Reducing nitrogen loss

What is nitrogen?

Nitrogen (N) is an essential part of a farm’s biological system with pasture, crops and cows needing it to function.

Nitrogen inputs from fertiliser, feed, and the atmosphere (through clover fixation) cycle from the soil to the plant and then into the animal, which uses a small amount for maintenance, growth, and milk production. There is more nitrogen in pasture than animals can use, so the excess is returned to the soil, mainly as urine.

Microbes in the soil transform nitrogen into a range of different outputs. Some is given off as ammonia gas (NH₃) through a process called volatilisation.

Most of the nitrogen in the urine patch is initially in the form of urea, which is rapidly hydrolysed to ammonium (NH₄). Soil microbes called nitrifiers then convert this ammonium to nitrate (NO₃) through a process called nitrification. Nitrate is highly soluble and is the form of nitrogen that plants can most readily use. A small amount of nitrous oxide (N₂O), a greenhouse gas, is also produced during the nitrification process. Additional nitrous oxide is also produced by a separate group of microbes through a process called denitrification.

Finally, some of the nitrate produced during nitrification isn’t taken up by plants or converted by denitrifiers into nitrous oxide, and sits in the soil as nitrate. This can leach into groundwater or run off in irrigation or rainwater.

A simplified version of this nitrogen cycle is shown in the diagram below.

How nitrogen loss is measured

Nitrogen loss can be measured in different ways on a dairy farm, but the most common is ‘Purchased N Surplus’ (PNS). This is calculated by subtracting nitrogen outputs (milk, meat, crops, and supplementary feeds) from nitrogen inputs (fertiliser and imported feeds). The lower the PNS number, the most efficiently nitrogen is being used. The higher the number, the more nitrogen is at risk of being lost to the environment.

How nitrogen loss affects the environment

When grazing livestock eat higher nitrogen pastures or crops, they use only a fraction of the nitrogen consumed to support the production of milk and meat. Most of it is excreted, around 70-80 percent through urine and 20-30 percent through dung. This can create concentrated nitrogen patches in the soil, which exceeds plant growth requirements. The concentration is about the same as applying 1,000kg of nitrogen per hectare. This nitrogen can’t be fully utilised and is lost (leached) in the form of nitrate below the plant’s root zone, or lost to the atmosphere as nitrous oxide.

Nitrogen can also be directly lost via leaching from effluent or nitrogen fertiliser applications, e.g. via heavy applications and/or by application to wet soils, although this is lower in concentration than nitrogen leached via urine patches.

Nitrogen cap regulations are in place to manage the impact of nitrogen loss on the environment, including limiting use to 190kg/ha/year, and requiring dairy farmers to report annual usage.

Options to reduce nitrogen loss

There are nine main ways to reduce nitrogen loss on a dairy farm:

1. Fertiliser and effluent use

Nitrogen application, no matter the form (fertiliser, effluent), should be in the right place, at the right time, and used in the right amount for plant requirements. This will help increase efficiency and minimise environmental impacts.

Urease-coated fertilisers reduce the losses of ammonia from urea use and maximise the nitrogen available for plant uptake. This means less nitrogen needs to be used and less nitrous oxide is emitted.

Effluent applications should be used as a substitute for fertiliser applications, enabling reduced fertiliser on effluent application areas.

Reducing the amount of nitrogen input immediately before and during higher risk drainage months can also help, for example using/applying effluent or nitrogen fertiliser in autumn.

It is possible to reduce nitrogen fertiliser use and still grow the same or more pasture on farm. This can be achieved by optimising pasture management and pasture utilisation. Feed budgets and feed wedges should be used in combination with nutrient budgets to match fertiliser applications to pasture and crop demands. For more on this, see strategies to reduce nitrogen fertiliser use.

2. Forages

DairyNZ research has identified several forage crops that can reduce the amount of nitrate being leached through the soil: catch crops, fodder beet and plantain.

Plantain, specifically the cultivars Tonic and Agritonic (marketed as a blend as Ecotain™), has consistently shown reduced nitrate leaching of up to 60 percent. It is recognised as a nitrogen leaching mitigation in Canterbury, Manawatu-Whanganui, and Bay of Plenty. It is also a mitigation in OverseerFM. The amount of leaching reduction achieved through plantain will depend on your soils, climate, and farm system set-up. These case studies show how other farmers have been incorporating plantain.

Catch crops, e.g. oats or Italian ryegrass after fodder beet, will take up excess nitrogen from the soil and reduce the risk of nitrate leaching in winter and spring.

3. Supplementary feed

Substitute supplementary feeds that have a higher nitrogen content with feeds that have a lower nitrogen content. For example, use less grass silage in autumn by feeding fodder beet or maize silage. For this strategy to make a difference to nitrogen loss, a significant amount of the total feed eaten needs to be substituted with lower nitrogen feeds. To identify the nitrogen content in common feeds, see the supplements page.

4. Genetics

Breeding cows with higher genetic merit can result in increased feed conversion efficiency. This means cows produce more from the same amount of feed consumed, including nitrogen. Less then goes to maintenance, improving the nitrogen-use efficiency of the animal. Note that this approach will only reduce total nitrogen loss if nitrogen eaten per hectare is reduced.

5. Reproduction

Improving the 6-week in-calf rate can improve rates of genetic gain, by increasing options for herd selection, and reduce the number of non-productive animals (replacements and carry-over cows). While this is a long-term option, it will mean less nitrogen is eaten, and excreted, because the same amount of milk can be produced by fewer cows, provided there is no increase in stocking rate.

6. Use stand-off facilities

Feed pads and stand-off pads can be used to capture effluent and reduce the amount of nitrogen deposited on pasture when plant growth is low and/or the drainage risk is high. The effluent from the facility can then be spread at a time which matches plant demand and/or has less risk of drainage and nitrate leaching. See the stand-off pad page.

7. Irrigation management

On irrigated farms, reduce soil saturation and therefore drainage through the soil profile by managing irrigation to ensure water is not over-applied, leading to drainage below the root zone (defined as 0-60 cm in OverseerFM). For tips see irrigation management.

8. Cultivation and cropping

Organic matter holds onto nitrogen in a stable form but cultivation can cause it to be converted to a soluble form and lost to the environment. Minimum tillage techniques such as direct drilling of crops and pastures will reduce nitrogen loss.

When managing crops, leaching via dung and urine patches can be reduced through paddock selection, forage crop selection, and grazing management.

Leaching during the winter months can be reduced though the use of cover crops. Avoid leaving ground fallow during winter by growing catch crops that grow actively in winter, e.g. oats, Italian ryegrass.

9. Riparian planting

Riparian areas act as filters to increase uptake of nutrients such as nitrogen before they enter the waterway. This can help reduce nitrogen loss and support water quality and biodiversity outcomes, as well as reducing the risk of algal blooms and nuisance aquatic plants. For more information see riparian planting.