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Chapter 6: Agriculture

6.1 Sector overview

In 2007, the agriculture sector contributed 36,430.0 Gg carbon dioxide equivalent (Gg CO2-e) (48.2 per cent) of New Zealand’s total greenhouse gas emissions. Emissions in this sector had increased by 3,918.9 Gg CO2-e (12.1 per cent) from the 1990 level of 32,511.1 Gg CO2-e (Figure 6.1.1). This increase is primarily due to a 1,507.4 Gg CO2-e (6.9 per cent) increase in methane (CH4) emissions from the enteric fermentation category and a 2,254.7 Gg CO2-e (22.4 per cent) increase in nitrous oxide (N2O) emissions from the agricultural soils category.

Figure 6.1.1 New Zealand agricultural sector emissions from 1990 to 2007

Figure 6.1.1 New Zealand agricultural sector emissions from 1990 to 2007

Year Gg CO2-equivalent
1990 32,511.1
1991 32,534.1
1992 31,943.2
1993 32,331.5
1994 33,310.4
1995 33,728.9
1996 34,070.0
1997 34,780.5
1998 34,171.1
1999 34,660.8
2000 35,836.0
2001 36,403.9
2002 36,509.4
2003 37,136.2
2004 37,188.7
2005 37,519.1
2006 37,491.2
2007 36,430.0

In 2007, CH4 emissions from enteric fermentation were 64.0 per cent (23,326.4 Gg CO2-e) of agricultural emissions and 30.9 per cent of New Zealand’s total emissions. Nitrous oxide emissions from the agricultural soils category were 33.8 per cent (12,298.1 Gg CO2-e) of agricultural emissions and 16.3 per cent of total emissions.

Agriculture is a major component of the New Zealand economy, and agricultural products comprise 56 per cent of total merchandise exports (Ministry of Agriculture and Forestry, 2008). This is facilitated by the favourable temperate climate, the abundance of agricultural land and the unique farming practices used in New Zealand. These practices include the extensive use of year-round grazing systems and a reliance on nitrogen fixation by legumes rather than nitrogen fertiliser.

Figure 6.1.2 Change in New Zealand’s emissions from the agricultural sector from
1990 to 2007

Figure 6.1.2 Change in New Zealand's emissions from the agricultural sector from

Category 1990
(Gg CO2-equivlaent)
2007
(Gg CO2-equivalent)
Enteric fermentation 21,819.0 23,326.4
Manure management 616.7 787.1
Rice cultivation Not occurring Not occurring
Agricultural soils 10,043.4 12,298.1
Prescribed burning of savannahs 3.2 1.0
Field burning of savannahs 28.7 17.5

Note: Rice cultivation does not occur (NO) in New Zealand.

Since 1990, there have been changes in the proportions of the main livestock species farmed in New Zealand. There has been an increase in dairy and deer production because of high world demand and favourable prices. This has been counterbalanced by land coming out of sheep production and decreasing sheep numbers. Beef numbers have remained relatively static. There have also been productivity increases across all major animal species and classes.

The land area used for horticulture also increased by 50 per cent since 1990 and the types of produce grown have changed (Ministry of Agriculture and Forestry, 2008). There is now less cultivated land area for barley, wheat, and fruit but more for grapes (for wine production) and vegetables than in 1990. There has also been a net increase in land planted in forestry, therefore taking land out of agricultural production.

There was a gradual increase in the implied emission factors for dairy cattle and beef cattle from 1990 to 2007. This is expected because the applied method uses animal performance data that reflects the increased levels of productivity achieved by New Zealand farmers since 1990. Increases in animal liveweight and productivity (milk yield and liveweight gain per animal) require increased feed intake by the animal to meet higher energy demands. Increased feed intake results in increased CH4 emissions per animal.

Changes in emissions between 2006 and 2007

Total agricultural emissions in 2007, were 1,061.2 Gg CO2-e (2.8 per cent) lower than the 2006 level. This was largely due to a decrease in the population of sheep (1,621,117 or 4.0 per cent), deer (190,895 or 12.0 per cent), and non-dairy cattle (45,519 or 1.0 per cent). There were also decreases in animal performance statistics. For example, milk yield per head decreased by 4.0 per cent, non-dairy cattle weight decreased by 10.6 percent, and sheep weight decreased by 2.7 per cent. The drought that affected many regions of New Zealand throughout the summer and autumn of 2007/2008 was the main cause for these decreases in animal performance (Ministry of Agriculture and Forestry, 2008).

6.1.1 Methodological issues for the agriculture sector

New Zealand uses a June year for all animal statistics as this reflects the natural biological cycle for animals in the southern hemisphere. The models developed to estimate agricultural emissions work on a monthly time step, beginning on the 1 July of one year and ending on the 30 June of the Next year. To calculate emissions for single calendar years (January–December), emission data from the last six months of a July–June year are combined with the first six months’ emissions of the Next July to June year.

To ensure consistency, a single livestock population characterisation and feed-intake estimate is used to estimate CH4 emissions from the enteric fermentation category, CH4 and N2O emissions from the manure management category, and N2O emissions from the pasture, range and paddock manure subcategory.

Information on livestock population census and survey procedures is included in Annex 3.1.

6.2 Enteric fermentation (CRF 4A)

6.2.1 Description

Methane is a by-product of digestion in ruminants eg, cattle, and some non-ruminant animals such as swine and horses. Within the agriculture sector, ruminants are the largest source of CH4 as they are able to digest cellulose. The amount of CH4 released depends on the type, age and weight of the animal, the quality and quantity of feed, and the energy expenditure of the animal.

In 2007, CH4 emissions from the enteric fermentation category were identified as the largest key category for New Zealand in the level assessment (excluding land use, land-use change and forestry (LULUCF)). In accordance with Intergovernmental Panel on Climate Change (IPCC) good practice guidance (IPCC, 2000), the methodology for estimating CH4 emissions from enteric fermentation in domestic livestock was revised to a Tier 2 modelling approach for the 2003 inventory submission. All subsequent inventory submissions have used this Tier 2 approach.

In 2007, enteric fermentation was the largest single emissions category of New Zealand’s inventory, contributing 23,326.4 Gg CO2-e. This represented 30.9 per cent of New Zealand’s total CO2-e emissions and 64.0 per cent of agricultural emissions. Cattle contributed 13,824.3 Gg CO2-e (59.3 per cent) of emissions from the enteric fermentation category, and sheep contributed 8,789.4 Gg CO2-e (37.7 per cent) of emissions from this category. Emissions from the enteric fermentation category in 2007 were 1,507.4 Gg CO2-e (6.9 per cent) above the 1990 level of 21,819.0 Gg CO2-e. Since 1990, there were changes in the source of emissions within the enteric fermentation category. The largest increase came from emissions from dairy cattle. In 2007, dairy cattle were responsible for 8,531.2 Gg CO2-e, an increase of 3,519.8 Gg CO2-e (70.2 per cent) from the 1990 level of 5,011.4 Gg CO2-e. Meanwhile, there have been decreases in emissions from sheep and minor livestock populations such as goats, horses and swine. In 2007, emissions from sheep were 8,789.4 Gg CO2-e, a decrease of 2,490.6 Gg CO2-e (22.1 per cent) from the 1990 level of 11,280.0 Gg CO2-e.

6.2.2 Methodological issues

Emissions from cattle, sheep and deer

New Zealand’s Tier 2 method (Clark et al, 2003) uses a detailed livestock population characterisation and livestock productivity data to calculate feed intake for the four largest categories in the New Zealand ruminant population (dairy cattle, beef cattle, sheep and deer). The amount of CH4 emitted per animal is calculated using CH4 emissions per unit of feed intake (Figure 6.2.1).

Livestock population data

The New Zealand ruminant population is separated into four main categories: dairy cattle, beef cattle, sheep and deer. Each livestock category is further subdivided by population models (Clark et al, 2003; Clark, 2008). The populations within a year are adjusted on a monthly basis to account for births, deaths and transfers between age groups. This is necessary because the numbers present at one point in time may not accurately reflect the numbers present at other times of the year. For example, the majority of lambs are born and slaughtered between August and May and, therefore, do not appear in the June census or survey data.

Livestock numbers are provided by Statistics New Zealand from census and survey data conducted in June each year.

Figure 6.2.1 Schematic diagram of how New Zealand’s emissions from enteric fermentation are calculated

Figure 6.2.1Schematic diagram of how New Zealand's emissions from enteric fermentation are calculated

Note: GEI is the gross energy intake and DMI is the dry-matter intake.

The schematic illustrates an overview of the enteric fermentation model. All of the details in the figure are contained within the text of this chapter (Agriculture).

Livestock productivity data

Productivity data comes from Statistics New Zealand and industry statistics. To ensure consistency, the same data sources are used each year. This ensures the data provides a time series that reflects changing farming practices, even if there is uncertainty surrounding the absolute values.

Obtaining data on the productivity of ruminant livestock in New Zealand, and how it has changed over time, is a difficult task. Some of the information collected is complete and collected regularly. For example, the slaughter weights of all livestock exported from New Zealand are collected by the Ministry of Agriculture and Forestry from all slaughter plants in New Zealand. This information is used as a surrogate for changes in animal liveweight over time. Other information, for instance liveweight of dairy cattle and liveweight of breeding bulls, is collected at irregular intervals, from small survey populations, or is not available at all.

Livestock productivity and performance data are summarised in the time-series tables in the MS Excel worksheets available for download with this report from the Ministry for the Environment’s website (here). The data includes average estimated liveweights, milk yields and milk composition of dairy cows, average liveweights of beef cattle (beef cows, heifers, bulls and steers), average liveweights of sheep (ewes and lambs), and average estimated liveweights of deer (breeding and growing hinds and stags).

Dairy cattle: Data on milk production is provided by Livestock Improvement Corporation Ltd, a dairy-farmer-owned company providing services to the dairy, beef and deer industries (2008). This data includes the amount of milk processed through New Zealand dairy factories and milk for the domestic market. Annual milk yields per animal are obtained by dividing the total milk produced by the total number of milking dairy cows and heifers. Milk composition data is taken from the Livestock Improvement Corporation’s national statistics. For all years, lactation length is assumed to be 280 days.

Average liveweight data for dairy cows is obtained by taking into account the proportion of each breed in the national herd and its age structure based on data from the Livestock Improvement Corporation. Dairy cow liveweights are only available from the Livestock Improvement Corporation from 1996 onwards. For earlier years in the time series, liveweights were estimated using the trend in liveweights from 1996 to 2007, together with data on the breed composition of the national herd.

Growing dairy replacements at birth are assumed to be 9 per cent of the weight of the average cow and 90 per cent of the weight of the average adult cow at calving. Growth between birth and calving (at two years of age) is divided into two periods: birth to weaning, and weaning to calving. Higher growth rates are applied between births and weaning, when animals receive milk as part of their diet. Within each period, the same daily growth rate is applied for the entire length of the period.

No data is available on the liveweights and performance of breeding bulls. An assumption is made that their average weight is 500 kg and that they are growing at 0.5 kg per day. This is based on expert opinion from industry data. For example, dairy bulls range from small Jerseys through to larger-framed, European beef breeds. The assumed weight of 500 kg and growth rate of 0.5 kg/day provides an average weight (at the mid-point of the year) of 592 kg. This is almost 25 per cent higher than the average weight of a breeding dairy cow but it is realistic given that some of the bulls will be of a heavier breed (eg, Friesian and some beef breeds). Total emissions are not highly sensitive to these assumed values, as breeding bulls only make a small contribution to total emissions eg, breeding dairy bulls contribute less than 0.1 per cent of emissions from the dairy sector.

Beef cattle: The principal source of information for estimating productivity for beef cattle is livestock slaughter statistics provided by the Ministry of Agriculture and Forestry. All growing beef animals are assumed to be slaughtered at two years of age and the average weight at slaughter for the three subcategories (heifers, steers and bulls) is estimated from the carcass weight at slaughter. Liveweights at birth are assumed to be 9 per cent of an adult cow weight for heifers and 10 per cent of an adult cow weight for steers and bulls. As with dairy cattle, growth rates of all growing animals are divided into two periods: birth to weaning, and weaning to slaughter, as higher growth rates are applied before weaning when animals receive milk as part of their diet. Within each period, the same daily growth rate is applied for the entire length of the period.

The carcass weights obtained from the Ministry of Agriculture and Forestry slaughter statistics do not separate carcass weights of adult dairy cows and adult beef cows. Therefore, a number of assumptions2 are made in order to estimate the liveweights of beef breeding cows. A total milk yield of 800 litres per breeding beef cow is assumed.

Sheep: Livestock slaughter statistics from the Ministry of Agriculture and Forestry are used to estimate the liveweights of adult sheep and lambs, assuming killing-out percentages of 43 per cent for ewes and 45 per cent for lambs. Lamb liveweights at birth are assumed to be 9 per cent of the adult ewe weight, with all lambs assumed to be born on 1 September. Growing breeding and non-breeding ewe hoggets are assumed to reach full adult size at the time of mating when aged 20 months. Adult wethers are assumed to be the same weight as adult breeding females. No within-year pattern of liveweight change is assumed for either adult wethers or adult ewes. All ewes rearing a lamb are assumed to have a total milk yield of 100 litres. Breeding rams are assumed to weigh 40 per cent more than adult ewes. Wool growth (greasy fleece growth) is assumed to be 5 kg/annum in mature sheep (ewes, rams and wethers) and 2.5 kg/annum in growing sheep and lambs.

Deer: Liveweights of growing hinds and stags are estimated from Ministry of Agriculture and Forestry slaughter statistics, assuming a killing-out percentage of 55 per cent. A fawn birth weight of 9 per cent of the adult female weight and a common birth date of mid-December are assumed. Liveweights of breeding stags and hinds are based on published data that has liveweights changing every year by the same percentage change recorded in the slaughter statistics for growing hinds and stags above the 1990 base. It is assumed there is no pattern of liveweight change with any given year. The total milk yield of lactating hinds is assumed to be 240 litres (Kay, 1995).

Dry-matter intake calculation

Dry-matter intake (DMI) for the major livestock classes (dairy cattle, beef cattle, sheep and deer) and sub-classes of animals (breeding and growing) is estimated by calculating the energy required to meet the levels of animal performance and dividing this by the energy concentration of the diet consumed. For dairy cattle, beef cattle and sheep, energy requirements are calculated using algorithms developed in Australia (CSIRO, 1990). These algorithms are chosen as they specifically include methods to estimate the energy requirements of grazing animals. This method estimates a maintenance requirement (a function of liveweight, the amount of energy expended on the grazing process), and a production energy requirement influenced by the level of productivity (eg, milk yield and liveweight gain), physiological state (eg, pregnant or lactating), and the stage of maturity of the animal. All calculations are performed on a monthly basis.

For deer, an approach similar to that used for cattle is adopted using algorithms derived from New Zealand studies on red deer. The algorithms take into account animal liveweight and production requirements based on the rate of liveweight gain, sex, milk yield and physiological state.

Monthly energy concentrations

A single data-set of monthly energy concentrations of the diets consumed by beef cattle, dairy cattle, sheep and deer is used for all years in the time series. This is because there is no comprehensive published data available that allow the estimation of a time series dating back to 1990. The data used is derived from farm surveys on commercial cattle and sheep farms.

Methane emissions per unit of feed intake

There are a number of published algorithms and models3 of ruminant digestion for estimating CH4 emissions per unit of feed intake. The data requirements of the digestion models make them difficult to use in generalised national inventories and none of the methods have high predictive power when compared against experimental data. Additionally, the relationships in the models have been derived from animals fed indoors on diets unlike those consumed by New Zealand’s grazing ruminants.

Since 1996, New Zealand scientists have been measuring CH4 emissions from grazing cattle and sheep using the SF6 tracer technique (Lassey et al, 1997; Ulyatt et al, 1999). New Zealand now has one of the largest data-sets in the world of CH4 emissions determined using the SF6 technique on grazing ruminants. To obtain New Zealand-specific values, published and unpublished data on CH4 emissions from New Zealand were collated and average values for CH4 emissions from different categories of livestock were obtained. Sufficient data was available to obtain values for adult dairy cattle, sheep more than one year old and growing sheep (less than one year old). This data is presented in Table 6.2.1 together with IPCC (2000) default values for per cent gross energy used to produce CH4. The New Zealand values fall within the IPCC range and are applied in this submission. Table 6.2.2 shows a time series of CH4 implied emission factors for dairy cattle, beef cattle, sheep and deer. Measurements using open-circuit respiration chamber techniques that provided complete gas balances were conducted to further confirm the SF6 tracer technique.

The adult dairy cattle value is assumed to apply to all dairy and beef cattle, irrespective of age, and the adult ewe value is applied to all sheep greater than one year old. An average of the adult cow and adult ewe value (21.25g CH4/kg DMI) is assumed to apply to all deer. In very young animals receiving a milk diet, no CH4 is assumed to arise from the milk proportion of the diet. Not all classes of livestock are covered in the New Zealand data-set and assumptions are made for these additional classes.

Table 6.2.1 Methane emissions from New Zealand measurements and IPCC defaults

  Adult dairy cattle Adult sheep Adult sheep < 1 year
New Zealand data (g CH4/kg DMI) 21.6 20.9 16.8
New Zealand data (%GE) 6.5 6.3 5.1
IPCC (2000) defaults (%GE) 6 ±0.5 6 ±0.5 5 ±0.5

Table 6.2.2 Time series of New Zealand’s implied emission factors for enteric
fermentation (EF) (kg CH4 per animal per annum)

Year Dairy cattle Beef cattle Sheep Deer
1990 69.4 50.7 9.3 18.8
1991 71.6 51.9 9.4 19.3
1992 72.0 52.4 9.4 20.2
1993 73.9 53.5 9.5 20.4
1994 71.8 54.2 9.6 19.7
1995 71.6 53.3 9.4 20.6
1996 74.1 54.6 9.8 20.9
1997 74.7 55.1 10.2 21.0
1998 73.2 55.1 10.2 21.2
1999 75.9 54.1 10.2 21.3
2000 77.2 56.1 10.7 21.8
2001 77.6 57.0 10.7 21.7
2002 76.8 56.6 10.7 21.6
2003 79.8 56.1 10.7 21.6
2004 78.1 57.0 11.0 21.7
2005 79.1 57.7 11.1 22.2
2006 78.8 58.6 10.9 22.2
2007 77.2 57.4 10.9 22.3

Emissions from other farmed species

A Tier 1 approach is adopted for minor livestock such as goats, horses and swine using either IPCC default emission factors (horses and swine) or New Zealand-derived values (goats). These minor species comprised 0.2 per cent of total enteric CH4 emissions in 2007.

Livestock population data

The populations of goats, horses and pigs are reported using the animal census (or survey) data from Statistics New Zealand.

Livestock emissions data

Horses and swine: Enteric CH4 from these classes of livestock were not a key category in 2007 and in the absence of data to develop New Zealand emissions’ factors, IPCC default values were used.

Goats: Enteric CH4 from goats was not a key category in 2007. There is no published data available to attempt a detailed categorisation of the performance characteristics, as has been done for the major livestock categories. New Zealand uses a country-specific value of 9 kg CH4/head/yr. This was calculated by assuming a default CH4 emission value from goats for all years that is equal to the per head value of the average sheep in 1990 (ie, total sheep emissions/total sheep population). The goat emission factor is not indexed to sheep over time because there is no data to support the kind of productivity increases that have been seen in sheep.

6.2.3 Uncertainties and time-series consistency

Livestock numbers

Many of the calculations in this sector require livestock numbers. Both census and survey data are used. Surveys occur each year between each census. Detailed information from Statistics New Zealand on the census and survey methods is included in Annex 3.1.1.

Methane emissions from enteric fermentation

In the 2003 inventory submission, the CH4 emissions data from domestic livestock in 1990 and 2001 were subjected to Monte Carlo analysis using the software package @RISK to determine the uncertainty of the annual estimate (Clark et al, 2003). For the 2007 data, the uncertainty in the annual estimate was calculated using the 95 per cent confidence interval determined from the Monte Carlo simulation as a percentage of the mean value ie, in 2001, the uncertainty in annual emissions was ±53 per cent.

Table 6.2.3 New Zealand’s uncertainty in the annual estimate of enteric fermentation emissions for 1990, 2001 and 2007, estimated using Monte Carlo simulation (1990, 2001) and the 95 per cent confidence interval (2007)

Year Enteric CH4 emissions (Gg/annum) 95% CI min 95% CI max
1990 1,038.6 488.1 1,589.1
2001 1,123.0 527.8 1,718.2
2007 1,110.8 522.1 1,699.5

Note: The CH4 emissions used in the Monte Carlo analysis exclude those from swine and horses.

Uncertainty in the annual estimate is dominated by variance in the measurements of the “CH4 per unit of intake” factor. For the measurements made used to determine this factor, the standard deviation divided by the mean is equal to 0.26. This uncertainty is predominantly due to natural variation from one animal to the Next. Uncertainties in the estimates of energy requirements, herbage quality and population data are much smaller (0.005–0.05).

6.2.4 Source-specific QA/QC and verification

In 2007, CH4 from enteric fermentation was identified as a key category (level and trend assessment). In preparation for this inventory, the data for this category underwent Tier 1 quality checks.

Methane emission rates measured for 20 dairy cows and scaled up to a herd have been corroborated using micrometeorological techniques. Laubach and Kelliher (2004) used the integrated horizontal flux technique and the flux gradient technique to measure CH4 flux above a dairy herd. Both techniques are comparable, within estimated errors, to scaled-up animal emissions. The emissions from the cows measured by integrated horizontal flux and averaged over three campaigns are 329 (±153) g CH4/day/cow compared to 365 (±61) g CH4/day/cow for the scaled-up measurements reported by Waghorn et al, (2002; 2003). Methane emissions from lactating dairy cows have also been measured using the New Zealand SF6 tracer method and open-circuit respiration chamber techniques (Grainger et al, 2007). Total CH4 emissions were similar, 322 and 331 g CH4/day, when measured using chambers or the SF6 tracer technique respectively.

6.2.5 Source-specific recalculations

All activity data was updated with the latest available data (Statistics New Zealand table builder and Infoshare database, 2008).

6.2.6 Source-specific planned improvements

A national inter-institutional ruminant CH4 expert group was formed to identify the key strategic directions for research into the CH4 inventory and mitigation, and to develop a collaborative approach to improve the certainty of CH4 emission data. This expert group is funded through the Ministry of Agriculture and Forestry.

The Pastoral Greenhouse Gas Research Consortium has been established to carry out research, primarily into mitigation technologies and management practices but also on improving on-farm inventories. The consortium is funded from both public and private sector sources. The implementation of the Tier 2 approach for CH4 emissions from enteric fermentation and manure management is a consequence of the research identified and conducted by the expert group.

6.3 Manure management (CRF 4B)

6.3.1 Description

In 2007, emissions from the manure management category comprised 787.1 Gg CO2-e (2.2 per cent) of emissions from the agriculture sector. Emissions from manure management had increased by 170.3 Gg CO2-e (27.6 per cent) from the 1990 level of 616.7 Gg CO2-e.

Livestock manure is composed principally of organic material. When the manure decomposes in the absence of oxygen, methanogenic bacteria produce CH4. The amount of CH4 emissions is related to the amount of manure produced and the amount that decomposes anaerobically. Methane from manure management was identified as a key category (level assessment) for 2007.

The manure management category also includes N2O emissions related to manure handling before the manure is added to the soil. The amount of N2O emissions depends on the system of waste management and the duration of storage. With New Zealand’s extensive use of all-year-round grazing systems, this category contributed a relatively small amount of N2O – 58.0 Gg CO2-e – in 2007. In comparison, N2O emissions from the agricultural soils category totalled 12,298.1 Gg CO2-e in 2007.

In New Zealand, dairy cows only have a fraction (5 per cent) of their excreta stored in anaerobic lagoon waste systems. The remaining 95 per cent of excreta from dairy cattle is deposited directly onto pasture. These fractions relate to the proportion of time dairy cattle spend on pasture compared to the time they spend in the milking shed. All other ruminant species (sheep, beef cattle, goats, deer and horses) graze outdoors all year round and deposit all of their faecal material (dung and urine) directly onto pastures. This distribution is consistent with the revised 1996 IPCC guidelines (IPCC, 1996) for the Oceania region. New Zealand scientists and Ministry of Agriculture and Forestry officials consider the default distributions are applicable to New Zealand farming practices. Table 6.3.1 shows the distribution of livestock in animal waste management systems in New Zealand.

Table 6.3.1 Distribution of livestock across animal waste management systems in New Zealand

  Percentage of animals in each animal waste management system
Livestock Anaerobic lagoon Pasture, range and paddock Solid storage and dry-lot Other
Non-dairy cattle _ 100 _ _
Dairy cattle 5 95 _ _
Poultry _ 3 _ 97
Sheep _ 100 _ _
Swine 55 _ 17 28
Goats _ 100 _ _
Deer _ 100 _ _
Horses _ 100 _ _

6.3.2 Methodological issues

Methane from manure management

The IPCC Tier 2 approach to calculate CH4 emissions from ruminant animal wastes in New Zealand has been used since the 2006 inventory submission. The Tier 2 approach is based on the methods recommended by Saggar et al (2003) in a review commissioned by the Ministry of Agriculture and Forestry.

The approach relies on (1) an estimation of the total quantity of faecal material produced; (2) the partitioning of this faecal material between that deposited directly onto pastures and that stored in anaerobic lagoons; and (3) the development of New Zealand-specific emission factors for the quantity of CH4 produced per unit of faecal dry-matter deposited directly onto pastures, and that stored in anaerobic lagoons. Table 6.3.2 summarises the key variables in the calculation of CH4 from manure management.

Table 6.3.2 Derivation of CH4 emissions from manure management in New Zealand

Animal species Proportion of faecal material deposited on pasture CH4 from animal waste on pastures (g CH4/kg faecal dry-matter) Proportion of faecal material stored in anaerobic lagoons Water dilution rate (litres water/kg faecal dry-matter) Average depth of a lagoon (metres) CH4 from anaerobic lagoon(g CH4/m2/
year)
Dairy cattle 0.95 0.98 0.05 90 4.6 3.27
Beef cattle 1.0 0.98 0.0
Sheep 1.0 0.69 0.0
Deer 1.0 0.92 0.0

New Zealand-specific emissions’ factors are not available for CH4 emissions from manure management for swine, horses and poultry. These are minor livestock categories in New Zealand and IPCC default emission factors are used to calculate emissions.

Dairy cattle

Faecal material deposited directly onto pastures: The quantity of faecal dry-matter produced is obtained by multiplying the quantity of feed eaten by the dry-matter digestibility of the feed, minus the feed retained in product. These feed intake and dry-matter digestibility estimates are used in the enteric CH4 and N2O Tier 2 model calculations. Consistent with the N2O inventory, 95 per cent of faecal material arising from dairy cows is assumed to be deposited directly onto pastures (Ledgard and Brier, 2004). The quantity of CH4 produced per unit of faecal dry-matter is 0.98 g CH4/kg. This value is obtained from New Zealand studies on dairy cows (Saggar et al, 2003; Sherlock et al, 2003).

Faecal material stored in anaerobic lagoons: Five per cent of faecal material arising from dairy cows is assumed to be stored in anaerobic lagoons. The method adopted here is to assume that all faeces deposited in lagoons are diluted with 90 litres of water per kilogram of dung dry-matter (Heatley, 2001). This gives the total volume of effluent stored. Annual CH4 emissions are estimated using the data of McGrath and Mason (2002). McGrath and Mason (2002) calculated specific emissions values of 0.33–6.21 kg CH4/m2/year from anaerobic lagoons in New Zealand. The mean value of 3.27 CH4/m2/year of this range is assumed in the New Zealand Tier 2 calculations.

Beef cattle, sheep and deer

The quantity of faecal dry-matter produced is obtained by multiplying the quantity of feed eaten by the dry-matter digestibility of the feed, minus the feed retained in product. These feed intake and dry-matter digestibility estimates are used in the enteric CH4 and N2O Tier 2 model calculations.

Beef cattle, sheep and deer are not housed in New Zealand and all faecal material is deposited directly onto pastures.

No specific studies have been conducted in New Zealand on CH4 emissions from beef cattle faeces and values obtained from dairy cattle studies (0.98 g CH4/kg) are used (Saggar et al, 2003; Sherlock et al, 2003).

The quantity of CH4 produced per unit of sheep faecal dry-matter is 0.69g CH4/kg. This value is obtained from New Zealand studies on sheep (Carran et al, 2003).

There are no New Zealand studies on CH4 emissions from deer manure and values obtained from sheep and cattle are used. The quantity of CH4 produced per unit of faecal dry-matter is assumed to be 0.92 g CH4/kg. This value is the average value obtained from all New Zealand studies on sheep (Carran et al, 2003) and dairy cattle (Saggar et al, 2003; Sherlock et al, 2003).

Nitrous oxide from manure management

This subcategory reports N2O emissions from the anaerobic lagoon, solid storage and dry-lot, and other animal waste management systems. Emissions from the pasture range and paddock animal waste management system are reported in the agricultural soils category.

The calculations for the quantity of nitrogen in each animal waste management system are based on the nitrogen excreted (Nexx) per head per year multiplied by the livestock population, the allocation of animals to animal waste management systems (Table 6.3.1), and a N2O emission factor for each animal waste management system.

The Nexx values are calculated from the nitrogen intake less the nitrogen in animal products. Nitrogen intake is determined from feed intake and the nitrogen content of the feed. Feed intake and animal productivity values are the same as used in the Tier 2 model for determining CH4 emissions (Clark et al, 2003). The nitrogen content of feed is estimated from a review of over 6000 pasture samples of dairy and sheep and beef systems (Ledgard et al, 2003).

The nitrogen content of product is derived from industry data. For lactating cattle, the nitrogen content of milk is derived from the protein content of milk (nitrogen = protein/6.25) published annually by the Livestock Improvement Corporation. The nitrogen content of sheep meat and wool and beef, and the nitrogen retained in deer velvet, are taken from New Zealand-based research.

Table 6.3.3 shows Nex values increasing over time reflecting the increases in animal productivity in New Zealand since 1990.

Table 6.3.3 Nex values for New Zealand’s main livestock classes over time (kg/head/year)

  Sheep N Non-dairy
Cattle N
Dairy
Cattle N
Deer N
1990 12.61 65.51 103.87 24.88
1991 12.83 66.98 107.02 25.64
1992 12.80 67.78 107.49 26.83
1993 12.93 69.26 109.80 27.06
1994 12.96 70.15 106.85 26.20
1995 12.81 68.95 106.50 27.42
1996 13.32 70.82 109.81 27.76
1997 13.89 71.48 110.46 27.84
1998 14.01 71.48 108.50 28.06
1999 13.97 70.00 111.96 28.24
2000 14.58 72.60 113.41 28.91
2001 14.72 73.71 113.95 28.85
2002 14.68 73.14 113.12 28.74
2003 14.66 72.38 117.40 28.67
2004 15.07 73.61 115.23 28.88
2005 15.20 74.60 116.43 29.47
2006 15.02 75.81 115.67 29.58
2007 14.97 74.05 113.62 29.68

6.3.3 Uncertainties and time-series consistency

The main factors causing uncertainty in N2O emissions from manure management are the emission factors from manure and manure management systems, the livestock population, nitrogen excretion rates, and the use of the various manure management systems (IPCC, 2000).

New Zealand uses the IPCC default values for EF3 (direct emissions from waste) for all animal waste systems except for EF3(PR&P) (manure deposited on pasture, range and paddock). The New Zealand-specific emission factor for EF3(PR&P), is 0.01 kg N2O-N/kg N (further details in section 6.5.2). The IPCC default values have uncertainties of –50 per cent to +100 per cent (IPCC, 2000).

The overall inventory uncertainty analysis shown in Annex 7 (Table A.7.1) demonstrates that the effect of uncertainty in annual emissions from manure management is relatively minor compared to the effect of the uncertainty in CH4 emissions from enteric fermentation and N2O from agricultural soils.

6.3.4 Source-specific QA/QC and verification

Methane from manure management was identified as a key category (level assessment) in 2007. In preparation for this inventory submission, the data for this category underwent Tier 1 quality checks.

6.3.5 Source-specific recalculations

All activity data was updated with the latest available data (Statistics New Zealand table builder and Infoshare database, 2008).

6.3.6 Source-specific planned improvements

The National Institute of Water and Atmospheric Research (NIWA) has recently completed a project on continuous measurement of anaerobic lagoon emissions over a full year. New Zealand will assess whether this research can be used to update the manure management estimates in the future.

6.4 Rice cultivation (CRF 4C)

6.4.1 Description

There is no rice cultivation in New Zealand. The “NO” notation is reported in the common reporting format tables.

6.5 Agricultural soils (CRF 4D)

6.5.1 Description

In 2007, the agricultural soils category contributed 12,298.1 Gg CO2-e (16.3 per cent) to New Zealand’s total emissions and 95.7 per cent to total N2O emissions. Emissions were 2,254.7 Gg CO2-e (22.4 per cent) above the 1990 level of 10,043.4 Gg CO2-e. The category comprises three subcategories. Each of these subcategories has been identified as a key category. The subcategories are:

  • direct N2O emissions from agricultural soils as a result of adding nitrogen in the form of synthetic fertilisers, animal waste, biological fixation in crops, inputs from crop residues and sewage sludge. Direct N2O soil emissions contributed 1,680.7 Gg CO2-e (13.7 per cent) to emissions from the agricultural soils category in 2007. This was an increase of 1,193.5 Gg CO2-e (245.0 per cent) from the 1990 level of 487.2 Gg CO2-e. Direct N2O emissions from agricultural soils were identified as a key category (level and trend assessment)

  • indirect N2O from nitrogen lost from the field as NO3, NH3 or NOx. In 2007, indirect N2O emissions from nitrogen used in agriculture contributed 3,270.7 Gg CO2-e (26.6 per cent) to emissions from the agricultural soils category. This was an increase of 567.5 Gg CO2-e (21.0 per cent) from the 1990 level of 2,703.1 Gg CO2-e. Indirect N2O emissions from agricultural soils were identified as a key category (level assessment)

  • direct N2O emissions from animal production (the pasture, range and paddock animal waste management system). Nitrous oxide emissions from animal production contributed 7,346.7 Gg CO2-e (59.7 per cent) to emissions from the agricultural soils category. This is an increase of 493.6 Gg CO2-e (7.2 per cent) from the 1990 level of 6,853.1 Gg CO2-e. Direct N2O emissions from agricultural soils were identified as a key category (level and trend assessment). Direct N2O emissions from animal production were identified as a key category (trend and level assessment).

Carbon dioxide emissions from limed soils are reported in the LULUCF sector.

6.5.2 Methodological issues

The two main inputs of nitrogen to the soil are excreta deposited during animal grazing and the application of nitrogen fertilisers. Emission factors and the fraction of nitrogen deposited on the soils are used to calculate N2O emissions.

Three New Zealand-specific emission factors and parameters are used in the inventory: EF1, EF3(PR&P) and FracLEACH. The emission factor, EF1 (direct emissions from nitrogen input to soil), was reviewed during 2006 and the recommendation by Kelliher and de Klein (2006) to use a country-specific factor of 1 per cent was adopted for agriculture inventory calculations in the 2006 inventory submission. The EF3(PR&P) emission factor of 1 per cent and FracLEACH (0.07) were extensively reviewed and first included in the 2001 inventory submission.

The emission factors and other parameters used in this category are documented in Annex 3.1. The calculations are included in the MS Excel worksheets available for download with this report from the Ministry for the Environment’s website (here).

Animal production (N2O)

Direct soil emissions from animal production refers to the N2O produced from the pasture, range and paddock animal waste management system. This system is the predominant regime for animal waste in New Zealand as 95 per cent of dairy cattle excreta and 100 per cent of sheep, deer and non-dairy cattle excreta are allocated to it (Table 6.3.1).

The emissions calculation is based on the livestock population multiplied by nitrogen excretion (Nexx) values and the percentage of the population on the pasture, range and paddock animal waste management system. The Nexx values and allocation to animal waste management systems are discussed in section 6.3. The Nexx values have been calculated based on the same animal intake and animal productivity values used for calculating CH4 emissions for the different animal classes and species in the Tier 2 model. This ensures the same base values are used for both the CH4 and N2O emission calculations.

New Zealand uses a country-specific emission factor for EF3(PR&P) of 0.01 (Carran et al, 1995; Muller et al, 1995; de Klein et al, 2003; Kelliher et al, 2003). Considerable research effort has gone into establishing a New Zealand-specific emission factor for EF3(PR&P). Field studies have been performed as part of a collaborative research effort called NzOnet. The EF3 (PR&P) parameter has been measured by NzOnet researchers in the Waikato (Hamilton), Canterbury (Lincoln) and Otago (Invermay) regions for pastoral soils of different drainage classes (de Klein et al, 2003). These regional data are comparable because the same measurement methods were used at the three locations. The percentage of applied nitrogen (in urine or dung) emitted as N2O, and relevant environmental variables, were measured in four separate trials that began in autumn 2000, summer 2002, spring 2002 and winter 2003. Measurements were carried out for up to 250 days at each trial site or until urine-treated pasture measurements dropped back to background emission levels.

Kelliher et al (2003, 2005), assessed all available EF3(PR&P) data and its distribution to pastoral soil drainage class, to determine an appropriate national annual mean value. The complete EF3(PR&P) data set of NzOnet was synthesised using the national assessment of pastoral soil drainage classes. These studies recognise that:

  • environmental (climate) data is not used to estimate N2O emissions using the IPCC (1996) methodology

  • the N2O emission rate can be strongly governed by soil water content

  • soil water content depends on drainage that can moderate the effects of rainfall and drought

  • drainage classes of pastoral soils, as a surrogate for soil water content, can be assessed nationally using a geographic information system.

  • drainage classes of pastoral soils, as a surrogate for soil water content, can be assessed nationally using a geographic information system.

An earlier analysis in New Zealand showed that the distribution of drainage classes for pasture land is highly skewed with 74 per cent well-drained, 17 per cent imperfectly drained, and 9 per cent poorly drained (Sherlock et al, 2001).

The research and analysis to date indicates that if excreta is separated into urine and dung components, EF3 for urine and dung could be set to 0.007 and 0.003, respectively. However, it is recognised the dung EF3 data is limited. Combining urine and dung EF3 values, the dairy cattle total excreta EF3 is 0.006. Conservatively rounding the total excreta EF3 of 0.006 provides a New Zealand-specific value of 0.01 for EF3(PR&P). The IPCC default value of EF3(PR&P) is 0.02 (IPCC, 1996).

Incorporation of the mitigation technology DCD into the agriculture inventory

A methodology to incorporate a N2O mitigation technology, the nitrification inhibitor dicyandiamide (DCD), into the agriculture sector of the inventory has been developed. A detailed description of the methodology can be found in Clough et al (2008). The N2O emissions reported in the agricultural soils category for 2007 take into account the use of nitrification inhibitors on dairy farms using the methodology described in Clough et al (2008). For the 2007 calendar year, DCD mitigated 28.8 Gg CO2-e, a 0.2 per cent decrease in total agricultural N2O emissions.

Dicyandiamide is an environmentally safe, and long researched, nitrification inhibitor that has been demonstrated to reduce N2O emissions and nitrate leaching in pastoral grassland systems grazed by ruminant animals. There have been 28 peer-reviewed, published New Zealand studies on the use and effects of DCD.

The method to incorporate DCD mitigation of N2O emissions into New Zealand’s agricultural inventory is by an amendment to the existing IPCC methodology. Activity data on livestock numbers is drawn from Statistics New Zealand’s annual agricultural survey. This survey has recently included questions on the area that DCD is applied to on grazed pastures.

The DCD product is applied to pastures based on research that has identified good management practice to maximise N2O emission reductions. This is at a rate of 10kg/ha of DCD applied twice per year in autumn and early spring within seven days of the application of excreta or fertiliser nitrogen. “Good practice” application methods are by slurry or granule.

Changes to the emission factors EF3PR&P, EF1 and parameter FracLEACH were established for use with DCD application. These emission factors and parameters were modified based on comprehensive field-based research that showed significant reductions in N2O emissions and nitrate leaching where DCD was applied.

The peer-reviewed literature on DCD use in grazed pasture systems was critically reviewed and it was determined that on a national basis, reductions in EF1, EF3PR&P, and FracLEACH of 67 per cent, 67 per cent and 53 per cent could be made respectively (Clough et al, 2008).

The reductions in the emission factors and parameters are used along with the fraction of dairy land treated with DCD to calculate DCD weighting factors.

DCD weighting factors

The appropriate weighting factor is then used as an additional multiplier in the current methodology for calculating indirect and direct emissions of N2O from grazed pastures. The calculations use a modified EF3PR&P of 0.0098 and FracLEACH of 0.0687 for dairy grazing area in the months that DCD is applied (May to September). The modified emission factors are based on information from the agricultural census that 3.5 per cent of the effective dairying area in New Zealand received DCD in 2007.

Table 6.5.1 Emission factors and parameters for DCD calculations

  NZ emission factor or parameter value for untreated area
(kg N2O-N/kg N)
Reduction from DCD treatment (%) Proportion land treated with DCD (%) Final modified emission factor or parameter
EF3PR&P 0.01 67 3.5 0.0098
EF1 0.01 67 3.5 0.0098
FracLEACH 0.07 53 3.5 0.0687

All other emission factors and parameters relating to animal excreta and fertilizer use (FracGASM, FracGASF, EF4 and EF5) remain unchanged when DCD is used as an N2O mitigation technology. Based on the physico-chemical reaction of DCD in the soil, DCD should have no effect on ammonia volatilisation during May to September when DCD is applied. This is supported by the results of field studies (Clough et al, 2008).

The derivations of the modified emission factors and the resulting calculations are included in the MS Excel worksheets available for download with this report from the Ministry for the Environment’s website (here).

The method will be refined over time to reflect any updated information that may arise from ongoing research into this area.

Indirect N2O from nitrogen used in agriculture

Nitrous oxide is emitted indirectly from nitrogen lost from agricultural soils through leaching and run-off. This nitrogen enters water systems and eventually the sea, with N2O being emitted along the way. The amount of nitrogen that leaches is a fraction (FracLEACH) of that deposited or spread on land.

Research studies and a literature review in New Zealand have shown lower rates of nitrogen leaching than are suggested in the revised 1996 IPCC guidelines (IPCC, 1996). A New Zealand parameter for FracLEACH of 0.15 was used in inventories submitted before 2003. However, using a FracLEACH of 0.15, IPCC-based estimates for different farm systems were found on average to be 50 per cent higher than those estimated using the OVERSEER® nutrient-budgeting model (Wheeler et al, 2003). The OVERSEER® model provides average estimates of the fate of nitrogen for a range of pastoral, arable and horticultural systems. In pastoral systems, nitrogen leaching is determined by the amount of nitrogen in fertiliser, in dairy-farm effluent and that excreted in urine and dung by grazing animals. The latter is calculated from the difference between nitrogen intake by grazing animals and nitrogen output in animal products, based on user inputs of stocking rate or production and an internal database with information on the nitrogen content of pasture and animal products.

The IPCC estimates were closer for farms using high rates of nitrogen fertiliser, indicating that the IPCC-based estimates for nitrogen leaching associated with animal excreta were too high for New Zealand. When the IPCC method was applied to field sites where nitrogen leaching was measured (four large-scale, multi-year animal grazing trials), it resulted in values that were double the measured values. This indicated that a value of 0.07 for FracLEACH more closely followed actual field leaching in New Zealand (Thomas et al, 2005). The 0.07 value has been adopted and is used for all years as it best reflects New Zealand’s national circumstances.

New Zealand uses the IPCC default EF4 emission factor for indirect emissions from volatilisation of nitrogen in the form of ammonia (NH3) and oxides of nitrogen (NOx).

Direct N2O emissions from agricultural soils

The N2O emissions from the direct soils emissions subcategory arise from synthetic fertiliser use, spreading animal waste as fertiliser, nitrogen fixing in soils by crops, and decomposition of crop residues left on fields. All of the nitrogen inputs are summed together and a New Zealand-specific emission factor of 0.01 kg N2O–N/kg N (Kelliher and de Klein, 2006) is applied to calculate total direct emissions from non-organic soils.

Data on nitrogen fertiliser use is provided by the New Zealand Fertiliser Manufacturers’ Research Association (FertResearch) from sales records for 1990 to 2007. There has been a six-fold increase in elemental nitrogen applied through nitrogen-based fertiliser over the 1990–2007 time series from 51,633 t in 1990 to 315,920 t in 2007. The calculation of N2O that is emitted indirectly through synthetic fertiliser and animal waste being spread on agricultural soils is shown in the MS Excel worksheets available with this report from the Ministry for the Environment’s website (here). Some of the nitrogen contained in these compounds is emitted into the atmosphere as ammonia (NH3) and nitrogen oxides (NOx) through volatilisation, returning to the ground during rainfall and then re-emitted as N2O. This is calculated as an indirect emission of N2O. A review by Sherlock et al (2008) suggests that the NH3 emission factor may be 50 per cent too high. These results are being internationally peer reviewed.

The calculation for animal waste includes all manure that is spread on agricultural soils, irrespective of the animal waste management system it was initially stored in. This includes all agricultural waste in New Zealand except for emissions from the pasture range and paddock animal waste management system. New Zealand uses a country-specific value for EF1 of 0.01 kg N2O-N/kg N (Kelliher and de Klein, 2006).

Direct N2O emissions from organic soils are calculated by multiplying the area of cultivated organic soils by an emission factor (EF2). Analysis identified 202,181 hectares of organic soils in New Zealand. Kelliher et al (2002) estimated 5 per cent (ie, 10,109 ha) of organic soils are cultivated on an annual basis. New Zealand uses the IPCC default emissions factor (EF2 equal to 8 kg N2O-N/kg N) for all years of the time series.

6.5.3 Uncertainties and time-series consistency

Uncertainties in N2O emissions from agricultural soils were assessed for the 1990 and 2002 inventory using a Monte Carlo simulation of 5000 scenarios with the @RISK software (Kelliher et al, 2003) (Table 6.5.1). The emissions’ distributions are strongly skewed, reflecting pastoral soil drainage whereby 74 per cent of soils are classified as well drained and 9 per cent are classified as poorly drained. For the 2007 data, the uncertainty in the annual estimate was calculated using the 95 per cent confidence interval determined from the Monte Carlo simulation as a percentage of the mean value ie, in 2002, the uncertainty in annual emissions was +74 per cent and –42 per cent.

Table 6.5.2 New Zealand’s uncertainties in N2O emissions from agricultural soils for 1990, 2002 and 2007 estimated using Monte Carlo simulation (1990, 2002) and the 95 per cent confidence interval (2007)

Year N2O emissions from agricultural soils (Gg/annum) 95% CI min (Gg/annum) 95% CI max (Gg/annum)
1990 32.4 18.8 56.4
2002 39.6 23.0 68.9
2007 39.8 23.1 69.2

The overall inventory uncertainty analysis shown in Annex 7 demonstrates that the uncertainty in annual emissions from agricultural soils is a major contributor to uncertainty in the total estimate and to the uncertainty in the trend from 1990. The uncertainty between years was assumed to be correlated. Therefore, the uncertainty is mostly in the emissions’ factors and the uncertainty in the trend is much lower than uncertainty for an annual estimate.

The Monte Carlo numerical assessment is also used to determine the effects of variability in the nine most influential parameters on uncertainty of the calculated N2O emissions in 1990 and 2002. These parameters are shown in Table 6.5.2, together with their percentage contributions to the uncertainty. There was no recalculation of the influence of parameters for the 2007 data. The Monte Carlo analysis confirmed that uncertainty in parameter EF3(PR&P) has the most influence on total uncertainty, accounting for 91 per cent of the uncertainty in total N2O emissions in 1990. This broad uncertainty reflects natural variance in EF3, determined largely by the vagaries of the weather and soil type.

Table 6.5.3 Percentage contribution of the nine most influential parameters on the uncertainty of New Zealand’s total N2O emissions for 1990 and 2002

Parameter 1990 2002
  % contribution to uncertainty % contribution to uncertainty
EF3(PR&P) 90.8 88.0
EF4 2.9 3.3
Sheep Nex 2.5 1.8
EF5 2.2 2.8
Dairy Nex 0.5 0.7
FracGASM 0.5 0.5
EF1 0.3 2.4
Beef Nex 0.2 0.3
FracLEACH 0.1 0.2

6.5.4 Source-specific QA/QC and verification

In 2007, N2O emissions from the direct soil emissions and pasture range and paddock manure subcategories were key categories (level and trend assessment), and N2O from the indirect emissions category was also a key category (level assessment). In preparation for this inventory, the data for these categories underwent Tier 1 quality checks.

6.5.5 Source-specific recalculations

All activity data was updated with the latest available data (Statistics New Zealand table builder and Infoshare database, 2008).

6.5.6 Source-specific planned improvements

New Zealand scientists are continuing to research N2O emission factors for New Zealand’s pastoral soils. This includes development of New Zealand-specific emission factors for sheep and cattle dung and emission factors for New Zealand hill country pastures. New Zealand is also continuing research to refine the methodology used to estimate N2O emission reductions using dicyandiamide (DCD) nitrification inhibitors.

An extensive review of FracGASM and FracGASF emission factors has been carried out assessing international and New Zealand data. Preliminary results suggest the emission factors are currently too high. The review will be internationally peer assessed.

6.6 Prescribed burning of savanna (CRF 4E)

6.6.1 Description

In 2007, prescribed burning of savanna was not a key category in New Zealand. The inventory includes burning of tussock (Chionochloa) grassland in the South Island for pasture renewal and weed control. The amount of burning has been steadily decreasing over the past 50 years as a result of changes in lease tenure and a reduction in grazing pressure. In 2007, prescribed burning emissions accounted for 1.0 Gg CO2-e, a 2.2 Gg CO2-e (68.0 per cent) reduction in emissions from the 3.2 Gg CO2-e reported in 1990.

The revised 1996 IPCC guidelines (IPCC, 1996) state that in agricultural burning, the CO2 released is not considered to be a net emission as the biomass burned is generally replaced by regrowth over the subsequent year. Therefore, the long-term net emissions of CO2 are considered to be zero. However, the by-products of incomplete combustion – CH4, CO, N2O and NOx – are net transfers from the biosphere to the atmosphere.

6.6.2 Methodological issues

New Zealand has adopted a modified version of the IPCC methodology (IPCC, 1996). The same five equations are used to calculate emissions. Instead of using total grassland and a fraction burnt, New Zealand uses statistics of the total area of tussock grassland that has been granted consent (a legal right) for burning, under New Zealand’s Resource Management Act (1991). Only those areas with consent are legally allowed to be burned. Expert opinion obtained from local government is that approximately 20 per cent of the area allowed to be burnt is actually burnt in a given year.

Current practice in New Zealand is to burn in damp spring conditions, reducing the amount of biomass consumed in the fire. The composition and burning ratios used in calculations are from New Zealand-specific research (Payton and Pearce, 2001) and the revised 1996 IPCC guidelines (IPCC, 1996).

6.6.3 Uncertainties and time-series consistency

The same emission factors and sources of data were used for the whole time series. This gives confidence in comparing emissions through the time series. The major sources of uncertainty are the percentage of consented area actually burnt in that season, extrapolation of biomass data from two study sites for all areas of tussock, and that many of the other parameters are the IPCC default values (ie, the carbon content of the live and dead components, the fraction of the live and dead material that oxidises, and the nitrogen to carbon ratio for the tussocks). Uncertainty in the New Zealand biomass data has been quantified at ±6 per cent (Payton and Pearce, 2001). However, many IPCC parameters vary by ±50 per cent and some parameters do not have uncertainty estimates.

6.6.4 Source-specific QA/QC and verification

There was no source-specific QA/QC for this category in 2007.

6.6.5 Source-specific recalculations

All activity data was updated with the latest available data (Statistics New Zealand table builder and Infoshare database, 2008).

6.7 Field burning of agricultural residues (CRF 4F)

6.7.1 Description

Burning of agricultural residues produced 17.5 Gg CO2-e in 2007. This was a decrease of 11.3 Gg CO2-e (–39.2 per cent) below the level of 28.7 Gg CO2-e in 1990. Burning of agricultural residues was not identified as a key category in 2007.

New Zealand reports emissions from burning barley, wheat and oats residue in this category. Maize and other crop residues are not burnt in New Zealand.

Burning of crop residues is not considered to be a net source of CO2, as the CO2 released into the atmosphere is reabsorbed during the Next growing season. However, the burning is a source of emissions of CH4, CO, N2O and NOx (IPCC, 1996). Burning of residues varies between years due to climatic conditions.

6.7.2 Methodological issues

The emissions from burning agricultural residues are estimated using the equation on page 4.82 of the revised 1996 IPCC guidelines. This calculation uses crop production statistics, the ratio of residue to crop product, the dry-matter content of the residue, the fraction of residue actually burned, the fraction of carbon oxidised, and the carbon fraction of the residue. These parameters were multiplied to calculate the carbon released. The emissions of CH4, CO, N2O and NOx were calculated using the carbon released and an emissions ratio. Nitrous oxide and NOx emissions’ calculations also used the nitrogen to carbon ratio.

IPCC good practice guidance suggests that an estimate of 10 per cent of residue burned may be appropriate for developed countries, but also notes that the IPCC defaults: “are very speculative and should be used with caution. The actual percentage burned varies substantially by country and crop type. This is an area where locally developed, country-specific data is highly desirable (IPCC, 2000).” For the years 1990 to 2003, it was estimated that 50 per cent of stubble was burnt. For the years 2004 to 2007, experts assessed this to have decreased to 30 per cent. These values were developed from opinions of the Ministry of Agriculture and Forestry officials working with the arable production sector (M. Doak, pers com).

6.7.3 Uncertainties and time-series consistency

No numerical estimates for uncertainty are available for these emissions. The fraction of agricultural residue burned in the field was considered to make the largest contribution to uncertainty in the estimated emissions.

6.7.4 Source-specific QA/QC and verification

There was no source-specific QA/QC for this category in 2007.

6.7.5 Source-specific recalculations

All activity data was updated with the latest available data (Statistics New Zealand table builder and Infoshare database, 2008).

2  The number of beef breeding cows was assumed to be 25 per cent of the total beef breeding cow herd and other adult cows slaughtered were assumed to be dairy cows. The carcass weight of dairy cattle slaughtered was estimated using the adult dairy cow liveweights and a killing-out percentage of 40 per cent. The total weight of dairy cattle slaughtered was calculated (carcass weight × number slaughtered) and then deducted from the national total carcass weight of slaughtered adult cows. This figure was then divided by the number of beef cows slaughtered to obtain an estimate of the carcass weight of adult beef cows. Liveweights were calculated assuming a killing-out percentage of 50 per cent.

3  For example, Blaxter and Clapperton, 1995; Moe and Tyrrel, 1975; Baldwin et al, 1988; Djikstra et al, 1992; and Benchaar et al, 2001 – all cited in Clarke et al, 2003.