The Gemara in Rosh Hashanah on 11a says:
ר' יהושע אומר מנין שבניסן נברא העולם שנא' (בראשית א, יב) ותוצא הארץ דשא עשב מזריע זרע ועץ עושה פרי איזהו חדש שהארץ מליאה דשאים ואילן מוציא פירות הוי אומר זה ניסן ואותו הפרק זמן בהמה וחיה ועוף שמזדווגין זה אצל זה שנאמר (תהלים סה, יד) לבשו כרים הצאן וגו'
Rabbi Yehoshua says: From where is it derived that the world was created in the month of Nisan? As it is stated: “And the earth brought forth grass, herb yielding seed after its kind, and tree yielding fruit” (Genesis 1:12). Which is the month in which the earth is full of grass and the trees begin to bring forth fruit? You must say that this is Nissan. And further proof that the world was created in Nissan is that when the world was first created, the animals had to breed in order to fill the world, and the period beginning with Nisan is a time when cattle, and beasts, and birds mate with one another, as it is stated: “The flocks are clothed in the meadows, and the valleys are wrapped in grain; they shout for joy, they also sing” (Psalms 65:14).
Yet earlier in Rosh Hashanah (8a) the Gemara establishes that this text demonstrates that animals primarily mate in Adar:
אלא אמר רבא דכ"ע לבשו כרים הצאן בזמן שעמקים יעטפו בר באדר
Rather, Rava said that the dispute must be explained as follows: Everyone is of the opinion that “the flocks are clothed in the meadows,” i.e., the sheep conceive, primarily at the time when “the valleys are wrapped in grain,” in the month of Adar.
It's important to note that the Gemara on 8a seems to assume a significant difference between Adar and Nissan. We see this from a machloket there about whether R"H for ma'aser beheima is in Elul or Tishrei. This is based on an argument from this passuk about whether or not the animals primarily mate in Adar or Nissan. Thus it appears that Adar and Nissan are different enough in terms of when the mating period is.
Therefore, my question is how can our Gemara use this passuk as a proof for R' Yehoshua that creation happened in Nissan?