Nitrogen fertilizer decisions are considered important for the agronomic, economic and environmental performance of cereal crop production. Despite good recommendation systems large unpredicted variation exists in measured N requirements. There may be fields and farms that are consistently receiving too much or too little N fertilizer, therefore losing substantial profit from wasted fertilizer or lost yield. Precision farming technologies can enable farmers (& researchers) to test appropriate N rates, through tramline comparisons and analysis of yield maps. We report findings from the LearN initiative in the UK to help enable farmers to test N rates on their farm, to learn whether the N rates they are using are about right, too high or too low. A group of 18 farmers were supported to conduct simple tramline trials with single replicates of 60kgN/ha more and less fertilizer in alternate tramlines, on 3 fields per year from 2014-2017.

We found strong engagement from farmers in conducting tramline trials, which are in principle easy to set-up, manage & harvest. However, there are many challenges that are not always fully appreciated; ensuring comparable treatment areas; precise recording of tramline wheelings and treatment boundaries; non-linear application by spinning disc spreaders; protocols for combine harvest; transfer, processing & cleaning of data; data analysis & statistics to achieve robust conclusions in the face of confounding variation and appropriate interpretation.

Overall yields were 11.43, 11.07 and 11.74 t/ha for farm-standard, +60kgN/ha and -60kgN/ha respectively, giving differences in margin of -£8.75/ha and +£0.51/ha. Differences in yield between two ‘farm standard’ tramlines was used to give a crude measure of error and infer confidence. Firm conclusions on N management could be made on around half of the 142 tramline experiments. Underlying spatial variation in yield was usually much greater than the nitrogen effect. A few fields and farms were found with sub-optimal N rates, but lost profits were modest. We conclude that nitrogen is not the major cause of variation in yield within & between fields and farms. Tramline trials are useful for understanding variation in yields; for maximum impact they should be networked and employ spatial analysis.