Agronomy Advice

How to determine the optimum N rate for cereals

With only 50% of wheat crops having an optimum N rate close to the average figure and the other 50% varying by more than 15%, good determination of the required N rate is needed to avoid under or over fertilising fields.

How to determine the optimum N rate for cereals
How to determine the optimum N rate for cereals

Over the last 15 years, Yara has conducted many nitrogen response trials with the average response to nitrogen being very consistent.  However, the response of individual fields can vary considerably around the average depending on soil conditions and other factors. 

Nitrogen response of winter wheat.png

As can be seen from the chart below in at least half of the fields the optimum rate of nitrogen (N Opt) was more than 30 kg higher or lower than the average.  The challenge for agronomists and growers is to identify these fields prior to making nitrogen application to ensure the correct rate is applied to optimise crop yield while minimising the spend on nitrogen.

  Optimum N rate in wheat N response trials

Furthermore, as well as variation between fields, there is often as much variation within fields between different areas which is where variable rate applications can be a useful option to apply the correct rate of nitrogen to different areas of the field.

Application rates for first nitrogen splits on cereals should generally vary according to the condition of the crop coming out of the winter. Nutrition is a great way to manipulate tiller numbers with higher rates applied to less well-tillered crops, where tiller survival is of greater importance and slightly lower rates to the more forward crops.

Second applications will largely be the same year on year with the objective being to ensure the crop has sufficient nitrogen to meet requirements, without over-application which risks losses through runoff or leaching. The best time to make adjustments is when making final applications to take into account the growing conditions during spring.

Use the crop as the best indicator of N uptake

Average optimum rates for winter wheat have generally stayed relatively flat over the last decade of trials, at 230 kgN/ha, however with the recent changes in both crop and fertiliser prices this figure may need to be adjusted based on the actual prices paid for fertiliser and received for wheat.  To calculate the the effect of recent changes in prices on the optimum N rate check out our wheat N-response calculator

This average is a very good starting point and any adjustments in either direction need to be carefully considered and backed by sound rationale. It is difficult to understand when to make adjustments, as the main influencing factors are mineralisation and efficiency of utilisation, two variables that cannot be accurately measured. 

Using the crop as an indicator of nitrogen uptake is the most efficient means; which can either be through tissue analysis or through the use of tools such as the Yara N-Tester. These identify the levels of nitrogen in the plant to help understand whether the crop is running short to influence final application rates and more accurate nitrogen timings.

The N-Tester measures the level of chlorophyll in the leaf and from this can determine the nitrogen content.  This figure is then used to calculate a recommended nitrogen rate to achieve the optimum economic return.  Currently, the calculation is made using 'historic' values for crop and nitrogen prices and break-even ratios, so depending on current or actual prices the actual recommendation may need to be revised accordingly.

The Yara N-Sensor or Atfarm are the two solutions for variable rate nitrogen to address within-field variation in nitrogen requirement.  The N-Sensor does this by sensing the variation in the crop in real-time at the time of application, whereas Atfarm uses satellite images to create variable rate application maps



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