Who are the many shepherds?
Rashi: They are the head troops of Nebuchadnetzar's army - "Eleha Yavo'u Ro'im v'Edreihem" (6:3).
Radak, Malbim, Targum Yonasan: They are kings who came with Nebuchadnetzar. A king is like a shepherd of flock.
What does "My vineyard" allude to?
Radak: It is Yisrael - "Kerem Hashem Tzevakos Beis Yisrael" (Yeshayah 5:7).
What is the meaning of "Bosesu"?
Rashi: It is an expression of trampling, like "v'Hayu ch'Giborim Bosim b'Tit Chutzos" (Zecharyah 10:5).
What is the meaning of "Chelkas"?
Rashi: It is a field. Malbim - after they destroy the vines, and the vineyard became like a field without vines, they trampled the field and destroyed the ground.
How did they make it like a Midbar?
Malbim: This is like "Gafris va'Melach Sereifah Chol Artzah Lo Sizara v'Lo Satzmi'ach" (Devarim 29:22). There are three expressions of destruction, corresponding to the three times that Nebuchadnetzar came against Yehudah.