Overall is a decent set of photos, harvest has to be one of my favourite time of year, and I'm really getting the feel from these, that second one would really have been made if there had been some interest on the track as it feels a little empty, be that a tractor coming down the track, or a couple of walkers say, but just something to add a little extra interest...I'd also be tempted to clone out the wires from the sky..
On the first of the combine photos it could do with a little bit of a boost to the exposure, you could stick on a digital grad onto the sky to retain the detail you've got in the sky when boosting the overall exposure, I think however what would really have made this photo would have been a lower PoV getting down on a similar level as what the crop was
The last one I really like, I don't mind the fact the sky's a bit over exposed as the main subject is well exposed, I would however be tempted to just warm the white balance a touch
Matt