I don't think Dany burned all of the food
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I mean, think about it. The Reach's entire grain supply would be hundreds and hundreds of wagons, not the few dozen or so that were burned. I remember when Olenna was talking to Tyrion about how much food they were supplying to King's Landing and she was talking about millions of bushels.
I'll be shocked if it's revealed that all, or even a majority of the grain was lost. It seems like she burned some wagons as a show of force like she did with the masters when she took out some ships before taking the rest of them. I would guess she's doing the same thing, and will either keep the grain for her army or just hoard it to starve King's Landing.