Moed Katan 6B

רובע זרע ממין אחר ⁠— ימעט.

1/4th-kav or more of seeds of a different type, i.e., 1/24th of the mixture is a type of seed other than the main type, one must reduce the other type of seeds in the mixture by uprooting the shoots.

והתניא: התקינו שיהו מפקירין כל השדה כולה! לא קשיא: כאן קודם תקנה, כאן לאחר תקנה.

With regard to the halakha that inspectors must go out and uproot the shoots of diverse kinds of seeds that grew in the fields, the Talmud asks: But isn’t it taught in a baraita that the rabbis ordained that they should pronounce the crop of the entire field in which diverse kinds was found ownerless, rather than uprooting the diverse kinds? The Talmud responds: This is not difficult. Here, in the Mishnah, where it says that the inspectors go out and uproot the diverse kinds, it is referring to the time before the institution of the new ordinance; there, in the baraita, where it says that the entire field is pronounced ownerless, it is referring to the time after the institution of that ordinance.

דתניא: בראשונה היו עוקרין ומשליכין לפני בהמתן, והיו בעלי בתים שמחין שתי שמחות: אחת שמנכשין להם שדותיהן, ואחת שמשליכין לפני בהמתם,

The Talmud explains this ordinance as it is taught in another baraita: At first, the agents of the court would uproot the diverse kinds and cast them before the livestock belonging to the owners of the fields. However, the property holders would rejoice for two reasons: One, that the agents of the court weeded their fields for them when they uprooted the plants of the other type; and another one, that they cast the diverse kinds before their livestock, thereby saving them from having to feed them. Accordingly, the field owners took no steps to keep their fields free of diverse kinds of seeds.