New ADAS research has revealed that varying nitrogen fertiliser rates within a field delivered only minor improvements to yield, gross margin, and greenhouse gas emissions compared to a flat optimum nitrogen rate for winter wheat, winter barley, and oilseed rape.
The research, conducted across 11 commercial farms in the UK over 5 years, found that on average, variable rate nitrogen (VRN) applications would have increased yields by only about 0.05 t/ha and improved profits by roughly £10/ha The environmental benefits of VRN were also modest, with only small reductions in greenhouse gas (GHG) emissions per tonne of crop produced.
ADAS divided single fields into hundreds of small square plots with different amounts of nitrogen applied to each plot, then fitted nitrogen response curves and calculated the “optimal” nitrogen rate for each plot. Besides nitrogen, all other inputs and applications remained the same.
Within a single field, yields varied by up to 2.5 t/ha (wheat), and the optimal nitrogen requirement differed by more than 200 kg/ha (oilseed rape) between areas highlighting the large within-field variation.
ADAS project lead Dr Kate Storer said: “These differences highlight exactly why precision farming tools are considered promising – crop needs are not uniform, even within the same field”
Later data analysis revealed that applying nitrogen rates variably within the field would result in only small benefits compared with applying the same average nitrogen rate as a flat application to the whole field (‘flat optimum nitrogen rate’).
They were also compared with other rates, including the UK average nitrogen rate from the British Survey of Fertiliser Practice, and the RB209 recommended nitrogen rate for each field.
Kate added: “We found that using VRN would result in some small yield and gross margin gains, but ultimately most benefits came from being closer to the field optimum nitrogen rate.”
Environmental impacts
Environmental impacts followed a similar pattern – adjusting nitrogen rates within fields had very little impact on overall modelled GHG emissions when compared to using a flat optimal nitrogen rate for the whole field. When a flat optimum nitrogen rate was used, the GHG emissions from applying above the optimum in some areas of the field were cancelled out by reduced GHG emissions when applying below the optimum in other areas.
Kate explained: “The findings indicate that tailoring nitrogen applications by variably applying nitrogen within a field offers relatively little gain over applying an accurate flat optimum nitrogen rate across the field. But estimating the perfect optimum nitrogen rate is difficult, so it’s important to utilise all relevant methods, including in season adjustments, to be as accurate as possible.”
Adjusting fertiliser applications
Matching fertiliser applications to crop demand remains critical for optimising fertiliser use, but estimating the optimum nitrogen rate accurately remains a challenge.
Co-author Dr Sarah Kendall reflected: “There is a renewed focus on fertiliser rates as nitrogen prices rise. Yet, the results were similar when tested under increased fertiliser prices, suggesting that VRN will not necessarily provide greater benefits with a higher ratio of nitrogen cost to grain prices.”
“Prioritising understanding field level fertiliser requirements through soil assessments, crop monitoring, and decision support tools is key. We urge growers to make use of the Plan, Check & Adjust, Review approach for nutrient management to help optimise decisions. The greatest value of VRN is likely to result from helping to get as close as possible to the field average optimum nitrogen rate, rather than the actual delivery of different nitrogen rates within a field.
