Rashi on Deuteronomy 14:22/רש"י על דברים י״ד:כ״ב

1THOU SHALT TRULY TITHE [ALL THE INCREASE OF THY SEED] — What has this matter to do with that? (Why are they placed in juxtaposition)? But the Holy One, blessed be He, says in effect to Israel: “Do not compel Me to blast by heat the tender kernels of the grain, whilst they are yet in their mother’s womb (i.e. in the husks), for if you do not tithe your products as is proper, when they are near ripening I shall bring forth the east wind and it will blast them”, as it is said, (2 Kings 19:26) “[Therefore … they were] as the corn blasted before it be grown up” (Midrash Tanchuma, Re'eh 17). A similar reason may be given for בכורים (for its juxtaposition to לא תבשל גדי (Exodus 34:26).

2שנה שנה [THOU SHALT TRULY GIVE TITHE] YEAR BY YEAR — From here we may derive that one must not give tithe from the new grain for the old (Sifrei Devarim 105:1).