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Variable-rate nitrogen applications could be a valuable tool for managing uneven growth of winter cereals and oilseeds this spring. Some crops sown before the... Variable nitrogen could prove its worth this spring

Variable-rate nitrogen applications could be a valuable tool for managing uneven growth of winter cereals and oilseeds this spring.

Some crops sown before the weather broke last October established well and show good potential. But others have struggled where seedbeds were compromised or land suffered prolonged waterlogging or flooding.

“There’s huge variation out there and some pretty sensitive crops need managing carefully,” says Omnia digital farming manager Aidan Monaghan. “Variable nitrogen applications to even up growth and optimise yield potential has a stronger role in a year like this.

“All winter crops can show a response from variably applying nitrogen, but the biggest benefits are when there is more variation. This year is almost the perfect opportunity for variable nitrogen, even if you’re already using variable rates elsewhere, such as for seed.”


When planning nutrition strategies, Mr Monaghan recommends first establishing a baseline for each field and crop. Omnia users are able to do this by accessing NDVI satellite imagery for any fields saved on the system.

The system generates a regular supply of images throughout the year when the skies are clear. They provide a useful starting point to show in-field variation before anything starts growing, says Mr Monaghan.

Early mapping can highlight areas where crops are damaged beyond repair and are better being re-sown, rather than receiving further inputs. Soil Mineral Nitrogen (SMN) testing and later tissue, sap or in-field chlorophyl can also help build a picture.

There are different approaches to variable nitrogen, depending on the requirements of individual crops and field situations.

The traditional approach in winter wheat is often to variably apply the first couple of doses, upping rates on thinner areas of crop to build biomass, while cutting rates on thicker parts, to generate more even canopies.

Most fields will benefit from a good early nitrogen dose to get crops moving because residual nitrogen levels this spring are likely to be lower than normal after the wet winter, says Mr Monaghan. This will have increased leaching losses.

“The best approach might therefore be to apply a flat rate first, then vary the second application to even crops up, before tailoring the final nitrogen applications to the yield potential of specific areas of crop.”

Many trials over multiple years show a clear benefit from front-loading nitrogen at the start of the season to build biomass and support tiller retention during the crucial foundation phase.

This then allows subsequent doses to be varied according to yield potential and end user requirements. But it isn’t just soil nitrogen that could be much lower this spring, says Hutchinsons fertiliser and crop nutrition specialist Rob Jewers.

“Other nutrients are likely to be in shorter supply given how wet it has been, so there could be a strong case for using a nitrogen, phosphorus, potassium and sulphur compound fertiliser to give crops a bit of an all-round nutritional boost.”

Mr Jewers points out that NDVI imagery could be useful this spring for highlighting any areas of crop have that have failed to establish completely, or have been lost to flooding, before fertiliser is applied.

Fields should be walked to double-check whether there is any viable crop remaining, and if not, application plans can be set so that no fertiliser is applied to bare areas.


But saving fertiliser is not the main aim of variable rate applications, he notes. “It’s about using the technology to even-up crop growth through the season, tailoring inputs precisely to crop requirements to optimise yield potential.”

This can bring other management benefits, such as reduced lodging or more even harvesting, and help growers improve overall nitrogen use efficiency (NUE).

“Typically, NUE is around 55-60%, but a good target is 75-80%; any more than that and there is a risk that you’re starting to ‘mine’ the soil’s reserves. Omnia is a really good tool for helping you calculate the NUE for individual fields.”

Variable fertiliser applications may be slightly different in oilseed rape, as crops often receive just two main applications, says Mr Monaghan. Crops can be evened up by variably applying the first dose, then adjusting the second according to expected yield.


“The key is to establish that early baseline, then keep reviewing crops and their yield potential as the season progresses, using the NDVI imagery in Omnia, and any other information, to plan the most appropriate strategy.”