Adopting variable rate nitrogen application could help growers offset the cost of the UK’s new carbon border tax without sacrificing yield.
Variable rate applications can trim unnecessary spend by targeting fertiliser where crops are most likely to respond, rather than applying a flat rate across every field, say advocates.
Industry estimates suggest imported nitrogen prices are already £40/tonne higher this spring because the EU’s carbon border adjustment mechanism (CBAM) is already in force on most fertiliser coming into the UK.
Cost pressure
Further increases are expected when the UK introduces its own regime next year. “It isn’t ideal considering the current grain prices and that nitrogen is already at a high price,” says Ben Foster, product manager for digital agronomy company Rhiza.
Variable-rate application could offset much of that increase. “Although the technology has been available for a long time, some farmers have tried it and decided it wasn’t for them, while others have the required machinery but have never tried it.”
Newer digital tools have made variable rate nitrogen cheaper, easier and more accurate, explains Mr Foster.
“I’d encourage all farmers with the capability of variable rate spreading to look at the technology this season, at least in a trial area, to examine its results on their bottom line.”
Field evidence
Yield maps from combines provide a starting point, he says, particularly as more second-hand machines with mapping capability enter the market.
“I’d recommend looking at the variation in satellite imagery from a platform like Contour in March or April, and overlaying it with historical yield maps,” says Mr Foster.
“In my experience, the variation in spring satellite images will likely correlate very closely with yield data from the summer. This should give farmers the confidence to tweak late spring nitrogen applications to match field potential.”
Peter Cartwright, farms manager at the Revesby Estate in Lincolnshire, adopted variable-rate nitrogen on wheat last season after seeing it trialled through Agrii’s digital technology programme.
“In oilseed rape, it’s straightforward because it’s linked to the green area index,” says Mr Cartwright. “With wheat, we didn’t know whether to push a backwards crop or hold back on it.
“We are still asking similar questions, but we have a better grasp of it with the information available to us. We signed up for the variable rate Sustainable Farming Incentive action and we are using it across the whole estate now.”
Changing maths
He says the economics have shifted. With nitrogen expensive and grain values lower, the risk of over-application now outweighs the potential reward. “The economic viability of that last 10% of yield has changed.”
Total nitrogen use was similar last season – but it was redistributed. Better-performing areas received more, while poorer zones received less.
Early results, particularly on lighter land in dry conditions, suggest savings are possible without compromising output.
Tools such as Contour allow farmers to choose between strategies focused on maximising yield in stronger areas or levelling crop canopies across a field.
“There is no definitive answer to this,” says Mr Foster. “It depends on the season and the farmer’s knowledge of the field.”
With drier springs becoming more common, understanding soil type and underlying geology is increasingly important, he adds. Clay soils or chalk tend to cope better with drought and justify higher nitrogen rates than sandier ground.
For those considering a trial this season, Rhiza offers a pay-as-you-go model. Farmers can access the software across their whole farm but pay only for the area where a variable-rate plan is produced.