We reuse ours, as do many farmers. The big round ones make great water buckets, storage buckets, an upturned one was my milking stool for years! The smaller oblong ones are just the right size for a Box to Carry Things In on the quad bike, in the boot, and so on, and also get used as feed buckets for animals. I have about 30 with lids I use as mothproof lightproof yarn and fibre storage boxes.
A sizeable farm often collects up more than they can reuse themselves of course and many sell surplus in lots of 10 at farm sales and so on.
I think most farmers would say "help yourself" if you asked - but it would be polite to ask, if you can identify the farmer in question - which I know isn't always easy.