Everyone agrees that nitrogen should be used efficiently and that we need to find ways to reduce nitrogen inputs whilst maintaining or increasing crop yield and quality. However it can be misleading to make one ratio from two desirable outcomes; sometimes it is better to keep the two separate: high yield and low N input.
Farmers are often advised to calculate NUE to help monitor their efficiency. If a ratio is required, several definitions of NUE are used, each with a different nuance. Broadly, they have one of two broad purposes, to sumarise:
- Nutrient capture: The output of crop N per unit of N applied or available (%)
- Nutrient productivity: The output of crop dry matter per unit of N applied or available (kg DM/ kg N)
Neither is right or wrong, and both definitions are widely used. But it is important to be clear what is meant by NUE in any given context or by any individual or organisation, and what exactly is involved in the calculation. This is especially true if comparing metrics between farms or enterprises. NFU and Defra are currently looking at appropriate KPIs for NUE.
An important consideration is whether nitrogen fertiliser is applied with the objective of raising yields, or for the return of nitrogen as protein in harvested products. For most crops (feed wheat, barley, oilseed rape, linseed, sugar beet, potatoes, forage, grass) the prime role of nitrogen is to provide and sustain the photosynthetic machinery in the leaves to fix carbon and grow yield, stored as carbohydrate or oil. Some nitrogen is required for protein in the seed or harvested product, but mostly the farmer is not paid for this, and can even be penalised for it (eg malting barley, sugar beet). So maximising offtake of nitrogen in the crop is not usually the farmer's objective, agronomically its best to keep as much N in the field as possible - Nitrogen in crop residues generally has minimal risk of loss where C:N ratio is high (eg straw). Most leaching of N from arable soils occurs where soil N mineralises before winter, without being taken up by a crop.
Everybody wants as much of any fertiliser N applied to get into the crop as possible. Ideally we would simply measure and calculate Fertiliser N Recovery. Unfortunately this is complicated by the uncertainties about N provided by the soil, and by variation in grain and straw nitrogen contents. Some of the widely used definitions and calculations for NUE are listed below:
- kg dry matter harvested / kg N available from all sources (soil + fertiliser + manure)
- N output / N input (not necessarily including soil N)
- Expressed as %. Advocated by Brentrup et al. (2015) EU Nitrogen Expert Panel
- N removed in harvested crop / N supply from all sources (soil N + manure N + fertiliser N)
- Expressed as %.
- Basis of NUE calculation in Yara NUE calculator
- Advocated in June 2022 FACTS newsletter
- Apparent Fertiliser N Recovery: kg N in crop minus N acquired from the soil (usually measured as N in unfertilised crop) / kg fertiliser N applied
- Referred to as fertiliser NUE (NfUE) by CF
Components of many of the NUE calculations above which support understanding nitrogen dynamics and balances are:
- Crop N concentration (N% of harvested crop on DM basis)
- Crop N offtake (DM yield x grain N%)
- Crop N uptake (Crop N offtake + N in above ground residues (i.e. straw DM yield x straw N))
- Nitrogen Harvest Index (proportion of N in the crop that is in the harvested product (crop N offtake/crop N uptake))
- NHI is rarely measured on-farm, but a range of typical values can be used to estimate Crop N uptake from crop N offtake/NHI
Grain analysis is usually necessary for calculating N uptakes, N offtakes and to calculate N balances. Grain analysis can also prove useful in diagnosing whether supplies of N (and other nutrients) to the crop have been deficient or excessive.
Highly efficient N use is not necessary a good thing; it may reflect that a crop is 'mining' N from the soil's organic matter. Brentrup et al. (2015) provided the diagram below to show how both high and low NUE outcomes (i.e. N capture) can be undesirable in crop-soil systems:
N Balances: Going beyond crop-soil systems, it is important to note that, for agriculture as a whole, N balances may be calculated at field, enterprise, farm and region scales. N balances are used across EU, including for NVZ and regulatory requirements. Key to this is that N harvested in crops is predominantly fed to livestock or people, who only retain a minority of what they consume. Thus, maximising N harvested from crops can ultimately prove counter-productive for the environment. Ultimately the objective of crop producers must be to only export proteins that are essential to their end-users, for their crops to rely on recycled nitrogen as much as is possible, and for their farms to import as little 'new' nitrogen as possible. Thus, farm system design and management of organic manures are both vital aspects of developing efficient use of N across all farms.
Routes to improving NUE
Good nutrient management - right product, right rate, right place, right time
N efficient crop species and varieties
Human dietary choices, livestock nutrition, manure treatment, management, storage and spreading.