Radak: Eat it like a cake of pure barley. The prefix Kaf is omitted, like "Lev Soma'ech Yeitiv Gehah" (Mishlei 17:22; it means k'Gehah). The mixture of all these species is harsher than pure barley. It does not say like a wheat cake, for it is unreasonable that one could eat the mixture with appetite like a cake of pure wheat.

Malbim: After the days of the siege end, he will eat a barley cake, which is animal food, to hint to the days of Galus which will be after the siege. Their food will be animal food, just like they lost their Jewish form and regressed to be like animals, which do not speak.

Eruvin 81a #1: We read "Se'orim" like 'Shi'urim.' (Eat it via measure, i.e. sparingly.)

Rashi, from Eruvin 81a #2: The loaves should be shaped in a way (Radak - it should be kneaded in a trough ¹ ) appropriate for barley ² , and not [as nice as] for wheat.

Rashi: You will bake it on coals of human excrement that was dried and burned.

Radak: "V'Hi" supports our Perush that it is like a barley cake. Without "v'Hi", I would say that "Ugas Se'orim" is the mixture; it is called so because it was mostly barley. Dry excrement is called Galal, because it is Misgalgel (rolls).

Radak #1: It is feminine plural (women will bake it).

Radak #2: The Nun Hei at the end is a pronoun. This is like Ta'ug Osah. Yonason translates Tachararinah (roast it on coals); this is why a cake is called Chararah. You will make it a cake via baking it.

Radak: He commanded to bake it on something Tamei ¹ and repulsive, for a sign that Bnei Yisrael will eat their food Tamei among Goyim.

Malbim: This shows that even though their food is animal food, their excrement and Tum'ah is human - it is Tamei and putrefies ² . This is because they sinned in their form - the Nefesh that speaks, They were Metamei it.

Radak: Do so in front of Bnei Yisrael, so they will see that it is a sign for them.