Voluntary Greenhouse Gas Reporting Feasibility Study
2 Agricultural sector GHG emissions in NZ
2.1 Introduction
This section describes the key agriculture sources of greenhouse gas (GHG) emissions in New Zealand; methane (CH4) and nitrous oxide (N2O). This section includes an overview of the sources of GHG emissions from agriculture, the biological pathways of CH4 and N2O and the factors that influence these emissions and the methods to mitigate these emissions. This section concludes with recommendations for minimum data requirements required to calculate CH4 and N2O emission estimates at a farm level, for a VGGR in New Zealand.
2.2 Background to New Zealand agriculture
Agriculture plays a major role in the New Zealand (NZ) economy, earning NZ $15.25 billion per annum or 53% of total merchandise exports (Ministry of Agriculture and Forestry (MAF) 2005). In New Zealand, the agriculture and horticulture sector comprises 70,000 individual farms, of these 26% are individually owned, 54% are owned in partnership, 14% are registered limited liability companies and 6% owned by trusts/estates (MAF 2002). Pastoral livestock farming has approximately 45,000 farms, with 13,000 of these being less than 20ha in size. The total number of farms decreased by 10,000 between 1999 and 2002 and there was an approximate 10% fall in the number of livestock farms, although a change in the categorisation of farm types between these two dates makes it impossible to estimate the decline precisely. In terms of the number of animals kept, sheep numbers have been declining in New Zealand since 1982 although the indications are that numbers have now stabilised. Beef cattle numbers are relatively stable whereas deer and dairy cattle numbers have increased in the last 15 years (Table 2-1).
Agriculture is responsible for almost 50% of NZ’s greenhouse gas (GHG) emissions and these emissions have been rising at approximately 1% a year since 1990 (MfE, 2006). Current industry targets are for year on year productivity gains in the 2-4% range, meaning that agricultural GHG emissions are projected to continue rising into the near future. The Ministry of Agriculture and Forestry released the public discussion document Sustainable Land Management and Climate Change in November 2006
(MAF, 2006). In the document, MAF notes that New Zealand needs to act to protect its economic, trade and environmental interests.
At present, although emissions are estimated annually at the national level there is no formal system in place for individual farmers to quantify emissions at the individual farm scale, although several models are available to allow this to happen. Better quantification by individual farmers is an essential component of any programme designed to reduce emissions and is a pre-requisite for a number of the proposals outlined in MAF’s discussion document (MAF, 2006).
Table 2-1 Farm number, animal populations and GHG emissions by sector in New Zealand
| Number of farms (2002) 1 | Number of animals, June 30th 2006 (1990)2 | Number of animal classes3 | CH4 emissions (Gg*/annum) 2004 (1990)2 | N2O emissions (Gg/annum) 2004 (1990)2 | ||
|---|---|---|---|---|---|---|
| Dairy | 14000 | 5,221,400 (3,440,815) | 4 | 406.9 (237.7) | 12.8 (7.6) | |
| Beef | 13000 | 4,430,200 (4,593,161) | 11 | 256.7 (235.5) | 7.2 (6.6) | |
| Sheep | 13000 | 40,106,800 (57,852,192) | 7 | 430.0 (535.2) | 12.8 (15.8) | |
| Beef & Sheep | 2000 | Included above | ||||
| Deer | 2300 | 1,597,600 (976,291) | 7 | 37.8 (18.5) | 1.1 (0.52) | |
*1 gigagram (Gg) = 1,000 tonnes = 1 kilotonne (kt)
2.3 Greenhouse gas emissions from New Zealand agriculture
As a signatory to the United Nations Framework Convention on Climate Change (UNFCCC) and the Kyoto Protocol, New Zealand is obliged to compile annual inventories of its GHG emissions (refer to Section 3). In the latest nationally published inventory, for the 2004 calendar year (Ministry for the Environment (MfE) 2006), GHG emissions from the New Zealand agricultural sector totalled 36,867 Gg CO2 equivalent.. This represents 49.4% of all New Zealand’s emissions. Agriculture emissions are dominated by methane (CH4) and nitrous oxide (N2O). CH4 emissions account for 32.8% of total emissions (24,473Gg CO2 equivalent) and N2O 16.6% of total emissions (12,394 CO2 equivalent).
CH4 emissions arise principally as a by-product of the digestion of feedstuffs by farm animals via a process known as enteric fermentation. In 2004, enteric fermentation emissions accounted for 96.9% of all New Zealand’s agricultural CH4 emissions (23,715 Gg). Other sources of agricultural CH4 are those arising from stored and pasture deposited animal wastes (746Gg CO2 equivalent), the burning of tussock grassland in the South Island (0.7Gg CO2 equivalent) and the burning of crop residues from arable farming (11.3Gg CO2 equivalent). Since 1990 emissions from enteric fermentation and manure management have risen by 10 and 27% respectively while emissions from the burning of tussock grassland and crop residues have fallen by 75 and 40% respectively. Emissions from the burning of tussock and crop residues are so minor that they will not be discussed further (MfE, 2006) (refer figure 2-1).
N2O emissions in New Zealand are completely dominated (98.6%) by those arising from nitrogen (N) in animal wastes (10,330 Gg CO2 equivalent) and synthetic fertilisers deposited onto agricultural soils (1,976 Gg CO2 equivalent). Small quantities arise from stored animal wastes (63.5Gg CO2 equivalent), the burning of tussock grassland (0.1 Gg CO2 equivalent), the burning of arable crop residues (3.7Gg CO2 equivalent) and the return of N in legume fixing crops, crop residues and the cultivation of organic soils (99 Gg). Since 1990 emissions from agricultural soils have risen by 24.3% and those from manure management by 67%. In contrast emissions from tussock and arable crop residue burning have declined by 75.3% and 42.4% respectively. Since emissions from any source other than agricultural soils are negligible they will not be discussed further.
In addition to CH4 and N2O emissions resulting from agricultural activities, NZ agriculture also emits CO2 and trace quantities of other gases (N2O, CH4, CO) from the combustion of fossil fuels. In the national inventory these are not reported under agricultural emissions but as a sub-category (agriculture, forestry and fisheries) under energy emissions. CO2 emissions under this sub-category in 2004 were 1143 Gg CO2, an increase of 3.1% since 1990. There are no data available to separate agricultural emissions from the burning of fossil fuels from those of the fishery and forestry sectors. Agricultural activity related CO2 emissions will not be discussed further as they are outside the scope of the proposed voluntary greenhouse gas recording system.
Figure 2-1 GHG emissions from the agricultural sector in 2004 (source MfE, 2006)
2.4 Biological pathways of emissions
2.4.1 Methane (CH4)
The principle source of CH4 from ruminants (cattle, sheep, deer and goats) is enteric methane arising as a by-product of the fermentation of feed in the rumen and, to a lesser extent, the large intestine. The rumen contains a large and diverse population of micro-organisms which break down feed to produce volatile fatty acids (VFA’s), CO2 and CH4. The VFA’s produced in the rumen are absorbed and used as an energy source, but most of the CO2 and CH4 are removed from the rumen by eructation (belching). Typically more than 80% of the CH4 is produced in the rumen and the rest in the lower digestive tract (Immig, 1996; Murray et al., 1976). In sheep, 98% of the CH4 produced is released via the mouth and 2% via the flatus (Murray et al., 1976). The micro-organisms responsible for the production of CH4 synthesise it from hydrogen, although they do have the ability to use other substrates (Miller, 1995). The removal of hydrogen by methanogens helps maintain a low partial pressure of hydrogen in the rumen without which microbial growth and forage digestion are inhibited (Wolin et al., 1997). As a percentage of the gross energy consumed, 2 - 15% can be lost as CH4 (Johnson & Ward, 1996), although in temperate forages the range is typically 3.5 – 7.5% (O’Hara et al., 2003). Non-ruminant animals (horses and pigs) also produce CH4 through enteric fermentation in the large intestine. The quantities emitted by non-ruminant animals are much lower on a unit of feed eaten basis than those produced by ruminant animals because the capacity of the large intestine to produce CH4 is much lower. In New Zealand emissions from enteric fermentation are dominated by ruminant emissions (Table 2-2), and at the national level emissions from non-ruminant animals are negligible (0.16%).
Table 2-2 Enteric CH4 emissions from New Zealand livestock in 2004 (Source MfE 2006)
| Livestock type | Animal population (1000’s) | Emission factor (kg CO2 equivalent/head/ year) |
Total emissions from enteric fermentation (Gg CO2 equivalent /annum) |
|---|---|---|---|
| Dairy cattle | 5,119 | 1668 | 8,538 |
| Non-dairy cattle | 4,528 | 1180 | 5,343 |
| Sheep | 39,572 | 227 | 8,983 |
| Goats | 137 | 189 | 26 |
| Deer | 1,720 | 462 | 795 |
| Horses | 78 | 378 | 29 |
| Swine | 385 | 32 | 12 |
| Poultry | 23,183 | 0 | 0 |
| Total | 23,725 |
A secondary source of CH4 is that arising from the anaerobic fermentation of voided faecal material. In grazing animals, where faecal material is deposited directly onto pastures, only small amounts of CH4 are emitted per unit of deposited material but large amounts can be emitted per unit of faecal material that is stored prior to being deposited onto land. In New Zealand agriculture ruminant livestock graze outdoors for 365 days per year and most faecal material is deposited directly onto pastures, although in the dairy sector some faecal material is deposited in or around the milking shed and this may be stored for varying lengths of time. Faecal material from horses will be deposited mainly on pastures but that from pigs and poultry will often be stored prior to deposition onto land. Emission factors per head and per animal species per annum are presented in Table 2-3. CH4 emissions from manure management are negligible for non-dairy cattle and sheep, while pigs have the largest per head emissions, and dairy cattle the largest per annum emissions.
Table 2-3 CH4 emissions from manure management in New Zealand 2004 (Source: MfE 2006)
| Livestock type | Animal population (1000’s) | Emission factor (kg CO2 equivalent/head/ year) |
Total emissions from manure management (Gg CO2 equivalent /annum) |
|---|---|---|---|
| Dairy cattle | 5119 | 357 | 1,827 |
| Non-dairy cattle | 4528 | 15 | 68 |
| Sheep | 39,572 | 2 | 79 |
| Goats | 137 | 4 | 1 |
| Deer | 1720 | 4 | 7 |
| Horses | 78 | 44 | 3 |
| Swine | 385 | 420 | 162 |
| Poultry | 23,183 | 2 | 46 |
| Total | 2,193 |
2.4.2 Nitrous oxide (N2O)
Nitrous oxide emissions from agricultural soils arise from nitrification and denitrification processes
(Figure 2-2). Denitrification is the stepwise reduction of soil nitrate (NO3) to gaseous nitrogen compounds, with N2O being one of the intermediate products (Haynes & Sherlock, 1986). It is an anaerobic process that requires a NO3 substrate, a restricted oxygen supply and suitable pH and temperature conditions (Firestone, 1982; Mosier et al., 1996). Nitrification is an aerobic process which in most soils is controlled by the availability of ammonium (NH4) (Schmidt, 1982).
Figure 2-2 Production of nitrous oxide by nitrification and denitrification (adapted from: O'Hara et al, 2003)
There are two principal sources of nitrogen (N) substrate in grazed pastoral systems; recycled dietary N and applied synthetic fertilisers. Ruminants are relatively poor converters of ingested dietary N into products, and the retention of N in meat, wool or milk ranges from 3 - 25% of the N ingested (Whitehead, 1995). As a result, large quantities of N are re-cycled via excreta deposited directly onto pastures by grazing livestock. The relative importance of these two sources of N substrate to nitrous oxide production is likely to vary markedly from country to country. In New Zealand pastoral agriculture, where there is a strong reliance on the biological fixation of N by forage legumes rather than synthetic fertiliser N, the vast majority of emissions arise from excreta N deposited by grazing animals. Table 2-4 presents detailed data on N2O emissions by livestock type and emission pathway.
Table 2-4 Nitrous oxide emissions by livestock type and emission pathway (source: calculated from MfE 2006). All figures in Gg CO2equivalent/annum
| Dairy cattle | Non-dairy cattle | Sheep | Deer | Non-specific | Total | |
|---|---|---|---|---|---|---|
| Fertiliser applications | ||||||
| Synthetic N | 1,9761 | 1,976 | ||||
| N from lagoons | 171 | 18.61 | 189.6 | |||
| N from solid storage | 5.71 | 5.7 | ||||
| Pasture deposited animal waste | 3,809 | 2,219 | 3,971 | 331 | .252 | 10,330.25 |
| Manure management | ||||||
| N from lagoons | .15 | 1.62 | 1.75 | |||
| N from solid storage | 9.81 | 9.8 | ||||
| Other | 37 | 37 | ||||
| Other emissions | 1033 | 103 | ||||
| Total | 3,980.15 | 2,219 | 3,971 | 331 | 2,152 | 12,653 |
1 No information is available to allocate emissions to a particular sector
2 Pigs, poultry, goats & horses
3 Burning of crop residues & cultivation of organic soils
2.5 Factors influencing CH4 and N2O emissions
As enteric CH4 emissions arise as a by-product of a fermentation process the biggest influence on the quantity of CH4 produced is therefore the amount of substrate fermented i.e. the amount of feed eaten. Thus, at a farm level the biggest factor influencing how much CH4 a single animal produces is the level of productivity since this governs how much an animal will eat. Total farm CH4 emissions will therefore be determined largely by the number of animals kept and their level of productivity.
A secondary influence on enteric CH4 emissions is the type of feed consumed (Blaxter and Clapperton 1965); feeds that ferment rapidly to produce a high proportion of proprionic acid (e.g. cereals) produce less CH4 than fibrous feeds (e.g. fresh and dried grasses). In the New Zealand situation, where animals graze outside 365 days a year and have a diet that comprises mainly fresh forage, the type of feed eaten appears to have little influence on CH4 emissions per unit of intake (Clark 2006).
For any given feed, emissions per animal will increase as the quantity of feed eaten increases, although the amount of CH4 produced per unit of intake does not necessarily remain constant since at high levels of feed intake, with some feeds at least, the increased rate of passage of feed through the digestive tract results in a lowering of the quantity of CH4 produced per unit of feed (Blaxter & Clapperton 1965). This effect seems to be minor in ruminants fed fresh forage (Clark 2006).
A third factor is age; young sheep (i.e. <1 year old) produce less CH4 per unit of feed eaten (Ulyatt and Lassey 2005) although the reasons for this are not known.
This means that in practice the quantity of feed eaten and to some extent the age of the animal are the major determinants of estimated CH4 emissions (Clark et al 2004). However, our understanding of the influence of diet, especially fresh forage diets, on CH4 emissions is incomplete and measurements of emissions from individuals and groups of animals on a wide variety of diets show considerable variation (Clark et al 2004). At the farm level, the type of diet will have an indirect effect on CH4 emissions through its effect on feed intake; a higher quality feed (e.g. leafy grass) will have different intake and animal performance characteristics than a poor quality feed (e.g. stemmy grass)
Methane emissions from animal wastes deposited on pastures (faecal material) are determined by the amount of substrate deposited and, to a lesser extent temperature and moisture, which determine the rate at which it is fermented (IPCC 2000). Only small quantities of CH4 are produced from animal wastes deposited directly onto pastures; the IPCC default value for New Zealand climatic conditions is 1% meaning that very little CH4 is produced by this route. Much larger quantities of CH4 per unit of substrate are produced when animal wastes are stored in anaerobic conditions. Fortunately in New Zealand it is only in the dairy sector that animal wastes are stored to any extent; these are wastes deposited in and around the milking shed by lactating cattle and these comprise 5% of the total animal waste produced by dairy cattle. Pig and poultry wastes are stored but our populations of these species are small and emissions correspondingly small. Emissions from stored animal wastes are influenced by the conditions, under which the material is stored, the manner in which it is treated prior to storage and the length of the storage period. At the farm level, the quantity of faecal material deposited onto pastures or stored in some way is determined by the quantity of feed eaten and its digestibility; faecal material = intake x (1-digestibility). Other influences will be treatment prior to storage, type of storage, length of storage and location of the farm.
Nitrous oxide emissions from soils are principally determined by the quantity of synthetic N applied or the quantity of N deposited by animals; at the national scale emissions are calculated as a fixed proportion of the amount of N deposited. Ruminant diets in New Zealand have a high N content because of the presence of N fixing legumes in our pastures and these concentrations are higher than those needed by the animal. Hence a large proportion of the N consumed is deposited back onto pastures in the form of dung and urine. In general, low producing animals (non-growing, non-lactating) will retain a smaller proportion of ingested N than high producing animals (lactating, rapidly growing). At a farm level the N content of the diet will vary in both space and time and be determined by the % clover in the pasture, the quantity of synthetic N fertiliser applied and the non-clover pasture species balance. Synthetic N fertilisers influence the N content of plants (Simba & Alberda 1980), and hence the N content of the diet, although since there is an inverse relationship between legume content and applied N (Ledgard & Steele 1992) in New Zealand the increased use of N fertiliser may not change the N content of the diet in most situations.
In addition to the quantity of N deposited onto pastures the form in which it is deposited also has an influence on emissions; different emissions have been recorded from N deposited as urine or faecal material (see MfE 2006). However, not enough evidence is yet available for New Zealand to apply a differential emission factor for N deposited as urine or faecal material (MfE 2006). There is little farmers can do to influence whether N is deposited as urine or faecal material in grazing animals although some condensed tannin containing species, such as sulla, can affect N partitioning in the animal.
Environmental conditions have a very large influence on N2O emissions, in particular rainfall, and this interacts with soil type such that water filled pore space is a major determinant of emissions
(Kelliher et al 2003). At the national inventory level a standard methodology is applied irrespective of time of year, location or environmental conditions and hence the only influence on estimated emissions is the quantity of N returned/applied. At the farm level this will not be the case and N2O emissions from the same quantity of deposited N will vary in both space and time.
2.6 Mitigation of CH4 and N2O
The current and potential technologies for mitigating agriculture emissions of CH4 and N2O in
New Zealand have been extensively reviewed (see O’Hara et al 2003; Clark et al 2003; Clark et al 2005) and only a brief summary will be presented here. In New Zealand ruminant animals graze outdoors all year and receive little supplementary feed and so the management options for reducing emissions are constrained. In the northern hemisphere, efforts to reduce N2O emissions are concentrated more on those from stored manures which make up a tiny proportion of New Zealand emissions. Internationally in the developed world there is little focus on reducing enteric CH4 emissions from ruminants because (a) they tend to be small compared with industrial emissions and (b) because in many developed countries (e.g. USA, UK & Japan) they are going down due to decreases in livestock numbers.
Reducing the number of animals is the simplest and most effective method of reducing GHG emissions from agriculture although, because of economic considerations, it is unlikely to be the method of choice in New Zealand.
It is possible to manipulate GHG emissions on farm by balancing the number of animals and level of productivity of each animal. From a GHG perspective, it is better for farmers to produce a given amount of product from fewer high producing animals than a larger number of low producing animals. This is because a smaller proportion of the energy consumed by a high producing animal is used for maintenance. By keeping fewer high producing animals the total amount of energy required to produce a given amount of product is therefore less and GHG emissions are correspondingly reduced. For example, in lactating dairy cows O’Hara et al (2003) estimate that a doubling of milk yield from 12kg/d to 24kg/d increases intake per animal by only 48% and reduces emissions per unit of product by 26%. In addition, CH4 production per unit of intake by an individual animal goes down as the quantity of feed eaten goes up (Blaxter & Claperton 1965) which reinforces the premise that, from a GHG perspective at least, it is better to keep fewer high producing animals. Productivity per animal in New Zealand has increased consistently over time meaning that the amount of GHG emitted per unit of product has also been falling. In the sheep sector meat and wool output has increased since 1990 but emissions have fallen; productivity per animal, and hence emissions per animal have increased, but this has been more than compensated for by decreases in the sheep population. In the dairy sector, emissions per unit of product have decreased but total emissions have increased; emissions per animal have increased as productivity per animal has increased but this has also been accompanied by increases in the dairy cattle animal population. From an individual farmer perspective reducing farm GHG emissions by increasing the level of performance of each animal is only guaranteed to work if animal numbers and/or the quantity of produce produced is controlled.
Changing the type of feed can also reduce emissions; replacing some of the New Zealand forage diet with grain would directly reduce CH4 emissions and indirectly reduce N2O emissions (lower N concentration in the diet) but is unlikely to be economic at current grain prices. Manipulating forage species and/or forage quality can influence CH4 emissions from individual animals but the scope for reductions by these methods appears to be small. Feeding forages with lowered N content and/or feeding forages that change the partitioning of N between that retained and excreted in faeces/urine will reduce N2O emissions although again the scope for this seems small at the present time.
Modifying the rumen fermentation process can in theory reduce CH4 emissions. Certain additives (e.g. monensin, fumaric acid) have been found to reduce emissions in some circumstances but so far results in forage fed ruminants have been disappointing. Monensin is used widely in New Zealand as a bloat control agent but efforts to reduce emissions by feeding monensin to New Zealand’s pasture fed ruminants have met with very mixed success. Direct modification of the rumen microbial population may also be possible by methods such as vaccination or the introduction of non-CH4 producing hydrogen utilisers such as acetogens but these methods are in the early stages of development and are many years away from the market.
There is considerable variation between animals in the amount of CH4 they emit from the same quantity of feed and in the future it may also be possible to breed for low CH4 producing animals. At present it is possible to identify low CH4 producing animals experimentally but the stability of the low producing trait has not been established.
For a given management regime any process that reduces the quantity of N deposited or applied to soils will reduce N2O emissions. Dietary manipulation is difficult in many situations since animals graze
365 days a year but supplementary feeding of such things as grain and maize silage will reduce the quantity of N returned to pastures as these feeds have lower N concentrations than pasture. Maximising productivity per animal and keeping less animals (cf CH4 emissions) also reduces the quantity of N produced as less feed is needed to generate a given amount of product. Reductions by this method are subject to the same constraints as noted above for CH4 reductions.
Reductions in N2O emissions can be brought about by manipulating soil conditions (e.g. liming and draining, avoiding compaction) and by avoiding the deposition of N on pastures during wet periods (e.g. the use of stand-off pads in winter, timing of N fertiliser applications). The possible size of reductions by these methods ranges from 4-7% (Clark et al 2001). Reductions by these methods are available immediately although they may be difficult to quantify.
The most promising avenue for reducing N2O emissions is the use of nitrification inhibitors. Work in
New Zealand by Di & Cameron, (2002, 2003) has found that the nitrification inhibitor DCD can reduce N2O emissions from urine treated grassland by up to 80% following spring and/or autumn applications of urine with or without DCD. This suggests that DCD can be a potent method for reducing emissions from the main source of emissions in New Zealand, urine patches. Products containing DCD are readily available commercially in New Zealand but at present the product costs are such that adoption by farmers is low.
From the above it is clear that for CH4, although there are some potential methods for directly reducing emissions, the options available now for New Zealand farmers are limited. However, it is important to make a distinction between direct methods that can reduce emissions per unit of feed processed in the rumen and indirect methods that do not reduce emissions per unit of feed but can reduce total emissions. As already discussed above, an individual farmer can alter emissions by balancing stock number and stock performance. Improving the quality of the diet by better pasture management means that less intake is needed to meet a given level of performance and live weights gains are increased and target slaughter weights are achieved quicker. Other indirect methods of reducing emissions are improving reproductive performance so that less replacement animals are kept e.g. lambing/calving animals at a younger age, improving gestation rates. In the short term these indirect methods can be used by individuals to manipulate their farm GHG emissions.
At present nitrification inhibitors offer the best possibility of substantial decreases in N2O emissions although the costs appear to prohibit rapid and widespread uptake. At the individual farm level however there is scope for smaller manipulations (standoff pads, draining and the introduction of lower N feeds) as well as the animal management methods described in the previous paragraph for CH4.
2.7 Summary
Agriculturally derived CH4 and N2O comprise approximately half of the current national C02e emissions. Table 2-5 provides a summary of the main components of the agricultural emissions and the influences on these.
Table 2-5 Breakdown of Non CO2 agricultural emissions and influences on these
| GHG emission source | Percentage contribution to overall agricultural sector emissions | Influences |
|---|---|---|
| CH4 from enteric processes | 64.2% | Amount of feed, type of feed, age of animal |
| CH4 from pasture deposited wastes | 2% | Amount of feed, digestibility, pasture temperature and moisture |
| N20 from pasture deposited wastes | 28% | Diet, animal productivity, soil conditions, climate |
| N2O from fertiliser application | 5.3% | Soil conditions, climate |
| Other sources | 0.5% | |
| TOTAL | 100% |
Emissions from agriculture have risen by approximately 1% per year since 1990. On a sectoral basis dairy and sheep are the biggest emitters, followed by beef cattle. Deer and non-ruminant animals (pigs, poultry and horses) are minor emitters. Emissions from dairy cows have risen by 76% in the last 15 years while those from sheep have fallen by 20%. Beef cattle emissions have risen by 8%.
Methane emissions arise primarily as a by-product of the fermentation of feed in the digestive tract of ruminants. The biggest single influence on emissions per animal is the quantity of feed eaten. Feed type and age also affect emissions per animal.
Nitrous oxide is released from soils in the nitrification/ denitrification process. The quantity of N deposited onto soils is the major determinant of N2O emissions. Soil type, form of N and environmental factors (temperature, rainfall) are also important drivers.
At present there are no mitigation technologies available to farmers to reduce CH4 emissions in a practical and cost effective manner. Individual farmers can influence emissions to a limited degree through individual management decisions that affect individual animal productivity and through the number of animals kept. There may also be options for changing diets and modifying rumen fermentation activity but the research findings in this area are mixed and inconclusive.
Experiments have shown that nitrification inhibitors can substantially reduce N2O emissions but costs are not conducive to extensive farmer uptake. Individual farmers can also influence emissions through a range of management practices e.g. drainage, liming, the use of stand off pads and reducing the N content of diets although these mitigation practices are not easily captured in emission estimates. Individual farmers can influence estimated emissions through individual management decisions that affect individual animal productivity, since this affects N retention and excretion, and the number of animal kept.
2.8 Recommendations
A voluntary greenhouse gas reporting system for agriculture should focus on emissions of CH4 and N2O as these sources comprise nearly half of the current national C02e emissions. . Further the VGGR should focus on enteric CH4 emissions which account for 64% of the overall agricultural emissions and N2O emissions from pasture deposited wastes which account for 28% of the overall agricultural emissions.
The VGGR should also focus on the sectors that contribute the major sources of emissions, that is dairy, sheep and beef cattle.
Desired information inputs for a system to estimate CH4 and N2O emissions at the farm level will be:
• Diet (nature and volumes);
• Animal numbers, type and age;
• Animal productivity;
• Soil type
• Climatic factors (rainfall and temperature)
• Nitrification inhibitors.
1 http://www.maf.govt.nz/statistics/primaryindustries/business-types/index.htm
2 MfE New Zealand’s Greenhouse Gas Inventory 1990 – 2004 ; Http://unfccc.int/national_reports/annex_i_ghg_inventories/
national_inventories_submissions/items/3734.php
3 Clark et al (2003)
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