Project highlights challenges of optimising fertiliser for yield and carbon footprint

The Carbon Programme

Three years of trials on five farms tested how farmers can reduce the carbon footprint of wheat production by fine tuning the fertiliser programme. Sponsored by Bayer, the Carbon programme also benchmarked soil carbon levels and the benefit of tillage practices to increase carbon sequestration in the soil. 

“The five UK farms were part of a larger European project involving 27 farms in seven countries. It was very much a factfinding project about how to effectively reduce and measure the carbon footprint of crop production and to develop simple to use tools for recording and reporting,” says Nick Duncan of Bayer 

In Europe, there is now a commercial project with a partner in the supply chain to reward growers for reducing the carbon footprint of crop production. Bayer is currently investigating opportunities for further commercial schemes, including in the UK. 

“With the project now ending in the UK, there have been many valuable learnings, which are helping to shape the commercial programmes we are operating elsewhere in Europe, so we would like to thank the five farmers in the UK who took part. One area of the work in the project we undertook, was looking at the opportunities to lower the carbon footprint of cereal production through optimising nitrogen fertiliser use whilst still maintaining yield and margin.” 

Optimising nitrogen 

Over 50% of the greenhouse gas emissions from crop production are linked to the production and use of nitrogen fertiliser. Added to that, nitrogen use efficiency, the amount applied that is used by the crop, is typically around 60%. But reducing total nitrogen input and maintaining yield and margin is not straightforward. 

“When you first hear that there is only 60% utilisation of N, it seems like there should be some easy ways to reduce the carbon footprint. But improving nitrogen use efficiency is difficult,” says Independent Agronomist Antony Wade who advises on one of the participating farms. 

“Applications are made to a dynamic biological system with numerous potential limitations on crop growth, not just nitrogen. Seasonality has a massive impact too, there are lots of things you have to measure or estimate to get it right – soil nitrogen levels, rooting and uptake to name a few. Yield is always the first priority for farmers. If you cut nitrogen rates to reduce the carbon footprint there is the risk you miss out on yield. In any scheme the value of carbon needs to cover that risk.” 

ADAS managed the on-farm trials, and the results back up the importance of effective measurement and adjustments during the growing season. “In the Bayer project, the group of growers had already made good progress in optimising their crop nutrition and yields. So the scope for this specific group of growers to reduce the carbon footprint from fertiliser was small. But even small improvements are helpful because the production and use of nitrogen fertiliser is very carbon intensive,” says Dr Christina Baxter of ADAS. 

Correctly estimating soil nitrogen supply is a challenge which can make it hard to optimise the rate. Even when you do Soil Mineral Nitrogen (SMN) analysis, heavy rainfall after the sampling can cause nitrogen losses which appeared to be the case on some farms in 2023.  

Dr. Baxter recommends regular monitoring of crops and consideration of the weather to help adjust rates in the season. There are a variety of sensors and imaging software alongside crop walking that can help with this. Experience from the YEN project shows that grain analysis is also a useful tool to reflect on the season and identify areas for improvement. 

Sentry Farms which took part in the project has managed to put a lot of this into action. “We use RB209, an N tester and satellite imagery to try and get the correct application rate and then test the grain to see how well we have done,” says John Barrett, Director. “The project challenged us to try different things and better monitor what we have achieved. For me the carbon insetting seems like a great idea to reward farmers for best practice.” 

Improve use efficiency 

Achieving higher use efficiency and preventing losses from the system are important ways to reduce the carbon footprint. “Cover crops can help keep nitrogen in the system so it is available to future crops,” says Dr. Baxter. “Timing is also important, avoiding applications in warm and wet conditions which increase losses, and pay attention to the basics like calibrating spreaders. Also, urease inhibitors can reduce volatilisation losses for those using urea-based fertilisers.” 

Dr. Baxter emphasises the importance of optimising output on land already in production, this works to reduce the carbon footprint per tonne of output. “We had a few cases where farmers underestimated yield potential, so the crops were under-fertilised and yields were reduced. Tiller counts and biomass measurements early in the spring can help improve yield estimates. On the other hand, on one farm, soil condition was limiting yield rather than nitrogen so in this scenario nitrogen rates could be reduced without affecting output.” 

Trials also investigated whether foliar nitrogen products can replace conventional nitrogen to reduce the carbon footprint, due to their claimed improved use efficiency. “We compared the farm standard nitrogen rate with a lower rate (-40kg) with and without a foliar product which contains less than 10kg of nitrogen. In these trials, the foliar nitrogen products could not compensate for a 40kg reduction in rate. Whether they can compensate for a smaller reduction in the total nitrogen rate is yet to be tested.” 

There was a trend for an increase in grain N from using this type of product in four out of five trials, so there may be an opportunity to help milling wheat reach the protein specification with a lower carbon footprint.  

Dr. Baxter suggests that farmers considering these products should trial them first to see if they will fit into their system. ADAS has a free guide online to help farmers conduct their own trials. 

 “I think reducing total nitrogen use with foliar methylated urea fertiliser is worth looking into more,” says Antony Wade. “But results from the Bayer project show it is not simple, once again there several of things you have to get right.”  

Farmer focus: Andrew Williamson

Still a lot to learn about fertiliser and carbon: 

Three years of trials have shown that It’s not easy to improve nitrogen use efficiency and maintain yield. In year one, we tested 160kg of nitrogen compared to the farm standard of 220kg and saw a significant drop in yield and margin, even though it improved the use efficiency.  

Last season, we looked at 180kg/ha compared to the standard and that didn’t affect yield. But last year was a difficult growing season so there was another factor limiting yield not nitrogen.  

I think we can make improvements around timing by measuring soil and crop condition. Applying at the correct time will improve the use efficiency but there is no perfect solution because fertiliser is applied in anticipation of what happens next, but we cannot predict the weather accurately very far ahead. 

From the project, the biggest learning is that there is still a lot to learn. My impression is that it is better to focus on beneficial practices rather than only try to measure carbon. 

Fig. 1: Carbon footprint of YEN Zero winter wheat (feed) crop
Graph provided by ADAS. 

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