Variable rate application of fungicides on winter wheat (PhD)

Currently, the dominant practice is to apply fungicides at uniform dose rates to winter wheat irrespective of in-field variability.

However, at a fixed application rate, smaller plants receive a larger dose per unit plant area or biomass, compared to larger plants.

Variable rate application (VRA) of plant protection products (PPPs) to cereal crops is technically feasible with off-the-shelf commercial equipment.

VRA is already used to manage nitrogen and, more recently, plant growth regulator (PGR) applications.

How to achieve VRA of fungicides on winter wheat was the primary aim of this research.

At the start of the project, a research review found that the main barriers to VRA adoption are cost and perceived risk of inconsistency of effect.

In 2018, a laboratory trial studied the deposition of a dye (representative of fungicide) through the winter wheat growing season. Destructive sampling determined the variation in dose per gram of biomass for each fungicide application timing. This showed that although later growth stages received the greatest dose, the dose relative to the biomass was lower than earlier growth stages.

In 2019 (March to June), a field study on two farms measured the spatial and temporal variation in volumetric biomass to determine if normalized difference vegetation index (NDVI) information could be exploited for application decisions.

Observations showed highly variable biomasses spatially, even across distances of 100m. Monitoring at earlier growth stages (associated with T0 and T1 applications) best reflected canopy variability.

A new method was piloted to extract NDVI min and max values from 43,000 winter wheat fields in 2018. This was to understand if the range of variation, in NDVI values at T1, in winter wheat fields is enough to continue VRA investment.

Finally, a cost benefit analysis was performed. This compared a standard uniform dose and a variable dose, with a saving of £13.82/ha on one farm from one VRA spray (T1).

This study provides methodology for conducting VRA and a foundation for future VRA investigations.