Our Gemara quotes a baraita that teaches that if a person makes his vineyard hefker and the next morning he goes and harvests it, he will be obligated to leave the tithes that go to the poor (peret, olelot, shik’hah and pe’a) but is free from the obligation of ma’aser.
Generally speaking, only a farmer who harvests his own field is obligated in all of the tithes that are commanded in the Torah. A field that is truly hefker – it has no owner – will not need to have tithes taken from it by the person who comes to harvest it. In our case, however, it is clear that this vineyard was not truly hefker, which is why the Sages distinguished between some of the tithes – those distributed to the poor – that still need to be brought and others that are no longer obligatory.
Most of the commentaries explain the difference between tithes based on the Gemara (Bava Kamma 94a) that points to the repeated use of the word ta’azov – “leave behind” – specifically in those tithes that are to be left for the poor. The Gemara concludes from here that whenever there is some possibility that these tithes should be distributed, we are obligated to do so. The Rambam offers a different source for this ruling. According to him, we find the word sadkhah – “your field” – only in reference to tithes for the poor. From this he concludes that whenever someone is harvesting his field – even if it was taken from hefker – he is still obligated in these tithes. With regard to the tithes for the kohanim and the Levites, since the term sadkhah does not appear, the exemption of hefker will apply.