
Sounds like Urticaria or nettle rash & I see it quite a lot in my hounds. Firstly, I have to say why is your dog jumping around in a corn field? Presumably its not your corn or you would know what had been applied to it.....if there is a public right of way through it, then there should be a clear tramline for you to walk on, but your dog should NOT be in the corn.
That aside, if the corn is still standing then it will not have been sprayed in the last 8 weeks or so. If its been cut then it could be the stubble irritating your dog, although his pads would most likely be very sore too. Other likely causes are midges, something in your lawn, or nettles/hogweed if he has been rooting throughthe undergrowth. One of my bitches gets this every summer but there are no chemicals used either in the garden or on the fields this time of year, so put it down to midges.