What does Rashi mean (ד”ה חטין וחיפן בשעורים) when he says that when a person is supposed to protect a haystack, and it seems to be made of barley, that he will not watch it well?
One possiblity is that barley does not get burned as easily, thus it requires less carefullness.
Another possibility is that since the barley is not as significant, it does not give the same impetus to the one who is supposed to watch it to take care of it properly.
ונראה לי פשט שני
In either pshat, if he would have known there was wheat to be watched there, he would have taken better care not to create this destruction through his fire.