Gemara

(Bei Rav citing Rav): The Halachah follows R. Yosi regarding Androginus...

(Shmuel): The Halachah follows R. Yosi regarding Kidush (forbidding through Kil'ayim):

(Mishnah - R. Meir): If one draped his vine over his neighbor's grain, he forbids it and must pay;

R. Shimon and R. Shimon say, a person cannot forbid what is not his.

Question: How does Rav rule regarding Kidush?

Answer (Rav Yosef): Rav Huna said in the name of Rav that the Halachah does not follow R. Yosi.

Objection (Abaye): Why rely on that? Rav Ada said that Rav rules like R. Yosi!

Answer: Our question was according to Bei Rav, which refers to Rav Huna!

Bava Kama 100a (Question): Where do we find that R. Meir obligates for Garmi?

Answer #1 (Mishnah): If Reuven draped his vines over Shimon's grain, this forbids the grain, and Reuven is liable.

Rejection: This is not Garmi. He damaged through his action!

Answer #2 (Beraisa): The wall between Reuven's vineyard and Shimon's grain field was breached, and Shimon told him to fix it; it was breached, and he told him to fix it; Reuven decided not to fix it. This forbids the grain, and Reuven is liable.

Menachos 15a: Todah joins (with bread) to make (something else, i.e. the bread) Pigul, yet it does not become Pigul. Bread does not (join Todah to) make (the Todah) Pigul, all the more so it does not become Pigul!

Question (Beraisa): A case occurred, Reuven planted (his) seeds (of other species) in Shimon's vineyard. Chachamim forbade what grew from the seeds, and permitted the vines.

We do not make a Kal va'Chomer (like above) to say that since vines forbid but do not become forbidden, all the more so seeds, which do not forbid, do not become forbidden!

Answer: Normally we make such a Kal va'Chomer, but here is different. The Torah forbids Kilai ha'Kerem only with hemp and Luf (certain legumes). Chachamim fined to punish (only) Reuven, for he transgressed.

Rishonim

Rambam (Hilchos Kil'ayim 5:8): One who plants or is Mekayem Kil'ayim, e.g. he sees Kil'ayim sprouting in his vineyard and leaves it, forbids. One cannot forbid what is not his. Therefore, if one draped his vines over another's grain, he forbids his vines but not the grain. If he draped another's vines over his own grain, he forbids his grain but not the vines. If he draped Reuven's vines over Reuven's grain, he does not forbid either of them. Therefore, one who sows his vineyard in Shevi'is does not forbid (since it is Hefker).

Kesef Mishneh: Even though the Halachah follows Rav in Isurim, since Rav's opinion is unclear and Shmuel clearly rules like R. Yosi, we follow Shmuel. Also, the Yerushalmi connotes that Rav holds like R. Yosi. Also, R. Akiva holds like him, and ruled like him (about one who seeded his field in Shevi'is) in practice.

Radvaz: Perush ha'Mishnayos says that the Halachah does not follow R. Shimon or R. Akiva! It seems that at first the Rambam ruled like Rav against Shmuel, then retracted. (Note: Some versions of Perush ha'Mishnayos are after the retraction.) The verse supports R. Yosi ("Karmecha"); the Yerushalmi says that R. Meir expounds "Kil'ayim" to include another's field, but this is a poor defense. The Halachah follows Rav against Shmuel only in their own arguments, not when they argue about whom we follow.

Ri Korkus: In other cases one can forbid another's property, e.g. meat cooked with milk, Sha'atnez and bestiality (the animal is Pasul for a Korban, and is stoned if there were witnesses). Kil'ai ha'Kerem is different, for it depends on the consent of the owner. If one intends to uproot Kil'ayim as soon as he reaches it, it does not forbid. This is because the verse discusses seeding, which is willful. However, we find that one can disqualify another's Mei Chatas or Parah Adumah, even though it depends on consent. Rather, we must say that Kil'ai ha'Kerem forbids only "Karmecha", not another's vineyard. Therefore, one cannot forbid Reuven's property even if he seeds in Reuven's vineyard. The Mishnah discusses draping vines to teach the extremity of R. Meir, who forbids even in this case.

Tosfos (Yevamos 83a DH R. Yosi Omer): The Gemara (Menachos 15a) says that the Beraisa discusses Kil'ayim mid'Rabanan, but Kil'ayim mid'Oraisa forbids another's field. This is unlike R. Yosi. We do not explain the Beraisa like R. Yosi, who permits the vines because one cannot forbid another's property. Since R. Yosi permits the vines, he would also permit the Zera'im.

Rambam (3:16): One must make a separation between different crops in his own field. One may plant wheat next to his neighbor's barley. It says "Sadcha Lo Sizra Kil'ayim", not 'Ha'Aretz Lo Sizra Kil'ayim'. In this case, even if one planted barley at the end of his (wheat) field next to his neighbor's barley it is permitted, for it looks like the barley is the end of his neighbor's field.

Rebuttal (Ra'avad): Only R. Shimon, who says that one cannot forbid another's property, permits this. He says that likewise, one cannot Chovesh another (forbid him to grow something).

Defense (Ri Korkus): Indeed, the Rambam rules that one cannot forbid another's property!

Rejection (Avnei Nezer YD 388): One may plant 'Kil'ayim' (i.e. a different crop) next to his neighbor only if (in a corresponding case of Kil'ai ha'Kerem,) his own would not be forbidden. The Ra'avad holds that R. Shimon holds that since one cannot forbid another's property, also his own is permitted. The Rambam (5:8) rules like 'Chachamim' that his own property becomes forbidden, so he should forbid 'Kil'ayim' next to a neighbor! ('Chachamim' includes R. Yosi, who argues with R. Shimon about this in Kil'ayim 2:7.)

Rosh (Yevamah 8:8): Since Rav argues with Shmuel, the Halachah follows Rav in Isurim. We cannot bring a proof from the Pesak in Bava Kama like R. Meir who is Mechayev for Garmi, for R. Yosi agrees that if one was Mekadesh he is liable to pay. The Gemara merely sought a source that R. Meir is Mechayev for Garmi. Regarding Kil'ayim, the Halachah could follow R. Yosi.

Question (Gra YD 296:14): Elsewhere, the Rosh (Mo'ed Katan 1:13) rules like Shmuel because the Yerushalmi is like the version (of the Bavli) in which Rav does not argue with Shmuel!

Poskim

Shulchan Aruch (YD 296:4): One who plants or is Mekayem Kil'ayim, e.g. he sees Kil'ayim sprouting in his vineyard and leaves it, forbids. One cannot forbidden what is not his. Therefore, if one draped his vines over another's grain, he forbids his vines but not the grain. If he draped another's vines over his own grain, he forbids his grain but not the vines. If he draped Reuven's vines over Reuven's grain, he does not forbid either of them. Therefore, one who sows his vineyard in Shevi'is does not forbid.