been years since i been near a manure spreader. had to google it...
https://www.google.com/search?q=manure+spreader+ground+drive&ie=utf-8&oe=utf-8 ((manure spreader ground drive)) was shocked to find stuff being being made for ATV's and like.
would say, ya better off buying one. more so ones with a belt in the bottom, and then the beaters on the back. lots of metal fractionation.
i question use of ground driven. vs PTO version, and simply not having enough weight to keep the belt / beaters moving, and keeping the wheels from locking up and sliding across the ground almost.
you get into them thicker hay/stray piles and up with descent size chunks that do not bust up / come apart very easily.
you also have some liquid / urine / water to deal with.
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a compost spreader might be more reasonable for a DIY project. compost being more of a fine dirt, were stray / hay has broken down.
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it has been 2 to 5 years now, but local library, i was able to dig up some old "farm books" that had plans for various farm equipment. it was made back in like 70's i would guess. but might be worth checking out, without having to figure out every little detail, if going DIY route.