Why does it mention the mountains?
Rashi: The grain fields are there.
Malbim: The entire land will be thorns, but the tall mountains, where it is impossible to plow with plowing tools, only with digging tools, the thorns will not 'rule' there.
What is "Mader"?
Rashi: It is a shovel.
Why will there be no fear of thorns?
Rashi: They will engage in them to plant grain to eat, for one cannot manage without grain, but the vines will be abandoned, for the generation of Chizkiyah will return to Me to engage in Torah, and not to drink wine! It says (Sanhedrin 94b) that they checked from Dan to Be'er Sheva 1 , and did not find a man who was not expert in the laws of Isur v'Heter, and Tum'ah and Taharah 2 . About that generation, it says "Asher Hetiku Anshei Chizkiyah Melech Yehudah" (Mishlei 25:1).
The Gemara implies that this was when Sancheriv threatened to conquer Yerushalayim (after he exiled the 10 tribes), and Chizkiyah threatened to kill one who will not learn. Surely this was only within his reign (Yehudah and Binyamin); 'from Dan to Be'er Sheva' is not precise; it means, in all of Yisrael (from the most northern place to the most southern, like it says in Shoftim, Shmuel and Melachim - PF).
Our text says that they did not find an Am ha'Aretz there, and in all of Yehudah , every boy and girl was fluent in the laws of Tum'ah and Taharah. (PF)
What do we learn from "v'Hayah l'Mishlach Shor"?
Rashi: Their animals will graze there; it is a fat (fertile) pasture.
Radak: They will let animals graze there. They will not be concerned [lest they eat grain] due to the abundance of grain.. This is like "Meshalchei Regel ha'Shor veha'Chamor" (32:20).
Malbim: All the good places [for farming] will be desolate. The inferior places will be settled for cattle and flock.
