What is the meaning of "Sefi'ach Ketzircha"?
Rashi: 'Sefichim' are crops that grow from seeds that fell during the harvest - that one did not plant 1 - which are included in the Isur.
What are the implications of "Lo Siktzor?
Rashi: One may not harvest it in the way that an owner does, but declare it Hefker.
Ramban: R. Akiva forbids Sefichim mid'Oraisa. Rabanan forbid when there is concern lest people sow them and say that they grew by themselves. (See DH 've'Achshav' and 'ul'Kach' and subsequent Diburim; he elaborates at length.)
Moshav Zekenim (citing Rashi Pesachim 51b): R. Akiva forbids Sefichim mid'Oraisa after Bi'ur (when they are no longer available in the field); Rabanan forbid mid'Rabanan. 1
Moshav Zekenim (citing Ri): R. Akiva forbids Sefichim before Bi'ur; Rabanan permit then. All forbid after Bi'ur.
Moshav Zekenim: If so, how do Rabanan explain the Torah Isur of harvesting Sefichim?
What are "Nezirecha"?
Rashi: They are vines that one separated people from them; he did not make them Hefker.
Ramban: They are vines from which one separated oneself , as if one did not own them 1 - by not working, digging or pruning them. 2 Targum Onkelos calls them abandoned (to worms and desolation, or thorns will grow there), It is like a Nazir's vineyard; he abandons it, for he may not eat anything from the vine.
What are the implications of "v'Es Invei Nezirecha Lo Sivtzor"?
Rashi: One may not harvest the grapes that one separated people from them, 1 only from those that are Hefker.
Ramban: One must declare them Hefker; one has no more rights to them than the poor and animals.
Why does the Torah discuss first Sefichim, which are after Pesach, before Nezirecha (vine branches that one cuts off in Tishrei)? Shemitah begins in Tishrei!