What is the full scope of the La'av of "Lo Sacharosh be'Shor va'Chamor ... "?
Rashi and Targum Yonasan: It incorporates plowing with (or leading - Rashi) an ox and a donkey or any two species of animals 1 that are harnessed together to pull any load.
Which we learn via a Gezeirah Shavah "Shor" "Shor" from Shabbos. See Sifsei Chachamim.
Is the Isur of Kl'ai Beheimah confined to an ox and a donkey?
Bava Kama, 54b and Targum Yonasan: Based on a Gezeirah Shavah "Shor" "Shor" - in Va'eschanan, 5:14, in connection with Shabbos - it incorporates any two species of animal. 1
See Torah Temimah, note 103, who elaborates.
Seeing as the Isur applies to all species, why does the Torah mention specifically "Shor va'Chamor"?
Bava Kama 54b: Because they are the two most common animals with which one is likely to plow. 1
Yerushalmi Kil'ayim, 8:4: To preclude fom the prohibition plowing with an ox or a donkey together with a man. 2
Refer to 22:10:2:2.
Why does the Torah forbid plowing with two species of animals?
Ramban and Moshav Zekenim (in Vayikra, 19:19): It is actually an extension of the La'av of "Behemt'cha Lo Sarbi'a Kil'ayim", since farmers tend to place the two animals together in the stable and mate them. 1
Hadar Zekenim, Rosh and Moshav Zekenim: Because since the ox chews its cud, it appears to always be always eating, and when the donkey sees (hears ? Da'as Zekeinim) this, it is pained. 2
See also Ba'al ha'Turim. See Oznayim la'Torah as to why the te Torah does not forbid placing them together in the stable. In fact, according to the current reason, if it did, it would no longer be necessary to forbid plowing with them together.
This implies like the Rambam in Peirush ha'Mishnayos, Kil'ayim, 8:2, that the Torah forbids only Tahor and Tamei. (Since it depends on chewing the cud, the camel will be considered a Tahor animal in this regard ? PF) However, the Rosh (in Hilchos Kil'ayim 5) rejects this explanation, and proves that the Torah forbids any two different species of animals.
Why does the Torah insert the (otherwise superfluous) word " ? be'Shor u'va'Chamor Yachdav"?
Sifri: To incorporate threshing with two different animals, and being led by or leading two different animals - that are tied together to the same yoke. 1
See Torah Temoimah, note 101.
Why does the Torah mention specifically "Shor va'Chamor"?
Bava Kama 54b: Because they are the most two most common animals with which one was likely to plow. 1
Yerushalmi Kil'ayim, 8:4: To preclude plowing with an ox or with a donkey and a man fom the prohibition. 2
According to R. Yoshiyah, who says in Sanhedrin, 66a that a 'Vav' is inclusive, why does the Torah insert the word "Yachdav"?
Moshav Zekenim (citng Yerushalmi Kil'ayim, 8:4): To confine the Isur to where one animal carries (more) due to the other's laziness. 1
Sifri: To include) in the Isur of Kil'ayim where one ties a horse to the side of the wagon (that is being drawn by an ox) or behind it. 2
Alei Tamar (Kil'ayim 8:3): Where the stronger species pulls harder to compensate for the weaker, and presses it to go faster. The Yerushalmi applies this to a horse tied in back of a wagon led by an ox. If the horse will help push when the ox slackens, it is forbidden.
See Torah Temimah, note 106.
