The first of his strength, and how Ibn Ezra becomes a darshan

A father must acknowledge his firstborn son of his two wives, as we are told in parshat Ki Teitzei:

יז כִּי אֶת-הַבְּכֹר בֶּן-הַשְּׂנוּאָה יַכִּיר, לָתֶת לוֹ פִּי שְׁנַיִם, בְּכֹל אֲשֶׁר-יִמָּצֵא, לוֹ: כִּי-הוּא רֵאשִׁית אֹנוֹ, לוֹ מִשְׁפַּט הַבְּכֹרָה.

17 but he shall acknowledge the first-born, the son of the hated, by giving him a double portion of all that he hath; for he is the first-fruits of his strength, the right of the first-born is his. {S}

I would assert that on a peshat level, reishit ono is idiomatic. It means bechor, but specifically the first child that he has had. And bechor is not sufficient, for both of them are bechor, just from different wives. This idiom shifts the focus to the father's output.

But Ibn Ezra has a suprisingly midrashic comment on this pasuk:

כי הוא ראשית אונו . הנודע עם הישר ידבר וכל ישראל בחזקת ישרים

As Yahel Or (=Mechokekei Yehuda) and Avi Ezer write, this is just as we saw regarding Yaakov Avinu in Vaychi, in describing Reuven as reshit oni, in accordance with Rashi's explanation over there:

3. Reuben, you are my firstborn, my strength and the first of my might. [You should have been] superior in rank and superior in power.		ג. רְאוּבֵן בְּכֹרִי אַתָּה כֹּחִי וְרֵאשִׁית אוֹנִי יֶתֶר שְׂאֵת וְיֶתֶר עָז:
and the first of my might: That is, his first drop [of semen], for he had never experienced a nocturnal emission. — [from Yeb. 76a]		וראשית אוני: היא טפה ראשונה שלו, שלא ראה קרי מימיו:

What Ibn Ezra is thus saying, local to Ki Teitzei is that reshit ono in terms of the son of the senuah was the result of his first drop, and the audience are righteous upright people, with all of Israel being under the assumption of being upright.

This dores not strike me as peshat, however brilliant this comment is. Rather reshit ono is an idiom, which more or less parallels bechor. Indeed, I would assert that this is its role in parshat Vaychi as well -- firstly to provide a poetic repetition to bechori, and secondly to single out Reuven over three other brothers as the paternal bechor, rather than maternal bechor. The first of his strength can be so even if he experienced a seminal emission before.

Indeed, I get the sense that Ibn Ezra is in a more midrashic frame of mind in Ki Teitzei, from other statements he makes here -- perhaps because specifically in legal codes there are major arguments with Karaites. He refers several times to the divrei hamakchishim when presenting his contrary commentary in support of Chazal.

Looking to Ibn Ezra on Vaychi, we see that he specifically does not explain it as Rashi does. He thus does not endorse this as the peshat meaning of reishit on.

ראובן בכורי אתה בחי. בך נראה בתחלה כחי

(והבכור יקרא ראשית און וכמוהו ״ראשית אונים" (תה׳ עח נא

And I say this even though Mechokekei Yehuda, in his Yahel Or commentary, might try to bring this idea back in, as this "koach" is a melitza for semen.

Ibn Ezra referred to Tehillim 78:51, where it is used as a poetic parallel for first-born.

נא וַיַּךְ כָּל-בְּכוֹר בְּמִצְרָיִם; רֵאשִׁית אוֹנִים, בְּאָהֳלֵי-חָם.

51 And smote all the first-born in Egypt, the first-fruits of their strength in the tents of Ham;

Ibn Ezra has no comment there, which can help us out.