Or HaChaim on Deuteronomy 11:17/אור החיים על דברים י״א:י״ז

1 לא תתן את יבולה, "it will not yield its produce." Although Moses had already said that G'd would lock the heavens he repeated that the earth would not yield its produce to tell the people that even if they were to plant their fields adjacent to rivers and wells the earth would not produce a harvest.

2ואבדתם מהוה, "and you will perish quickly, etc." Even if you somehow find a harvest dating back to previous years you will still perish. We have an illustration of this curse in Gittin 56 where the Talmud tells how Jewish zealots deliberately destroyed a 21 year supply of grain owned by a certain Kalba Savua during the siege of Jerusalem by the Romans.