Interesting game.
At first, I didn't notice you had to collect the hay (I guess I read the tutorial too fast), so the game was a bit boring. After reading it again, the game got a whole lot better.
Maybe a visual tutorial (just some sprites, nothing animated) would improve it. I think it would also be nice (and not break the game) if you had full control of the tractors (up and down included), could make some interesting mechanics.
Overall solid gameplay and original ideia, good job!