What is "Eglah Melumadah"?
Rashi: It has many wounds from the goad of the plowshare. I.e. I brought many afflictions against it, and it was not humbled.
Radak: Above (4:16), Efrayim was compared to "Parah Sorerah Sarar Yisrael", and now the verse calls them a calf. In its youth, Hashem taught it to plow.
Malbim: Efrayim does not have strength like an ox to [bear] a yoke and plow land. They are like a calf used to light work.
What is "Ohavti Ladush"?
Rashi: It loves to thresh grain, where there is food. It was not humbled to plow. I.e. they were not humbled to Torah. They followed the freedom of their hearts. The Yud at the end of "Ohavti" is extra 1 , like "Genuvesi Yom" (Bereishis 31:39).
Radak: Plowing with seeding is doing deeds; harvesting is the fruits of the deeds. I taught it to do good deeds in order to eat well. It refused to plow (do good deeds). Rather, it wanted to eat and enjoy the good that I give to it, without good deeds, rather, for free. "Ohavti Ladush" - to thresh and not plow, so it will eat at the time of threshing. It does not do what its owner wants, only for its desire and pleasure!
Malbim: It loves threshing, which is [light] work proper for a calf. It dances on the grain and threshes it, and also eats at the time. Plowing is not work for a tender calf. It loves to bear a light servitude. I.e. [Efrayim] wanted to serve Melech Egypt - it is light work and not a heavy yoke, and with this they will eat from the good of Eretz Egypt and profit from trade there. They did not want to bear the yoke of Melech Ashur, to plow and do all strenuous work.
Radak: This is because the word is mi'L'eil (the penultimate syllable is accented. I.e. if it were mil'Ra (the last syllable is accented), it would be like "Ahavasi" (Tehilim 109:4). However, "Ahavti" (Bereishis 27:4) is mi'L'eil!) (PF)
What is the significance of passing over her good neck?
Rashi: Therefore I bring against them kings who will weaken their strength.
Radak #1: When I passed over it to put a yoke on its neck, since its neck was thick, I did not put a heavy yoke on it. Cows that they put a heavy yoke on their necks, their necks are thin. I.e. Hashem did not put a heavy yoke of Mitzvos on them.
Radak #2: When I went to apportion the yoke of kingship, i.e. when I spoke to Achiyah [ha'Shiloni] ha'Navi about her good neck, I let all Yisrael be serene, everyone under his vine and fig trees, all the days of Shlomo. After his death, I said to divide the kingship.
Malbim: I still had compassion on them. I saw their good, fat neck - they are pampered, and cannot bear a heavy yoke. I did not intend to hand them over to the nations, who will tie them in ropes to make the plow two furrows at once. I thought that they will enter their neck [in My yoke] to do My Avodah, which is easy, and not be handed over to a harsh master. I thought that [Efrayim] will join with Yehudah, and both of them will serve Hashem and plow their land Admas Kodesh.
Why does it say "Arkiv Efrayim Yacharosh Yehudah"?
Rashi: If you want that I will put Efrayim on the Nochrim 1 , Yehudah should plow for himself a plowing of good deeds - "Niru Lachem Nir" (12).
Radak: I will graft on Efrayim the yoke of Yaravam's kingship; he was from Efrayim. I said that he should do My will, and command the nation to serve Me and be under My authority - "v'Hayah Im Tishama Es Kol Asher Atzavecha v'Halachta vi'Drachai... u'Vanisi Lecha Vayis Ne'eman" (Melachim I, 11:38). Regarding Yehudah, Hashem already put a yoke on them. I [request] only that they plow like My desire. The king was already from Yehudah, and it remained so after the kingship was divided.
Malbim: I will join and connect Efrayim to Yehudah, that both of them will plow and work Hashem's field.
I.e. Malchus Efrayim will confront other nations, and not Yehudah. (PF)
What is the meaning of "Yeshaded Lo Yakov"?
Radak: Meshaded is crushing clumps of earth after plowing, to level the land and prepare it for plowing. The same applies to "Im Yeshaded Amakim Acharecha" (Iyov 39:10). If Melech Efrayim and Melech Yehudah would plow like Hashem wanted (warn the nation), the nation would Yeshaded (fix their ears and heart to the words of warning). They would accept on themselves to do the Mitzvos - this is seeding. Had Yaravam and Rechav'am warned the nation to do what is straight in Hashem's eyes, Yakov (Klal Yisrael] would have been Yeshaded (prepare their hearts) to accept the Mitzvos.
Malbim: I thought that Yehudah, who are used to plowing in Avodas Hashem, will plow (the hard work), and Yakov, who are not used to plowing, will be Yeshaded (crush clumps of earth after plowing), which is light work. I.e. Yehudah will prepare the Nefesh until it is prepared to accept seeding of Tzedakah and good deeds. Afterwards, also Yakov (the 10 tribes) will finish the plowing and preparation.