The Gemara searches for a source for the rule that limits the rights of workers regarding this law, and Rav Ahadevoi bar Ami points to the verse “Thou shalt not muzzle the ox when he treadeth out the corn” (Devarim 25:4), which he suggests implies permission only for mundane work and not for work performed on behalf of the Temple. Rashi explains this teaching as based simply on the context of the passage, which is talking about ordinary threshing. In his Commentary to the Mishnah, the Rambam suggests that it is based on the word disho – which emphasizes that it is his own threshing – implying that the permission does not apply in threshing done on behalf of the Temple..
Generally speaking, workers – and even animals – that are employed in harvest are permitted to partake of the produce with which they are working. These biblical laws are based on the passages in Sefer Devarim (see 23:25 and 25:4). Notwithstanding this rule, the Mishnah on today’s daf (=page) teaches that workers in the Temple cannot eat figs that belong to the Temple, nor can an animal working in fields belonging to the Temple be allowed to eat karshinin that they are threshing.
Karshinim are a type of legume – vicia ervilia – more commonly known as Bitter Vetch. Like most legumes, karshinim have a high nutritional value and can be digested easily by animals. It grows in the winter and spring in lands around the Mediterranean Sea, and in Arab villages it is still grown as animal feed. Although its bitter taste makes it appropriate only for animals, in times of famine or for people who are in dire financial straights it would be prepared for human consumption, as well. Since it can be eaten by people, the obligation of terumot u’ma’asrot – of tithes – applies to it. Still it was usually fed to animals belonging to a kohen.