In-field Variable Rate Nitrogen found to deliver 'limited' yield benefits
New ADAS research confirms that varying nitrogen (N) fertiliser rates within a field result in only minor improvements to yield, gross margin, and greenhouse gas emissions compared to a flat optimum N 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 tonnes per hectare and improved profits by roughly £10 per hectare. The environmental benefits of VRN were also modest, with only small reductions in greenhouse gas (GHG) emissions per tonne of crop produced.
Read the full research paperHow the trials were undertaken
To carry out the trials, ADAS divided single fields into hundreds of small square plots with different amounts of nitrogen applied to each plot, then fitted N response curves and calculated the “optimal” N rate for each plot. Besides N, all other inputs and applications remained the same.
Within a single field, yields varied by up to 2.5 tonnes per hectare (wheat), and the optimal N requirement differed by more than 200 kg per hectare (oilseed rape) between areas highlighting the large within-field variation.
Later data analysis revealed that applying N rates variably within the field would result in only small benefits compared with applying the same average N rate as a flat application to the whole field (‘flat optimum N rate’). They were also compared with other rates, including the UK average N rate from the British Survey of Fertiliser Practice, and the RB209 recommended N 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 N rate.”
Environmental impacts
Environmental impacts followed a similar pattern – adjusting N rates within fields had very little impact on overall modelled GHG emissions when compared to using a flat optimal N rate for the whole field. When a flat optimum N 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.
Adjusting fertiliser applications
Matching fertiliser applications to crop demand remains critical for optimising fertiliser use, but estimating the optimum N rate accurately remains a challenge.
“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 N rate, rather than the actual delivery of different N rates within a field.
Background information
- This work was funded by Innovate UK (project 101909), Defra and AHDB (project LK09134), CF Fertilisers and Defra.
- There were 6 winter wheat chessboard trials, 3 winter barley and 2 OSR trials. Each with between 200 and 528 plots. Field trial locations were spread across England including Yorkshire, Nottinghamshire, Oxfordshire, Bedfordshire, Norfolk and Suffolk.
- NUTRI-CHECK NET 3-step checking approach aims to help growers better estimate and manage their optimum N rates.
The full research can be found on the European Journal of Agronomy on Science Direct.
Read full research paper