I'd use bwareaopen() or bwareafilt() to get blobs that are small. Large blobs will be the big crop rows. Of course when the plants are very young it would be hard to tell between a small wheat plant and a small weed. However John Deere has farm machines that drive over the crop rows and use deep learning to spray only the weeds, so it is a solved problem, and probably one that has patents on the concept.
You forgot to attach your code. You must have modified a demo of mine somehow and made a new script called DP.m. Evidently the code is expecting a binary image - a 2D logical image - and you're not giving it that. Maybe you're giving it a color image or something. But I can't tell because you didn't supply your code modifications. Please attach your image and DP.m and anything else we need to run your code.
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