al. 2006). Pasture derived emissions of N₂O are positively correlated with nitrogen input (Jones et al. 2007; Cardenas et al. 2010; Rafique et al. 2011). Adoption of a fertilizer recommendation system, which includes a soil and plant nutrient analysis, would ensure the optimization of nutrient supply (O'Hara et al. 2003; Moorby et al. 2007). This would also account for the nitrogen content of the soil and any applied manure (as many farmers fail to account for the nutrient content of organic manures when applying fertilizers; Jones et al. 2007). Proper maintenance and calibration of spreader equipment will improve targeting of nutrients to crop needs. Precision in fertilizer timing can also reduce nitrogen losses. These are simple approaches including ensuring application coincides with periods of rapid crop growth; minimizing delays between application and crop uptake and splitting applications into several smaller applications to improve efficiency of nitrogen uptake (Eckard et al. 2006; Jones et al. 2007).

Pasture renovation and plant selection

Temporary pastures on sheep farms are periodically ploughed and either reseeded to grass to improve sward productivity or planted with a forage crop. Pasture renovation has been associated with temporary, but significant, increases in soil N₂O emissions (Davies et al. 2001; Estavillo et al. 2002; Vellinga et al. 2004). Velthof et al. (2010) found that renovation of intensively managed fertilized grasslands increased N₂O emissions by an average of 1·8–3 times compared with non-reseeded control grasslands. Possible explanations include increased mineral nitrogen content of soil through the incorporation of crop residues, mineralization of N from soil organic matter and limited uptake of nitrogen by crops post-ploughing. Careful management of pasture ploughing (i.e. method and timing) may reduce emissions, although the number of studies supporting this is limited. Contrary to what might be expected, MacDonald et al. (2011) showed that full inversion tillage (FIT) reduced N₂O emissions relative to soil NO₃⁻ levels by two or three times compared with a no-till/glyphosate (chemical fallow) regime on poorly drained grassland soils. They suggested that FIT may reduce N₂O emissions in a wet year by placing the most nutrient-rich soil surface at depth where lower oxygen levels lead to the complete reduction of NO₃⁻ to N₂. In the case of chemical fallow, carbon and nitrogen remain available close to the surface, where higher oxygen concentrations may hinder the full conversion of N₂O to N_2. Similarly Velthof et al. (2010) reported lower N₂O emissions from ploughed grassland than grassland renovated through chemical destruction of the sward, perhaps due to increased aeration of soils through ploughing. Grassland renovation in spring as opposed to autumn may reduce total nitrogen losses from soil because the new sward has a higher capacity to take up nitrogen during the growing season (Vellinga et al. 2004; Velthof et al. 2010). Davies et al. (2001) have also suggested that avoiding grazing and fertilizer application on pastures prior to ploughing can reduce emissions, however, further work is needed to quantify the overall benefits of this.

Pasture renovation provides an opportunity to select plant varieties that may reduce nitrogen losses over the long term. Mixed pastures of legumes and grass typically fix between 100 and 250 kg/N/ha/year, reducing the need for mineral fertilizer use (Rochon et al. 2004). In a life-cycle analysis model of lowland and upland sheep production systems in England, lamb production emissions from fertilized grasslands have been estimated to be 14·6 kg CO₂e/kg of meat compared with 13·1 kg CO₂e/kg produced from an unfertilized grass-clover sward (EBLEX 2009). However, some uncertainty relating to the mitigation potential of clover arises from the possibility that NO₃⁻ and dissolved organic nitrogen leaching may increase with the legume content of the sward and the level of nitrogen fixation (Rochon et al. 2004). Possible explanations include low soil nitrogen immobilization and high mineralization due to the low carbon-to-nitrogen ratio of clover litter; and enhanced soil structure (Loiseau et al. 2001; Rochon et al. 2004). Forage legumes also represent a small source of N₂O, directly from the process of biological fixation, but primarily as a result of the release of root exudates in the growing season and the decomposition of crop residues post-harvest (Rochon et al. 2004). Few studies have compared the overall nitrogen balance of grazed unfertilized grass–clover pastures with grazed fertilized pure grass pastures. In a review of available data, Ledgard et al. (2009) found total nitrogen leaching losses and N₂O emissions from nitrogen cycling of excreta to be similar in both pasture types with comparable total nitrogen inputs. However, due to fertilizer-specific CO₂ and N₂O emissions (such as increased denitrification losses) whole system GHG emissions were typically lower per unit of produce in grass–clover systems. Research on the comparative nitrogen balance of pure legume pastures is more limited. There is some evidence that nitrogen leaching from pure white clover pasture may be considerably higher than grass–white clover pasture, possibly as a result of high nitrogen concentrations in the clover leading to greater nitrogen excretion, which the pasture is unable to take up (Loiseau et al. 2001).

Plant breeding to improve the efficiency of nitrogen use holds promise for future mitigation through pasture plant and forage crop selection. One area of current research interest is ryegrass breeding for improved fertilizer recovery (Abberton et al. 2008). Some species hold interest for future breeding strategies because of features such as improved rooting depths that enable nitrogen uptake from deep in the soil profile; the production of natural NIs in the roots; and greater nitrogen immobilization in soil associated with the quality of the crop residues (Luo et al. 2010; Schils et al. 2011). Richardson et al. (2009) suggested that plant breeding to control exudates to the soil could be a means of manipulating denitrification to increase the ratio of N₂ to N₂O production. Although these rhizosphere strategies involving manipulation of the soil microbial community hold strong promise it is likely that this technology will not be readily transferable between soil types making its widespread adoption difficult.

Additions to soil

Nitrification inhibitors, urease inhibitors (UIs) and slow-release fertilizers influence the rate at which fertilizer or urine nitrogen is supplied to plants (Shaviv & Mikkelsen 1993). They provide a steadier supply of nutrients to pasture and forage crops and minimize losses of excess nutrients. Slow release fertilizers such as those coated to reduce solubility have been shown to reduce losses of applied nitrogen, avoiding large fluxes of N₂O after rainfall (following a fertilizer application), whilst maintaining yields (Ball et al. 2004). Despite confidence in their mitigation potential the cost of slow release fertilizers in terms of substitution for a conventional fertilizer and in terms of the cost per tonne of carbon abated is currently prohibitive (Ball et al. 2004; Moran et al. 2008). Although outreach programmes are increasing farmer awareness of GHG issues, overcoming the barriers to technology adoption will remain difficult without farm subsidies.

Nitrification inhibitors and/or UIs can be applied directly to the crop (e.g. as a spray), incorporated into fertilizers or even infused into the gastrointestinal tract of livestock for excretion onto pasture (Ledgard et al. 2008). Nitrification inhibitors reduce the rate of conversion of NH₄ to NO₃⁻ in the soil (Di et al. 2007), releasing NO₃⁻ at a rate which better matches crop uptake. Urease inhibitors slow the conversion of urea to NH₄⁺, reducing the potential for NH₃ volatilization (Watson & Akhonzada 2005). Numerous studies have demonstrated the efficacy of NIs (Di et al. 2007; Hoogendoorn et al. 2008; Ledgard et al. 2008) and UIs (Watson & Akhonzada 2005; Dawar et al. 2011) in reducing nitrogen losses from pastures and forage crops receiving urine and/or urea. A recent review of studies on the NI dicyandiamide (DCD) found that, when applied above the recommended minimum rate of 10 kg/ha, it reduced N₂O emissions from urine by an average 57% (compared with controls receiving no DCD) (de Klein et al. 2011). However, emission reduction potential varies depending on site-specific factors such as soil type, soil moisture, urine nitrogen application rate and whether or not urea fertilizer is also applied (Luo et al. 2010; de Klein et al. 2011).

Critical knowledge gaps remain for NIs, including their efficacy over the long-term and under non-ideal conditions (Suter et al. 2007). The validity of extrapolating data from small-scale experiments to whole farm potentials is also problematic (Suter et al. 2007). Most studies to date have been based in New Zealand; therefore efficacy under other climatic conditions is less certain. The UK, for example, has predominantly heavy texture soils and short growing seasons in comparison with the free draining soils and longer growing seasons in New Zealand (Moorby et al. 2007). In contrast, one UI (n-butyl thiophosphoric triamide (NBTPT)) is already available commercially in the UK. When applied with urea to four contrasting soil types (two arable, two grasslands) it inhibited NH₃ loss on average across all soils, temperatures and formulations by 61·2–79·8% (Watson & Akhonzada 2005).

The effect of biochar incorporation on soil nitrogen cycling is an emerging area of research. In addition to the primary objective of sequestering carbon, biochar incorporation in soil may also increase biological nitrogen fixation, reduce N₂O emissions and NO₃⁻ leaching and increase nitrogen retention as NH₃ and NH₄⁺ (Clough & Condron 2010). In the only field-based study to date on the effect of biochar incorporation on emissions from ruminant urine patches on pasture, Taghizadeh-Toosi et al. (2011) incorporated biochar into a renovated perennial ryegrass pasture. The grass was fertilized with urea after emergence, cut to simulate grazing and received an application of urine. Biochar addition at a rate of 30 t/ha was found to reduce cumulative N₂O emissions over a 65-day period by c. 50% compared with a urine-only treatment. This biochar treatment also had the lowest soil NO₃⁻ concentrations and the highest soil NH₄⁺ concentrations. Taghizadeh-Toosi et al. (2011) proposed that the biochar functioned as a sink for urinary NH₃, reducing the inorganic nitrogen pool available to nitrifiers, therefore reducing N₂O emissions and the subsequent formation of NO₃⁻. Work on forage crops and grassland destined for silage has also indicated increased N use efficiency in the presence of biochar; however, the effects were not consistent over a 3-year period, suggesting that it does not offer a reliable strategy for GHG emission reduction (Jones et al. 2012). In addition, the high production and transport cost of biochar, competition from other sectors for biochar feedstock (e.g. biomass energy), risks to humans and the environment from pollutants contained within the biochar (e.g. dioxins, PAHs), negative interactions with pesticides and current legislative barriers all limit its use in sheep-based agricultural systems (Jones et al. 2011). Further work is certainly needed to understand the mechanisms through which biochar affects soil nitrogen cycling, the soil conditions, which favour these mechanisms and cost-effective strategies for implementation.

The addition of lime to soil has been suggested as a mitigation option with small potential for reducing N₂O losses (Clark et al. 2001). The rates of both nitrification and denitrification are sensitive to soil pH (Dalal et al. 2003; Kemmitt et al. 2006). Bouwman et al. (2002) modelled the relationship between N₂O emissions and controlling environmental and management factors such as climate, soil type and fertilizer type based on 846 published N₂O emission measurements. Soil pH was a significant determinant of N₂O emissions, which were lowest in alkaline conditions. Recent studies by Zaman & Nguyen (2010) and Galbally et al. (2010) found that liming pasture soils with and without the addition of urine or nitrate fertilizer has no significant effect on N₂O emissions, demonstrating that understanding of the impacts of liming under different field conditions restricts its viability as an on-farm mitigation option at present. It must also be remembered that lime itself has a high intrinsic GHG cost associated with production, transport and its subsequent decarbonation in soil (Brock et al. 2012). As with any GHG intervention, it is therefore important that a full life-cycle assessment (LCA) is performed to evaluate the net GHG balance of the mitigation strategy in a truly holistic sense before blanket policy recommendations are made.

CURRENT AND FUTURE MITIGATION OPTIONS

The present review has highlighted the current research and development status of mitigation options applicable to sheep farms. A number of interventions have emerged, which are available for current application, which have broad agreement on their mitigation potential and are likely to be widely applicable across sheep farms. These are: increasing lambing percentages, lamb survival and ewe longevity; increasing diet digestibility and formulating diets to minimize nitrogen excretion; avoiding exceeding pasture and forage crop nitrogen requirements particular in wet conditions. Other more novel interventions are also becoming commercially available such as high WSC grasses, a UI and lipid supplemented feed (currently only available for dairy cows).

Many more interventions require significant research and development before deployment or need technological enhancement or farm payment subsidies to become cost-effective. Long-term field trials under a range of conditions are clearly needed for interventions such as dietary additives and NIs. An assessment of net impact on all GHGs is required for interventions such as the inclusion of legumes in pasture and faster growth rates in lambs. Furthering understanding of underlying biological processes will enable exploitation of the mitigation potential of interventions such as pasture drainage and vaccination against rumen methanogenesis. Research into the efficacy of interventions such as the incorporation of biochar and breeding for lower RFI is at an early stage and longer term trials are required urgently.

DEVELOPING A MITIGATION STRATEGY

Distilling the long list of mitigation options to produce a farm-specific shortlist is challenging. Mitigation strategies must be developed based on a whole farm approach to GHG accounting, i.e. ensuring all CO₂, N₂O and CH₄ fluxes and the effect of mitigation measures on interactions between fluxes are accounted for (Schils et al. 2005, 2007; Smith et al. 2008; Stewart et al. 2009; Eckard et al. 2010). Often only the most evident of interactions are accounted for (Schils et al. 2005) and in reality the full effect of numerous mitigation practices on the GHG budget are still to be explored. The GHG balance of buying in additional concentrates to creep-feed lambs for faster growth is one example of this.

Another crucial consideration is that mitigation strategies must be constructed using additive measures that act upon different elements of the production system. Putting together complimentary sets of interventions is challenging given that the effectiveness of an abatement measure may be diminished depending on the measures applied before or after it. A very limited number of studies touch upon interactions between interventions.

In any farm system, abatement potential is contingent on current baseline emissions and the extent to which good practice, such as optimal fertilizer management, have already been adopted. Lambing replacements at a younger age has been shown to be an effective mitigation option in self-replacing flocks. However, in flocks where replacements are purchased, lambing earlier can decrease lambing percentages and growth rates and subsequently increase emissions. This example affirms that the effect of any intervention is highly dependent on the baseline flock management scenario. Many interventions such as pasture drainage and selection of fertilizer form cannot be recommended at a regional or national scale because their mitigation potential is inextricably linked to soil and weather conditions in the locality of use.

Other considerations when designing a mitigation strategy include ease of adoption, financial commitment and the permanence of the effect of the interventions (Smith et al. 2008), for example, the long-term efficacy of NIs is unknown. It has also been argued that the uncertainty surrounding the calculated abatement potential figure of a mitigation measure should itself be used as a selection criterion in mitigation strategies (Schils et al. 2005).

A number of tools are now available which help with bringing together some of these selection criteria:

1. 1. Whole-farm GHG models quantifying all direct, indirect, upstream and on-farm GHG emissions are a crucial tool for developing emissions baselines and exploring the abatement potential of farm-level mitigation options. As a result of increased model sensitivity at a farm level (e.g. estimation of enteric CH₄ emissions based on diet composition), the GHG reduction potential of mitigation measures is continuously being refined.

2. 2. Some emissions mitigation studies have refined their strategies by farm type and locality. For example MacLeod et al. (2010) assessed the applicability of a shortlist of mitigation measures to specific farm types, sizes and locations using a qualitative scoring system and found that, across all regions, mitigation measures were typically most applicable to larger farms. Sintori & Tsiboukas (2010) grouped dairy farms through cluster analysis based on size, intensity and production orientation. This identified four farm types for which they were able to estimate the effects of varying levels of emissions reductions on the gross margin under optimal management. Applying this type of analysis to sheep farms will identify the mitigation options most suited to different production systems in different countries, for example, lowland, upland and hill farms in the UK.

3. 3. Final selection and implementation of mitigation measures relies upon the incorporation of a financial component into whole farm models (Schils et al. 2005, 2007; Weiske 2005). Gibbons et al. (2006) used a whole-farm model that maximized farm net margin by optimizing the crop, animal and labour mix over a year, and linked this with emissions data to determine the most cost-effective measures for reducing farm emissions. Marginal abatement cost curves plot the relationship between the costs per tonne of carbon abated against the abatement potential for individual mitigation measures. They provide a decision-making tool for selecting cost and emissions saving measures, or for selecting options that reduce emissions below a selected cost threshold.

Applying these tools that have primarily been developed and adopted in relation to beef and dairy systems to sheep farms is a critical next step in sheep farm-specific GHG mitigation research.

CONCLUSIONS

Incorporation of the most promising mitigation options into sensitive and holistic farm models is needed to develop robust sheep farm GHG mitigation strategies. Refining the full set of mitigation options is a function of each individual measure's estimated abatement potential, whole system effects and interactions, deployment stage, ease of adoption and cost to the farm business. One significant hurdle to overcome is accounting for the effect of interactions between interventions on the overall carbon footprint. This will enable complimentary sets of interventions to be developed. Modelling mitigation potential against baseline emissions specific to farm typology will ensure that interventions with the maximum mitigation benefit in those conditions can be selected. Costed mitigation strategies tailored to sheep farm typology will be a critical stage in the translation of research-based advice to farm-level action, and in the realization of agricultural emissions targets.