Matching interventions to the farm system

Strategies for mitigating N2O emissions can broadly be categorized into those that reduce the total amount of N returned to or cycling through the soil, and those that utilize the available N more efficiently. The first group of mitigation options will always reduce N2O emissions, regardless of the source of the N2O within a system. Examples of such mitigation options are

• breeding to produce animals that have a higher nitrogen use efficiency,

• reducing the amount of N fertilizer inputs,

• reducing the amount of N in the diet, and

• feeding animal amendments, such as tannins, that can reduce the amount of urine N excreted.

Mitigation options that utilize available N more efficiently include fertilizer and effluent management, balancing energy and protein intakes, wet-season grazing management and nitrification inhibitors. The most effective options for reducing N2O emissions are those that target the key source(s) of N2O and these thus vary with the livestock system in which they are used. The main characteristics of livestock systems determining the relative importance of N2O sources are (1) the number of grazing days, with effluent management being a more dominant N2O source with fewer grazing days; and (2) N fertilizer use, which becomes a less dominant source in legume-based pasture systems. Table 6.2 summarizes published results on N2O emissions and their relative importance within a system. It should be noted that comparisons between countries need to be treated with caution as each publication uses a different methodology for estimating the N2O emissions. For example, Lovett et al (2008) use EFs for animal urine and dung deposited during grazing in Ireland (0.56 per cent and 0.19 per cent, respectively) that are much lower than values used by others (generally between 1 and 2 per cent, with no distinction between urine or dung). As a result, the relative contribution of the urine and dung deposited during grazing is much lower and fertilizer use and indirect emissions are the main sources. However, if these reduced EFs are acceptable country-specific values for Ireland, mitigation options targeting the urine and dung deposition will have a lower impact in Irish dairy systems than options that target N fertilizer use.

Table 6.2 Relative contribution (%) of different sources to total N2O emissions from different livestock systems in New Zealand (NZ), The Netherlands (NL), Ireland and Germany

NZ

NL dairy

Ireland dairy

Germany

Grazing days

dairyab sheep/ beefac 365 365

grass Nd 195

grass/ clovere 203

250f

149s

beefh 0

organic cattle' 182

N fertilizer use

23

13

31

5

40

36

54

0

Urine/dung deposited during grazing

52

71

45

63

11

7

0

24

Effluent applied to land

8

0

11

14

9

14

46

76

Indirect emissions

17

16

13

18

40

42

ng

ng

Total N2O

100

100

100

100

100

100

100

100

Note: ng: not given. a estimated using OVERSEER® (Wheeler et al, 2003); b NZ dairy farm: year-round grazing grass/clover pasture; 100kg fertilizer N/ha/yr; 2.8 cows/ha; 350kg milk solids/cow i.e. around 4060 litres milk/cow;c NZ sheep/beef farm: year-round grazing on grass/clover pasture; 24kg fertilizer N/ha/yr; 16 stock units/ha;d From Schils et al (2005). NL dairy farm grass N: part housing, part grazing grass-only pasture; 275kg fertilizer N/ha/yr; 2.2 cows/ha; 8095kg fat and protein corrected milk production (FPCM)/cow; i.e. around 7600 litres milk/cow. e From Schils et al (2005). NL dairy farm grass/clover: part housing, part grazing grass/clover pasture; 69kg fertilizer N/ha/yr; 1.9 cows/ha; 8294kg FPCM/cow; i.e. around 7800 litres milk/cow;f From Lovett et al (2008). Ireland dairy system: 250 days of 24-hour grazing grass-only pasture; 330kg fertilizer N/ha/yr; 2.1 cows/ha; 6237kg milk/cow; g From Lovett et al (2008). Ireland dairy system: 149 days of 24-hour grazing grass-only pasture; 238kg fertilizer N/ha/yr; 1.7 cows/ha; 5753kg milk/cow; h From Flessa et al (2002). German cattle farm: year-round housing, 188kg fertilizer N/ha/yr;i From Flessa et al (2002). German organic cattle farm: six-month grazing, no synthetic N fertilizer.

Note: ng: not given. a estimated using OVERSEER® (Wheeler et al, 2003); b NZ dairy farm: year-round grazing grass/clover pasture; 100kg fertilizer N/ha/yr; 2.8 cows/ha; 350kg milk solids/cow i.e. around 4060 litres milk/cow;c NZ sheep/beef farm: year-round grazing on grass/clover pasture; 24kg fertilizer N/ha/yr; 16 stock units/ha;d From Schils et al (2005). NL dairy farm grass N: part housing, part grazing grass-only pasture; 275kg fertilizer N/ha/yr; 2.2 cows/ha; 8095kg fat and protein corrected milk production (FPCM)/cow; i.e. around 7600 litres milk/cow. e From Schils et al (2005). NL dairy farm grass/clover: part housing, part grazing grass/clover pasture; 69kg fertilizer N/ha/yr; 1.9 cows/ha; 8294kg FPCM/cow; i.e. around 7800 litres milk/cow;f From Lovett et al (2008). Ireland dairy system: 250 days of 24-hour grazing grass-only pasture; 330kg fertilizer N/ha/yr; 2.1 cows/ha; 6237kg milk/cow; g From Lovett et al (2008). Ireland dairy system: 149 days of 24-hour grazing grass-only pasture; 238kg fertilizer N/ha/yr; 1.7 cows/ha; 5753kg milk/cow; h From Flessa et al (2002). German cattle farm: year-round housing, 188kg fertilizer N/ha/yr;i From Flessa et al (2002). German organic cattle farm: six-month grazing, no synthetic N fertilizer.

The effect of fertilizer management to reduce total N2O emissions is most effective in non-legume-based systems, for example the hybrid, grass-only and confinement systems commonly employed in the Northern Hemisphere.

Similarly, effluent management options will have a bigger impact on reducing N2O emissions in hybrid and confinement systems, while improved wet-season soil and grazing management is a key option in pastoral systems. Nitrification inhibitors can be an effective mitigation option in all systems, targeting urine and dung in grazed pastures and effluent or fertilizer emissions in hybrid/ confined systems. Although nitrification inhibitor use is being promoted in New Zealand and Australia to reduce N2O emissions from urine patches, this technology has not been as readily adopted in the Northern Hemisphere as an effective N2O mitigation option targeting N fertilizer or effluent.

Table 6.3 Greenhouse gas emissions (t CO2-eq per farm system per year) from case study dairy farms (base farm) in four catchments in New Zealand and under two mitigation strategies (use of wintering pad or a nitrification inhibitor)

Catchment Greenhouse gas Base farm Base farm with mitigation strategy

Wintering Nitrification pad inhibitor

Catchment Greenhouse gas Base farm Base farm with mitigation strategy

Wintering Nitrification pad inhibitor

Toenepi

N2O

175

162

(-7)

92

(-47)

CH4

393

407

418

CO2

205

236

221

Total GHG

774

804

(+4) (-5)*

731

-6)(-

15)*

Waiokura

N2O

247

244

(-1)

134

(-46)

CH4

476

532

507

CO2

231

275

252

Total GHG

955

1051

(+10)( 2)*

893

(- 6)(-

-17)*

Waikakahi

N2O

766

751

(-2)

463

(- 40)

CH4

1305

1301

1567

CO2

872

916

1001

Total GHG

2943

2969

(+1)(+1)*

3031

(+3)(-

-15)*

Bog Burn

N2O

647

638

(-1)

309

(-52)

CH4

1277

1273

1410

CO2

682

718

742

Total GHG

2606

2629

(+1)(+1)*

2460

(+6)(-

-14)*

Note: GHG = greenhouse gas; * relative change in greenhouse gas emission intensity (greenhouse gas emissions/unit of product); values in brackets represent the relative change (per cent) in emissions compared to the case study farm

Source: de Klein and Monaghan (2005)

Note: GHG = greenhouse gas; * relative change in greenhouse gas emission intensity (greenhouse gas emissions/unit of product); values in brackets represent the relative change (per cent) in emissions compared to the case study farm

Source: de Klein and Monaghan (2005)

In the pastoral systems employed in New Zealand and southern Australia, improved wet season grazing management (for example the use of wintering pads) and the use of nitrification inhibitors are key N2O mitigation strategies (Di et al, 2007; Luo et al, 2008a). An assessment of their impact on whole-farm N2O emissions from four key New Zealand dairying regions showed that these strategies could reduce N2O emissions by 1-7 per cent (wintering pads) and 40-50 per cent (nitrification inhibitors) (Table 6.3). In hybrid systems in

Europe, key N2O mitigation strategies include reducing N fertilizer use, reduced grazing, diet manipulation and improved effluent management (for example Schils et al, 2005; Lovett et al, 2008). Schils et al. (2005) estimated that reduced N fertilizer use and reduced grazing times (from 20 to 16 hours/day) each could reduce N2O emissions from Dutch dairy farms by about 5 per cent. Mitigation options for confinement systems obviously need to focus on effluent or manure management systems, and examples include improved utilization of animal manure as fertilizer and (an)aerobic digestion of manure. Amon et al (2001) showed that manure from housed dairy cows emitted 3540 per cent less N2O when aerobically composted instead of being stacked.

Mitigation options that increase the efficiency of N within the soil/plant system are likely to increase pasture and/or animal productivity, which in turn is likely to increase methane emissions (for example increased stocking rates). Therefore, to fully assess the impact of N2O mitigation options on total greenhouse gas emissions requires a whole-farm system analysis that accounts for all greenhouse gas emissions. The whole-farm models discussed above are useful tools for assessing the farm level impact of N2O mitigation options. For example, Schils et al (2005) showed that although reduced N fertilizer use could reduce N2O emissions by about 5 per cent, total greenhouse gas emissions were reduced by 3 per cent. Reduced grazing times did not reduce total greenhouse gas emissions, even though N2O was reduced by 5 per cent. Similarly, de Klein and Monaghan (2005) estimated that the wintering pad N2O mitigation options could increase total greenhouse gas emissions by 1-10 per cent (Table 6.3) due to an increase in CO2 emissions associated with fuel use, supplementary feed production and fertilizer manufacturing and use. The effect of nitrification inhibitors on total (on-farm) greenhouse gas emissions ranged from a 6 per cent reduction to a 6 per cent increase (Table 6.3) due to an increase in methane emissions associated with the utilization of the increased pasture production. However, expressed per unit of product, total greenhouse gas emissions from the systems using nitrification inhibitors were reduced by around 15 per cent, indicating that nitrification inhibitors increased the efficiency of the farming systems. This is an important consideration for identifying management strategies that have the largest reduction in environmental emissions for a given production level. However, as production levels continue to increase, the reduction in greenhouse gas per unit of product needs to be greater than the increase in production (products per ha) to ensure that net greenhouse gases are reduced.

The effectiveness of a mitigation strategy also depends on how it is adopted within a system and the choices a farmer makes. For example, studies have shown that the use of nitrification inhibitors can potentially increase pasture growth by between 0 per cent and 36 per cent (Di et al, 2007, Kelly et al, 2008; Smith L. C. et al, 2008). Using standard inventory methods Eckard (2008) estimated that 72 per cent of total greenhouse gas emissions from an average dairy farm in Southern Australia was due to enteric CH4, while total N2O equated to 28 per cent of total CO2-equivalent emissions. Half of these N2O emissions derived from urine deposition on pasture. A nitrification inhibitor applied to this pasture may reduce N2O emissions from urine by 61 to 91 per cent (Di et al, 2007; Kelly et al, 2008; Smith L. C. et al, 2008), which equates to a whole-farm abatement of between 8.5 to 12.5 per cent CO2 equivalent (CO2-eq). If the nitrification inhibitor increased pasture yield by, say, 25 per cent, the farmer could either increase stocking rate by 25 per cent to utilize this extra DM or reduce N fertilizer inputs while keeping the stocking rate constant. With the former choice, whole-farm greenhouse gas emissions would increase by between 11.5 and 7.5 per cent as a result of increased enteric CH4 emissions. However, whole-farm emissions would reduce by between 12.0 and 16.0 per cent CO2-eq if the farmer chose to reduce N fertilizer inputs. Therefore, in the above example, recommendations for the use of nitrification inhibitors as an abatement strategy need to emphasize the potential to reduce N inputs, and/or to harvest a greater grass surplus for dry-season feeding, rather than importing additional feed or fertilizer onto the farm.

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