Converting NDVI to Zone Polygons

I would like to try to find a way to create "clean" zones or polygons from an NDVI image. From what I have tried, I always end up with too many small areas due to the resolution of the images. I would like something more simplified and easier to manage for creating management zones for applications of fertilizers, etc. I attached an example of the NDVI, and also of a typical type of zone that I would like created from it. I can tweak the zones manually if needed, but just don't want to have to trace each one manually. We have hundreds of fields that we need to do this with.

Thanks for any advice!
Bryan

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