Unless we have robotic or air-conditioned tractors to harvest the cereal crops (or sugarcane! it's C4), it'd be a moot point; before it gets too hot for C4 grasses, there won't be any living mammals in the region. Many insects would be dead too (bees kill wasps by huddling around them and warming them to 45 C). Without insects for pollination, the variety of C4 crops that could propagate would be reduced. We might end up with just lichens before it got too hot to actually kill off the grasses.

My grandfather's tractor didn't have air conditioning, just fans; it was sometimes too hot to harvest even in relatively cool Nova Scotian summers.