INTRODUCTION
The livestock sector faces major challenges in reducing its contribution to global environmental problems (Steinfeld et al. Reference Steinfeld, Gerber, Wassenaar, Castel, Rosale and De Haan2006). At the same time livestock numbers are increasing across the globe in response to rising demand for meat and dairy products, with sheep numbers forecast to increase by 60% to 2050 (Foresight 2010). According to the UK's 2012 greenhouse gas (GHG) inventory returns, agriculture was the source of about 0·44 of total UK emissions of methane (CH4), and of this c. 0·70 came from livestock enteric sources (mostly ruminants) (DECC 2014). Sheep meat makes up 0·10 of the total meat produced and consumed in the UK, but according to available data sheep account for over 0.16 of the GHG emissions from UK livestock species (DECC 2014). Improving the precision of emission data reporting is essential in order to identify key factors influencing the emission rates from different livestock systems and subsequently assessing the effectiveness of mitigation strategies. The current UK National GHG Inventory reports baseline emissions from sheep to the United Nations Framework Convention on Climate Change (UNFCCC) uses the most simplified approach to accounting (Tier 1) and relies on default emission factors (EFs) published by the Intergovernmental Panel on Climate Change (IPCC) (Dong et al. Reference Dong, Mangino, McAllister, Hatfield, Johnson, Lassey, Aparecida de Lima, Romanovskaya, Eggleston, Buendia, Miwa, Ngara and Tanabe2006). However, many of the observational data that these have been based upon have been collected from breeds or forages unrepresentative of pastures grazed by sheep within the UK and northern Europe (Murray et al. Reference Murray, Bryant and Leng1978; Ulyatt et al. Reference Ulyatt, Lassey, Shelton and Walker2002, Reference Ulyatt, Lassey, Shelton and Walker2005; Pinares-Patiño et al. Reference Pinares-Patiño, Ulyatt, Waghorn, Lassey, Barry, Holmes and Johnson2003).
In order to provide a robust basis for generating improved EFs for CH4 from sheep, baseline data are required for all stages of the production cycle. However, much of the sheep production in the UK is stratified into systems that utilize smaller, hardier breeds in the hills and heavier, more productive breeds and their crossbreds in the lowlands. It is possible that physiological and/or behavioural differences between different breed types may lead to differences in amounts of CH4 emitted. As part of a multi-centre research programme developing revised EFs for sheep and cattle the current experiment investigated the extent to which breed × diet relationships for growing lambs were explained solely by simple body weight and digestibility characteristics of the animal and diet, respectively, with no interactions. It quantified, for the first time, CH4 emissions from growing weaned lambs fed on pastures representative of those used in UK sheep production systems.
MATERIALS AND METHODS
Experimental design
The work described was conducted in accordance with the requirements of the UK Animals (Scientific Procedures) Act 1986 and with the approval of the Aberystwyth University Animal Welfare and Ethical Review Board. Data were collected in a 2 (breed) × 2 (diet) Latin square cross-over design experiment. Measurements were made on lambs of contrasting breed types: (1) Welsh Mountain (WM, small, hardy, hill breed), and (2) Welsh Mule (WM × Bluefaced Leicester) × Texel (TexX, prime lamb) (n = 8 per breed). The diets offered were: (1) herbage cut from an intensively managed ryegrass ley, and (2) herbage cut from an extensively managed, long-term permanent pasture
Eight ewe lambs of each breed type, aged c. 3 months old at the start of the experiment in June 2013, were selected from their respective flocks on the basis of live weight (LW) and body condition score (BCS) (MLC 2000) and pastured together on a mixed ryegrass/clover sward until the experiment began. All animals were drenched with an anthelmintic prior to the start of each experimental period. Initially half the lambs of each breed type were grazed together on the ryegrass sward, and the other half on the permanent pasture. Following a minimum of 1 week of grazing at pasture the lambs were housed in group pens and zero-grazed on their respective forages. After a further acclimatization period of at least 1 week they were individually penned for 3 days, during which time feed intake was measured. They were then individually housed in one of four calibrated CH4 chambers (Gardiner et al. Reference Gardiner, Coleman, Innocenti, Tompkins, Connor, Garnsworthy, Moorby, Reynolds, Waterhouse and Wills2015) and data collected for 3 consecutive days from each individual animal. Within a given breed type, animals were assigned randomly to an initial forage treatment. Likewise, during each run animals were assigned randomly to individual CH4 chambers. Following completion of the first experimental period the animals were swapped to the other pasture type, and the procedure was repeated.
Forages offered
Two forages were offered to the lambs: (1) ryegrass, a recently sown ley of monoculture perennial ryegrass (Lolium perenne), and (2) a long-term ley of a mixture of grass and forb species. During the zero-grazing period, when animals were inside in group pens, individual pens and CH4 chambers, forage was cut from the two pastures using a Haldrup plot harvester (J. Haldrup a/s, Løgstør, Denmark). Forage was cut three times per week and stored in a dark cool room at c. 4 °C prior to feeding. The lambs were fed on an ad libitum basis, with feed quantities designed to ensure a refusal margin of 0·10–0·15 each day, with two equal portions offered at 09·00 and 16·00 h. Fresh water was continually available.
Animal measurements
The LW and BCS of the lambs was recorded prior to them entering the CH4 chambers and again as they were removed. Methane production was determined by comparing the CH4 concentrations in the air entering and leaving the chambers at a known rate of airflow. An eight port single channel CH4 gas analyser (MGA3000; ADC Gas Analysis Ltd, Hoddesdon, UK) was used to determine CH4 concentrations in ambient air and in the exhaust gas leaving each chamber on a rotational basis. Ambient gas was sampled from points between each pair of chambers. The CH4 analyser was set to take a reading for each location at the end of a 3-min sampling interval before moving on to the next source. Air flow data was captured using mini-vane anemometers (MiniVane6, Schiltknecht Messtechnik, Switzerland) at each chamber attached to a data logger (MSR145, MSR Electronics GmbH, Switzerland).
Feed characterization
The weights of feed offered and feed refused were recorded on a daily basis. Representative sub-samples of the material offered each day were oven-dried at 80 °C to constant weight in order to determine dry matter (DM) content. A further sub-sample of the feed offered was collected at each feeding, bulked for each 3-day chamber period, and thoroughly mixed. Sub-samples were then taken to determine chemical and botanical composition. Sub-samples for chemical analysis were freeze-dried and milled through a 1 mm sieve. Ash was measured by igniting samples in a muffle furnace at 550 °C for 16 h. Gross energy (GE), included because these values were used in subsequent calculations of Ym coefficients (CH4-E/GE intake), was determined by adiabatic bomb calorimetry (Gallenkamp autobomb; Sanyo Gallenkamp PLC, Loughborough UK). Total nitrogen concentrations were determined using a Leco FP 428 nitrogen analyser (Leco Corporation, St. Joseph, MI, USA), and were expressed as crude protein (nitrogen × 6.25). Water-soluble carbohydrate (WSC) concentrations were measured by an automated anthrone technique (Thomas Reference Thomas1977). Neutral detergent fibre (NDF) and acid detergent fibre were determined using the method of Van Soest et al (Reference Van Soest, Robertson and Lewis1991), adapted for the Gerhardt Fibrecap detergent system (FOSS UK Ltd, Warrington, UK). Digestibility of organic matter in the DM was determined using the two-stage pepsin-cellulase in-vitro method described by Jones & Hayward (Reference Jones and Hayward1975). In order to characterize botanical composition a sub-sample from the bulked material from each experimental run was separated into the following categories: ryegrass, unsown grasses, white clover (Trifolium repens), other forbs and dead material. All separated material was then dried to constant weight at 80 °C and sward composition expressed on a proportional DM basis.
Data analysis
Metabolic live weight (MLW) was calculated as mean LW0·75. Tier 1 (Dong et al. Reference Dong, Mangino, McAllister, Hatfield, Johnson, Lassey, Aparecida de Lima, Romanovskaya, Eggleston, Buendia, Miwa, Ngara and Tanabe2006) equivalent EFs (kg/year) were calculated as: CH4 (g/day) × 365. Two-way analysis of variance with a blocking structure of chamber run/(animal × period) was used to investigate forage type × breed type interaction effects (Genstat 16; VSN International Ltd, Hemel Hempstead, UK).
RESULTS
Composition of the forages offered
Proportionally the ryegrass sward was made up of 0·92 perennial ryegrass, 0·02 unsown grasses and 0·06 dead material. In comparison the permanent pasture was more diverse. Only 0·05 of this sward was accounted for by perennial ryegrass, with unsown grasses (mainly Agrostis spp., Festuca spp., Poa spp and Holcus lanatus) making up 0·77 of the sward. Clover accounted for just under 0·04 of the sward, and other forb species for 0·03. This sward also contained a higher proportion of dead material (0·12).
The DM content of the two forages offered was similar, as was the crude protein concentration (Table 1). However, compared with the low-input permanent pasture the ryegrass had a higher WSC concentration, lower fibre concentrations and a higher digestibility.
Table 1. Chemical composition of the forages offered. All values g/kg DM unless otherwise stated
DM, dry matter; FM, fresh matter; CP, crude protein; WSC, water-soluble carbohydrates; NDF, neutral detergent fibre; ADF, acid detergent fibre; DOMD, digestible organic matter in the DM; GE, gross energy.
Role of breed type and pasture type on voluntary intake and methane emissions
Although the lambs of the two breed types had the same BCS, as expected the smaller, native WM lambs were significantly lighter than the TexX lambs at the start of the experiment despite being approximately the same age (Table 2). While total daily CH4 emissions were higher for the TexX lambs compared with the WM lambs, the yield of CH4 per unit of forage consumed was similar for the two breed types, as was the yield per unit MLW.
Table 2. Effect of breed type and pasture type on voluntary dry matter intake (DMI) and methane (CH4) emissions by weaned lambs (where WM, Welsh Mountain; TexX, Texel cross; MLW, metabolic live weight)
GE, gross energy
* There were no significant interaction effects.
† Estimated using the Tier 1 approach (Dong et al. Reference Dong, Mangino, McAllister, Hatfield, Johnson, Lassey, Aparecida de Lima, Romanovskaya, Eggleston, Buendia, Miwa, Ngara and Tanabe2006).
Pasture type had a highly significant effect on daily CH4 emissions (Table 2), in turn influencing the EFs estimated using the Tier 1 approach (Dong et al. Reference Dong, Mangino, McAllister, Hatfield, Johnson, Lassey, Aparecida de Lima, Romanovskaya, Eggleston, Buendia, Miwa, Ngara and Tanabe2006). Total output of CH4 per day was higher when offered ryegrass, but DMI was also higher on this forage type. The CH4 yield per unit of forage consumed and CH4 energy excreted per unit GE intake (Ym) were higher for lambs consuming the permanent pasture. No forage type × breed type interactions were identified.
DISCUSSION
The current experiment explored the potential for breed type and pasture type to influence enteric CH4 emissions from growing lambs. Data were collected for weaned lambs offered material from swards representative of the pastures used by commercial producers at the corresponding stage of the sheep production cycle. There are relatively few previous studies that have reported emissions for sheep <1 year old, and the majority of these have been based on animals aged 7–12 months, i.e. store lambs rather than finishing lambs.
Influence of breed type
While the potential to exploit within-breed variation in CH4 emissions within genetic improvement programmes has been recognized (Pinares-Patiño et al. Reference Pinares-Patiño, McEwan, Dodds, Cárdenas, Hegarty, Koolaard and Clark2011; Lambe et al. Reference Lambe, Wall, Ludemann, Bünger and Conington2014), there is little information available regarding the extent to which CH4 emissions vary between different breeds of sheep. For the current experiment two contrasting breed types of lamb were chosen. The WM is a hardy hill breed adapted to survive on low-quality forage in the exposed conditions of upland Wales. They play an important role in the stratified sheep industry within the UK because as well as producing purebred hill lambs WM ewes are popular for crossing with longwool rams to produce the type of larger crossbred ewe (‘mules’ and ‘halfbreds’) commonly used for fat lamb production (Pollott Reference Pollott2014). The TexX lambs were representative of the output from this final cross, in which a terminal sire such as the Texel is used to produce prime lamb meat (Pollott Reference Pollott2014).
Overall, breed type was found to have a limited role in influencing CH4 emissions. While the TexX lambs produced more CH4 per day, the amounts produced by the two different breed types was similar on a per unit intake and per unit MLW basis. Recent experiments in which measurements were made on different breed types of hill (Aubry et al. Reference Aubry, Annett and Yan2014a) and lowland (Aubry et al. Reference Aubry, Annett and Yan2014b) replacement ewes aged from 8 to 19 months also found no difference in CH4 emissions as a proportion of feed intake. It was not possible to measure LW gain meaningfully within the current experimental design, but prime lambs such as the TexX lambs would be expected to be growing faster and reach slaughter weight earlier than purebred hill lambs (Carson et al. Reference Carson, Irwin and Kilpatrick2001). Consequently, although the CH4 emissions per day may be higher for these animals, emissions intensity in terms of CH4 per unit weight gain would be expected to be lower.
Influence of pasture type
Studies with lambs to date have largely focussed on the effects of selected dietary ingredients and additives (Molano et al. Reference Molano, Knight and Clark2008; Mao et al. Reference Mao, Wang, Zhou and Liu2010; Li et al. Reference Li, Davis, Nolan and Hegarty2012, Reference Li, Silveira, Nolan, Godwin, Leng and Hegarty2013; Avila-Stagno et al. Reference Avila-Stagno, Chaves, He, Harstad, Beauchemin, McGinn and McAllister2013; Barnett & Hegarty Reference Barnett and Hegarty2014; El-Zaiat et al. Reference El-Zaiat, Araujo, Soltan, Morsy, Louvandini, Pires, Patino, Correa and Abdalla2014). The role of different pasture types has been limited to comparisons of grass and white clover (Hammond et al. Reference Hammond, Hoskin, Burke, Waghorn, Koolaard and Muetzel2011). The current study found that pasture type had a much stronger influence on CH4 emissions than lamb breed type. The pastures chosen for the experiment were typical of sward types grazed by comparatively intensively (re-seeded ryegrass) and extensively (permanent pasture) produced lambs during the post-weaning period. The main differences in nutritional value related to the concentrations of WSC and fibre within the forages, and related effects on digestibility. In turn these differences will likely have influenced voluntary feed intake, with the significantly lower DMIs for lambs offered the permanent pasture probably being due to longer rumen retention times (Thornton & Minson Reference Thornton and Minson1973). An increase in NDF concentration has been shown to increase CH4 yield (g CH4/kg DM intake) for dairy cattle (Yan et al. Reference Yan, Porter and Mayne2009), and the results obtained for growing lambs in the current trial reflect this relationship. In contrast, Hammond et al. (Reference Hammond, Hoskin, Burke, Waghorn, Koolaard and Muetzel2011) found no difference in CH4 yield from older (1–2 years) sheep offered fresh white clover or perennial ryegrass, despite marked differences in chemical composition of the forages.
Inventory and industry applications
The current approach to the UK GHG inventory is to assume the IPCC Tier 1 default EF for enteric fermentation for all mature sheep (i.e. >1 year old; Dong et al. Reference Dong, Mangino, McAllister, Hatfield, Johnson, Lassey, Aparecida de Lima, Romanovskaya, Eggleston, Buendia, Miwa, Ngara and Tanabe2006). Lambs have a lower average LW than mature sheep and the majority have a lifespan of <12 months, and would be expected to be associated with a lower EF than mature sheep. The UK therefore uses a country-specific EF for enteric fermentation for lambs at 40% of that of an adult sheep together with a reduction factor reflecting the reduced lifespan of lambs (Webb et al. Reference Webb, Broomfield, Cardenas, MacCarthy, Murrels, Pang, Passant, Thistlethwaite and Thomson2013), which is estimated as 8·1 months. No adjustment is made for the pre-weaning period. The equivalent EFs calculated from the daily emission rates for the lambs in the current study are higher than the value estimated by Webb et al. (Reference Webb, Broomfield, Cardenas, MacCarthy, Murrels, Pang, Passant, Thistlethwaite and Thomson2013) (Table 3). In situations where finishing lambs are supplemented with concentrate feeds the CH4 emissions would be expected to be reduced, as such animals should produce less CH4 per day (Gill et al. Reference Gill, Smith and Wilkinson2010) and have a shorter time to finish. However, poor economic returns from sheep production mean that most farmers are keen to avoid the additional costs associated with supplementary feeding. The overall treatment mean for Ym of 0·054 MJ/MJ is lower than the IPCC Tier 1 value for sheep of 0·065 MJ/MJ (Dong et al. Reference Dong, Mangino, McAllister, Hatfield, Johnson, Lassey, Aparecida de Lima, Romanovskaya, Eggleston, Buendia, Miwa, Ngara and Tanabe2006). This means that CH4 emissions for growing lambs are likely to be lower than currently calculated using the default IPCC Tier 1 methodology because a lower proportion of the GE consumed would be lost from the rumen as CH4.
Table 3. Estimated enteric fermentation emission factors (EF) for sheep. The ryegrass and permanent pasture values were generated based on measured mean emission rates per day for Welsh Mountain and Texel cross lambs aged 3–4 months old
Methane, CH4
* Assumes emission factor is 0·4 of that of an adult sheep and the average lifespan of lambs is 8·1 months.
The current study provides an emissions benchmark against which mitigation targets can be set and progress measured. It also provides representative data for use in wider comparisons of environmental impact. For example, carbon footprinting is increasingly used in the food supply chain to estimate the quantity of GHG emitted at different stages of the production process (Edwards-Jones et al. Reference Edwards-Jones, Plassmann and Harris2009; Jones et al. Reference Jones, Jones and Cross2014). Enteric CH4 emissions are estimated to contribute to over 40% of the mean footprint (Jones et al. Reference Jones, Jones and Cross2014), but current calculations are based upon the default IPCC EFs. By refining input data and EFs the precision of carbon footprint models can be improved both spatially and temporally and uncertainty in the carbon footprint estimate reduced (Basset-Mens et al. Reference Basset-Mens, Kelliher, Ledgard and Cox2009).
CONCLUSIONS
Although total daily CH4 emissions were higher for the prime lambs compared with the smaller hill lambs when offered fresh forage, the yield of CH4 per unit intake was similar for the two breed types. The total output of CH4 per day was higher when offered ryegrass compared with permanent pasture, but CH4 emissions per unit intake and Ym were higher on the permanent pasture. Overall the results indicate that forage type has a greater impact than breed type on CH4 emissions from weaned lambs.
This work was funded by Defra, the Scottish Government, DARD and the Welsh Government as part of the UK's Agricultural GHG Research Platform.
Open access
