Water use efficiency is widely recognised as a key performance indicator for crop production. Similarly nitrogen use efficiency (NUE) is an important indicator of resource use effectiveness and economic performance of this important crop input.
Why measure NUE?
Nitrogen fertilizer is frequently the biggest single cost within cropping variable costs. With slim cropping margins, it is important to achieve the best possible return on money invested in nitrogen fertiliser.
Measurement of inputs and removal of nitrogen over several seasons will assist in understanding NUE at a field or farm scale. Inputs include legume derived N, as well as fertilizer source, rate, time and place. The quantity of rainfall and its timing and other seasonal influences create variability that limits the value of a single season observation.
Developing strategies to improve NUE has two broad outcomes:
- Achieving a higher crop yield on the N supplied (Partial Factor Productivity and Agronomic Efficiency)
- Assessment of the effect of management on the balance of N in the field or farm (Partial Nutrient Balance).
How to measure NUE?
There are many ways to assess NUE, but three of the simplest methods of assessing NUE at the paddock scale are:
- Partial Factor Productivity (PFP)
- Agronomic Efficiency (AE)
- Partial Nutrient Balance (PNB).
These can be assessed at paddock levels; PFP and PNB can be assessed at farm to national scale. Each measure gives a different perspective on NUE.
1. Partial Factor Productivity (PFP) of N.
This is a broad measure of the efficiency of grain production in relation to the N rate used, and is affected by the N strategy (4Rs), seasonal conditions and subsequent yield achieved. It is calculated as the grain yield divided by the N applied or supplied and has the units of kg grain/kg N:
PFP NUE = Ywith N / Nrate
Low N fertilizer application rates can produce high PFP values because the crop is very responsive to season type or low native N supply. Because N is also being supplied from the soil, a high PNB is not necessarily desirable or sustainable in the long-term unless there is a high frequency of nitrogen-fixing crops in the rotation.
Low PFP values suggest that supply is approaching a yield plateau, so the grain return on the added N is declining.
Growers can include fixed N into the N budget and so account for this source in assessing PNB, but even so, crop water supply can have a large impact.
Generally, N management that produces PFP in the range of 40 – 70 kg grain/kg N for wheat is recognised as being reasonably efficient.
2. Agronomic Efficiency (AE).
This is a direct measure of the response of the N fertiliser applied after removing the impact of the soil-supplied N.
AE is calculated from the grain yield with N applied minus the grain yield with Nil N divided by the N rate applied and so requires a nil N fertilizer strip to be left in the paddock. It has the units of kg grain/kg N:
AE = (Ywith N – Ynil N) / Nrate
Leaving a strip or strips in a paddock is getting progressively easier with yield monitors in harvesters, variable rate applicators and GPS guidance systems.
Improvement in AE depends on a robust assessment of the soil’s N contribution to yield. Underestimating inherent soil N supply capability causes over fertilisation and lowers AE. Moderate over-estimation of soil N supply has less effect on AE as grain protein content, rather than yield, is the first nitrogen component to decline with under fertilisation.
AE is probably the best guide to the economic effectiveness of N application. In recent times, AE in the range 4 to 8 kg grain/kg N has been breakeven efficiency.
AE can be extended to $AE, by including prices for crop and fertilizer into the equation:
$AE = ((Ywith N – Ynil N) * Crop price) /( Nrate * Nprice )
3. Partial Nutrient Balance (PNB)
A third measure of efficiency is the partial nutrient balance, sometimes called the removal to use ratio. It quantifies the amount of N being removed in the produce relative to the amount supplied.
PNB is calculated from grain yield multiplied by grain N (protein content divided by 5.7 – for wheat) divided by the N rate applied and has the units kg N in grain/kg N supplied:
PNB = (Yield * Protein/5.7) / N applied (wheat)
The value 5.7 in the equation is the N concentration in proteins (~17%). The value is generally 6.25 for other crops.
A low PNB (less than 0.5) indicates that more N is being applied than is being removed, and this could represent an environmental hazard. Where PNB is more than 1.0, more N is being removed than is being supplied, so soil N status will decline as N is effectively mined. Sustainable N management is indicated by PNB in range of 0.7 to 1.0, although these values can vary over seasons and with grain yield.
PNB is accepted internationally as the measure of NUE at national levels, and is easily downscaled to fields, farms or regions.
Table 1. Average nitrogen rate, partial factor productivity and partial nutrient balance for wheat growing in different countries. No account is taken of fixed N or N derived from manures in these calculations. International figures for 2007-2009, Australian data for 2010.
1. Data sources from document listed in the citation at the end of the article
NUE trends over time
While the magnitude of NUE is important, improvement over time is more important. Figure 1 shows a desirable trend in PFP over time. Grain yields need to also be considered as a high efficiency can be achieved with no fertilizer application, but the yields will be compromised or the N extracted derived from soil reserves.
Figure 1. NUE over time and a trend line.
Measuring NUE in summer crops 2015
The Nutrition Extension and Training Project within the More Profit from Crop Nutrition Initiative (MPCN 2) is keen to hear from advisors who would like to measure NUE efficiency in the coming summer crop. Assistance with soil testing, harvesting and interpretation may be available. Leaving a strip or strips in the paddock with no N applied will be needed. A small field walk and discussion will occur late in the season of the summer crop with the aim of demonstrating the $ value of getting the N rate as close as possible to the optimum for the season.
Please contact: Howard Cox 07 46881381 or Chris Dowling 0407 692251
Fischer, RA, Byerlee D & Edmeades, GO 2014 Crop yields and global food security: will yield increase continue to feed the world? Canberra: Australian Centre for International Agricultural Research. Available from http://aciar.gov.au/publication/mn158 [15 May 2015].
Heffer, P 2011 Assessment of Fertilizer Use by Crop at the Global Level 2010- 20 10/11. Available from http://www.fertilizer.org/imis20/images/Library_Downloads [15 May 2015}
Grain Yearbook. 2008 Available from http://www.ausgrain.com.au/Back%20Issues/177ybgrn08/30_Wheat.pdf [15 May 2015].
Article written by Chris Dowling (Back Paddock Company) and Howard Cox (DAFFQLD), with technical input from Rob Norton (International Plant Nutrition Institute), and Kaara Klepper (DAFFQLD).