First, what the heck is that in your avatar, a giant carp? What ever it is, its a biggun.
I do not know, I have seen wheat germinate and dry up in two weeks, but have only seen that once in 20 years. That was very dry year, finally got .3" of rain to germinate, then it quit again.
I've seen deer eat it to the ground, but never turn brown and die like that without very cold temps.
Did you spray only roundup? How is the drainage? Is it worse in high spots or low spots? Possibly a fungus or insect problem. Have seen lightning kill spots out of a field, but that's a long shot.
Pics?