The very purpose of futures contracts is to ensure availability in the future at a predetermined price. I am not sure what your point is.

That there is no guarantee that the stuff will be available so, thing shappen. Also, just because futures are cheap, doesn't mean stuff is available now or in the near future. It only means the markets think grain will be plenty at one point, and thus cheap.

All a low futures price means is that money men are betting there won't be a shortage, it doesn't say anything about whether there will be a shortage or not.

Price is not solely determined by availability, but it is largely determined by availability, especially in the case of rapid shifts in availability (since other factors take time to adjust in response).