All I can say is I never pass up a washer or bolt lying in a parking lot. And I usually take what fasteners I can easily get out of whatever I am throwing away. I also have about 100 pounds of metal in the form of bolts, screws, washers, clamps, etc, etc.
It just makes good sense to be thrifty since that's the way I was raised. I hate having to drop what I am doing to chase a bolt down at the hardware store...which mostly now come prepackaged at an inflated price.
I think most farmers buy them in bulk by the pound at places like Tractor Supply or Agri Supply if they are close enough.
I'm not sure how the farmer can pass his costs to the consumer since the farmer is at the beginning of the food chain. He has to sell his produce to whomever will offer the best price or the going rate, or has locked in a price earlier in the season. Seems like a squeeze play if they don't have some kind of specialized product.