(Mishnah): If one's field was sown with wheat (that had not yet grown) and he decided to sow with barley, he should wait until the wheat grows narrow roots, uproot them and then sow the barley. If they had grown, he shouldn't say, "I'll sow (the barley) and then uproot (the wheat)''; rather first uproot and then sow.

Question: How much should he plow (to uproot them)?

Answer: Like the (width of the) furrows plowed after the first rains.

(Abba Shaul): So that there shouldn't remain enough wheat to produce a quarter Kav in a Beis Seah of barley.

(Gemara) Question: How long must the wheat remain in the ground to have grown these narrow roots?

Answer: If the ground is moist, three days; but if it is dry, it needs longer.

It's like the Baraisa - Part of a day is like a full day in a moist place (meaning that the 3rd day can be a partial day). And if the wheat grew and the owner took his animal there and it ate all that grew there, he may sow there other produce.

What's the reasoning of the Rabbanan (that he may plow furrows without plowing the entire field)? Once he showed that he isn't interested in what remains, it's enough.

Baraisa: He only needs to plow like the (width of the) furrows plowed after the first rains.

(R. Shimon ben Gamliel): The furrows of the plow were known as a horse's tail as when they finished one row, instead of lifting the plow they curved around to begin the next one, like this...

Question: What is Abba Shaul's reasoning (for saying that there shouldn't remain enough wheat to produce a quarter Kav in a Beis Seah)?

Answer: It suffices as long as he has visibly begun to reduce it.

Question: Does Abba Shaul follow the view of R. Yosi (who said in the first Mishnah that he should remove all of the quarter Kav, whether it contains one or two species)?

Answer: He actually follows the Rabbanan there, as according to R. Yosi, the entire quarter must be removed. But here, the Tannaim disagree as to whether to apply the opinion of the Rabbanan of that Mishnah. The Rabbanan here say that it's enough to plow like the (width of the) furrows plowed after the first rains, even though in that case he must specifically reduce it to less than 1/24.

(Abba Shaul): Just as over there he must reduce it to less than 1/24th, so too here.

(R. Yosi): He must remove the entire quarter.