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Fungicide: Does it Pay to Spray?


Introduction

Year after year growers often wonder if it is worth spending the money to spray fungicides. Though spraying cereals for fusarium almost always pays, many growers question if they will see a return from spraying canola for sclerotinia. For the growers who often question if it’s worth spraying fungicides, variable rate through on/off prescriptions can be the answer to saving money on fungicide applications along with applying products where they are most needed in a field.


NDVI

NDVI stands for Normalized Difference Vegetation Index. NDVI assess vegetation through the measurement of the difference between near-infrared and red light. Near infrared is reflected strongly by the crop canopy whereas red light is absorbed by the crop canopy. The formula in figure 1 shows how NDVI is calculated. Figure 2 shows an example of how to calculate NDVI with a healthy plant and a plant that is under some sort of stress.


Figure 1. NDVI formula; NIR stands for near-infrared

Figure 2. NDVI calculation example.

Using NDVI Imagry and SWAT MAPS to Make On/Off Fungicide Prescriptions

To make on/off fungicide prescriptions, we have the option to use NDVI imagery or the SWAT MAP for a mapped field. Typically you would not want to spray the areas with less crop biomass, such as flooded out areas, saline areas, or dry, sparse hills. These areas of the field do not have the crop biomass that is worth protecting with fungicide. Some diseases, especially Sclerotinia Stem Rot in canola, you would want to spray the areas of the field that have a higher crop biomass as this disease tends to occur in higher biomass areas. SWAT MAPS can be used to identify these areas in a field, however not every year is the same. For example, zones 1-4 may have less crop biomass due to drought in one year and the next year those zones might have the most crop biomass due to flooding. NDVI imagery of a field shows exactly what areas of the field have the most and the least crop biomass of that year, making it the more suitable option for making on/off prescriptions. On NDVI Imagery, the field would be broken into a ten zone map based on the amount of biomass. The field needs to be ground truthed before the prescription can be made. When ground truthing, the grower will determine which zones on the SWAT MAP or the NDVI zone map that are worth spraying fungicide on and which zones are not worth spraying the fungicide on. Once the ground truthing is completed, the prescription can be made.


Cost Benefits of Using On/Off Prescriptions

Figure 3 shows an example variable rate on/off fungicide perscription report for a 45 acre canola field using NDVI Imagry. When ground truthing, it was determined that zones 1-7 had a dense plant stand that would be worth spraying fungicide on, and zones 8-10 had thinner plant stands which would not be worth spraying with fungicide. Zones 1-7 would be turned on, receiving 10 gallons per acre and zones 8-10 would be turned off. Using this perscription, 32.4% of the field will not receive any fungicide which works out to be 14.6 acres. Depending on the areas, the cost of fungicide can vary. Proline is roughly $18/acre, which would work out to costing $810 to spray this whole 45 acre canola field with proline. By using an on/off perscription, a grower would save roughly $263 on their fungicide bill to spray this field with Proline. Multiply this by large acres across the farm, savings can be significant. This can also help mitigate the worry about the cost of products and can help growers achieve ROI quicker.


Figure 3. Example on/off variable rate fungicide prescription using NDVI imagery.

Summary

In summary, spraying fungicides, specifically in canola, can be a difficult decision to make since the chance of getting a good ROI is lower. On/off variable rate prescripitions can be made using NDVI imagery or SWAT MAPS to help save money on fungicide products by spraying the products on areas of a field that are worth protecting. These areas include dense crop populations and exclude thin plant populations such as saline areas, flooded areas, and dry hills with low biomass.



Sean Barath

Precision Agronomist

sean@swatmaps.com





Figure 1 and 2 retrieved from https://gisgeography.com/ndvi-normalized-difference-vegetation-index/.



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