From growing up in farm country, here are a few I can recall:
"That boy's brain don't have enough horsepower to pull a thought"
"Stay between the rows, boy!" (focus on the task)
"Keep your furrows straight" (do a quality job)
"I'd say he's about half a bubble off plumb"
"I might as well be talking to that fence post, boy"
"You can argue till the cows come home......"
"We need rain, the field's are dryer than a popcorn fart"
"Mess with the bull, get the horns"
"That boy's about as useless as teats on a boar hog"