Reduce N Loss

Loss of nitrogen to the environment is driven by nitrogen inputs into the farm system, how efficiently they are used and how are they are recycled.

Reducing nitrogen loss from your farm system can:

• Contribute to both reducing greenhouse gas emissions and improving water quality.
• Help improve efficiency and in turn have financial benefits.
• Help meet regulation requirements for national limit-setting

Where excess nitrogen comes from

Nitrogen intake in most pasture-based diets is surplus to the requirements of dairy cattle. This surplus is excreted as urine (70-80%) and dung (20-30%). Nitrogen in the urine patch exceeds plant requirements, and thus is leached below the root zone or lost to the atmosphere as nitrous oxide gas (N2O).

Read more about excess nitrogen

The majority of N in the urine patch is initially in the form of urea which is rapidly hydrolysed (within 48 hours) to ammonium (NH4+). This ammonium is converted to nitrate (NO3-) by soil microbes through a process called nitrification. Nitrate is highly soluble and is the form of nitrogen plants can most readily use.

In the highly concentrated urine patch (equivalent to between 300-1000 kg N/ha), plants often can’t use it fast enough before it is carried below the root zone by soil drainage and thus lost via leaching events. The nitrate then ends up as a pollutant in ground and surface water. Leaching events are more common in high rainfall areas, under irrigation, as well as in free-draining soils.

The nitrification process also produces nitrous oxide gas (N2O), and another group of soil microbes can facilitate further N2O production from nitrate through a process called denitrification. N2O gas is 300 times more potent as a greenhouse gas than CO2. Its formation is more common in wet and compacted soils.

Options to reduce N loss

Forages

Research in the Forages for Reduced Nitrate Leaching programme (FRNL) has shown that catch crops, fodder beet and plantain can be used to reduce nitrate leaching.

• More information on the Forages for Reduced Nitrate Leaching programme.
• More information on how plantain reduces nitrate leaching

Reduce N inputs

• Reduce N fertiliser. Grow similar or more pasture with less N fertiliser while optimising how much of the pasture grown can be harvested. See Strategies to reduce N fertiliser use.
• Reduce the amount of N in feed. Substitute high-N supplements with feeds with a lower N content, for example using less grass silage in autumn by feeding fodder beet or maize silage. For this strategy to make a difference in N loss, a significant amount of the total feed eaten needs to be substituted for lower N feeds. To identify the nitrogen content in common feeds see Common feed supplements.

Reduce N intake and ensure less N goes to urine

• Improve animal health and reproductive performance. Improving the 6-week in-calf rate will improve rates of genetic gain and reduce the number of non-productive animals (replacements and carry-over cows). Long term this will mean less N is eaten (and excreted) because the same amount of milk can be produced by fewer cows.
• Breeding cows with higher genetic merit. This will allow for more milk from fewer cows. They will also eat more kg DM and N but less of this N and energy goes to maintenance, improving the N use efficiency. This will only reduce N loss if N eaten per hectare is reduced.
• Breeding cows with lower urinary N excreted. Some animals naturally excrete less N via their urine. A new seven-year study is underway to determine if this can be incorporated into a genetic trait.
• Dilute the concentration of N
  • Plantain pastures have been shown to dilute the concentration of N in urine – spreading it over a larger area**.**  Plantain is now a mitigation in OverseerFM. Research is investigating the effects plantain has in the soil in slowing nitrification.
  • Salt has a diuretic effect reducing the N-load in the urine patch. Trials administering 200 g salt/day to non-lactating cows, showed cows drank 25% more and the average amount of N deposited in urine patches was reduced by 30% late autumn/ summer. There was no effect in winter. The practicalities of feeding salt can be challenging, and trials have only been short term and side effects of feeding salt on cow health are unknown.

Altering the timing of surplus mineral N

• Reduce the amount of N input immediately before and during the high drainage months – for example effluent or N fertiliser in autumn.
• Reduce N eaten per ha and therefore urinary N deposited immediately before and during the high drainage months **(**g. in autumn) through measures such as early culling, feeding low-N supplements (maize) or crops (fodder beet).
• Match N supply with plant demand. Catch crops (e.g. oats or Italian ryegrass after fodder beet) will take up excess N from the soil and reduce the risk of N leaching in winter/spring.
• Increase plant demand for N by using winter active species especially if planted after grazing a crop such as fodder beet, kale, or swedes.
• Reduce conversion of urea to nitrate in the soil (nitrification inhibitor effect). This will allow plants to take up a greater proportion of the deposited N as it stays in less mobile forms for a longer time. Plantain may be having this effect in the soil – research is underway to confirm
• Use stand-off facilities (e.g. feed pads, stand-off pads) to reduce the amount of N deposited on pasture when plant growth is low and/or the drainage risk is high reduces the risk of N leaching. The effluent from the facility can then be spread at a time which matches high plant demand and/or has less risk of drainage. See the Stand-off pad page.

Manage irrigation to reduce drainage and increase plant uptake

• On irrigated farms, reduce drainage through the soil profile by managing irrigation to ensure water is not applied that leads to drainage out of the root zone (defined as 0-60 cm in OverseerFM). For tips see Irrigation management.

Reduce mineral N loss from cultivation and cropping

• Organic matter holds onto N in a stable form but cultivation can cause the N to be converted to a soluble form and lost. Minimum tillage techniques such as direct drilling of crops and pastures will reduce N loss.
• The use of crop calculators and precision application can ensure appropriate timing and application. Soil test for the potential available N to the crop (AMN test or AN test).
• When managing crops, urine N leaching can be reduced through paddock selection, forage crop selection, and grazing management
• Winter fallow leaching can be reduced though the use of a cover crop. Avoid leaving ground fallow in winter by growing catch crops that grow actively in winter e.g. oats, Italian ryegrass.

Farm systems consideration

• Lower stocking rate does not always mean less N leaching. It is possible that N eaten per hectare might not change at a lower stocking rate as N eaten per hectare is the combination of three factors: feed eaten per cow, cows per hectare and the N % of the feed (pasture, crops and supplement). A higher stocking rate could reduce to a lower level of N leaching without reducing stocking rate if cows are culled or dried off early in autumn to reduce N intake.