Related for Chullin 267:39

An ear of grain that is among the standing grain, behold, it belongs to the homeowner. [An ear of grain] that is among the harvested [grain], behold, it belongs to the poor. [If] half is among the standing grain, and half is among the harvested [grain], he takes it and throws it behind him, because [in a case of] doubt [as to what is considered] gleanings, [the doubt is resolved in favor of its status being] gleanings (see also Peah 6:4 re: forgotten sheaves).