Pe'ah, Chapter Two, Mishnah 1. The Toras Kohanim, the halachic midrash on the book of Vayikra, derives from the verse "lo sechaleh pe'as sadcha" - that one may not finish off the corner of his field - that the obligation of pe'ah applies to each and every field on its own. Every individual field requires its own pe'ah. This raises the question our Mishnah addresses: what is it that defines a field as a separate unit?
"Eilu mafsikin l'pe'ah":
The following items create a break and a division between one field and another with regard to pe'ah, splitting them into two separate fields:
"HaNachal" - a river. It need not be a river that flows constantly; even a stream or wadi, which sometimes has water in it and sometimes does not, is considered a nachal, since its nature is to carry water.
"Ashlolis" - an irrigation channel that carries water from one field to another.
"V'derech hayachid" - a private road, four amos wide.
"V'derech harabim" - a public road, sixteen amos wide.
"U'shvil harabim u'shvil hayachid hakavua bimos hachamah u'vimos hageshamim" - these are not roads used by travelers, but merely access paths leading to the fields: a public path serves many people to reach their fields, and a private path serves just one person to reach his field. The condition is that both must be permanent - in the summer season and in the rainy season alike. If they are used all year round, they are fit to be considered a break between the fields.
"V'habor" - a field that is not worked and not sown, lying between two sown fields.
"V'hanir" - a plowed field that was not sown.
"V'zera acher" - a different crop found between two fields. For example, barley growing in the middle of a wheat field - the two portions of the wheat field are considered two separate fields.
"V'hakotzer l'shachas mafsik, divrei Rabbi Meir":
In Rabbi Meir's view, when a person harvests his grain for fodder - animal feed, which is generally cut early, before the grain is fit for human consumption - this early harvesting counts as a break between the two sides that were not harvested.
"VaChachamim omrim: eino mafsik ela im kein charash":
The Sages disagree and hold that harvesting for animal fodder does not create a break and does not separate the two fields, because they view it as the beginning of the final harvest that will eventually take in the entire area. Therefore there is no separate entity here - unless he plowed that area. Once he has harvested for fodder and also plowed, he has demonstrated that this really is a completely separate area, and the two fields on either side are then considered separate fields.
In summary: The Mishnah teaches that the obligation of pe'ah applies to each and every field, and it lists the items that create a break between one field and another: a river, an irrigation channel, a private road and a public road, a public path and a private path that are permanent in the summer season and in the rainy season, an unworked field, a plowed field, and a different crop. Regarding harvesting for fodder they disagree: according to Rabbi Meir it creates a break, and according to the Sages it does not create a break unless he plowed.