[I might think that two tithes obtain in that year, (the first-tithe and the second-tithe); it is, therefore, written (Ibid. 26:12) "the year of the tithe": one tithe (of the two obtains), and not two.] But I would not know which tithe is replaced by the poor-tithe — first-tithe or second-tithe? It is, therefore, written (Ibid. 14:29) "And the Levite shall come, for he has no portion and inheritance with you" — The Levite comes and takes his portion (first-tithe) in any event, (and there is no second-tithe). These are the words of R. Yehudah.
Which Tithe Gets Replaced in Poor-Tithe Years
Sifrei Devarim 109:10