Can I use "field" instead of "patch"?
Yes, you can. Patch and field are just size variances. Patch refers to a small piece of land, while a field would be something much bigger. So a cabbage field would be a large area only for growing cabbages, most likely a farm, while a cabbage patch is a smaller area like a garden. I think he just uses cabbage patch to exaggerate how silly he thinks the idea is.
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