Rosh Hashanah (8a) #1: Tzon become pregnant when "va'Amakim Ya'atfu Var" (the planted produce sprouts and is recognized), i.e. in Adar. They give birth five months later, in Av, so Rosh Hashanah for Ma'aser Behemah is [Rosh Chodesh] Elul.

Rosh Hashanah (8a) #2: Tzon become pregnant when "Yisro'a'u Af Yashiru", when sheaves 'sing' - in Nisan (the harvest time; grain is in its straw, and the wind blows and the sheaves knock each other). They give birth in Elul, so Rosh Hashanah is [Rosh Chodesh] Tishrei.

Rashi: The Sharon and Aravah (regions in Eretz Yisrael) are 'clothed' with flocks that come to graze on the grass that the rain caused to sprout.

Radak: This is like "Kar Nirchav" (Yeshayah 30:23), i.e. a plain. It mentions plains and valleys because it already mentioned hills. The plains are full of Tzon; it is as if they are clothed with flocks. They graze and cover the ground. Due to the great produce, they send the flocks to the fields to graze - "Meshalchei Regel ha'Shor veha'Chamor" (ibid., 32:20).

Malbim: This corresponds to "Yir'afu Ne'os Midbar" (verse 13), (in Hefker places].

Refer to 65:14:1:1.

Rashi: Via rain, the valleys are wrapped with grain, and people blow Teru'ah of Simchah and sing, due to the great Brachah.

Radak: Also the valleys are wrapped and covered due to the abundance of grain.

Malbim: This corresponds to "v'Gil Geva'os Tachgornah", in settled places.

Radak: It mentions them to teach that the great rain will not harm them.

Refer to 65:14:1:2.

Radak #1: The wind blows and the sheaves knock each other; they make noise. It is as if they shout and sing. This is a metaphor, like "v'Gil Geva'os Tachgornah" (verse 13), "v'Chol Etz ha'Sadeh Yimcha'u Chaf" (Yeshayah 55:12).

Radak #2: All people will sing with the animals.

Malbim: All this is the song for the rain.