While we focus mostly on soil variability with SWAT MAPS, we know that crop variability doesn’t end there. There are lots of opportunities to manage variability in season that is caused by temporal factors such as flooding, patchy emergence, drought, seeding misses, etc. For that reason, we shift to a strategy of using TOP MAPS. TOP MAPS take a strategy of utilizing NDVI imagery, a SWAT MAP, or some combination of both, to provide management zones that correctly delineate areas based on the factors that are important for the application. This can be difficult to explain so here are a couple examples:
For topdressing nitrogen or sulphur, for example, it would usually make the most sense to utilize the SWAT MAP to produce a prescription, since response to nutrients is primarily driven by soil properties and water relationships. So if conditions were relatively wet, and water driven yield potential increased in zones 1-5, you could apply more nitrogen in those areas that had the larger increase in yield potential, and less in lower landscape positions with higher mineralization rates, such as the example below. It all depends on the field, what was applied for a base rate, and the current conditions.
For desiccating peas, on the other hand, NDVI works well since it indicates relative biomass and green plant material. This strategy would work well for Reglone, where the water volume could range from 15 to 20 gal/ac, applying 0.52 L/ac at the lowest rate where the crop has essentially matured on its own, to 0.7 L/ac in areas of higher biomass and more green plant material to dry down. It has the added benefit of even higher water volume in these areas for more coverage. The SWAT MAP also helps us “ground-truth” the NDVI image to ensure it makes sense, since it tells us where wet depressions or dry knolls are.
Regardless of the application and what is used to create a TOP MAP, it should always be ground-truthed so the data and relative difference between zones makes sense. NDVI values for instance don’t easily tell you if the crop is good or bad in certain areas, it only tells you a relative difference, and sometimes the relative difference is not very significant. Other factors like heavy weed growth (i.e., cattails in the middle of a flooded depression) can cause high values that may look like good crop when in fact there’s no crop at all. So some “boots on the ground” agronomy is still needed for this! And viewing the maps in the CropRecords app makes this exercise very easy.
If you’re interested in a prescription for topdressing, fungicide, or desiccant give your CropPro Consulting agronomist a call and they will work with you to figure out the best approach for a TOP MAP. If you don't have SWAT MAPS or are not yet working with CropPro, please visit www.croppro.ca/topmaps and use the form to request TOP MAPS.
Senior Fertility Specialist