Gemara

The Zero'a is cooked with the ram. Even though the Zero'a is forbidden to Zarim, the rest of the ram is permitted to anyone. I.e., the Ta'am of the Zero'a is Batel in the ram.

98b (Rava): Regarding other Kodshim, Ta'am k'Ikar (if Isur mixed with Heter can be tasted, the Heter is forbidden.) Here, Ta'am k'Ikar of the Zero'a (absorbed in the ram) is Batel.

Zevachim 78a (Mishnah): If a dough was made with wheat flour and rice flour, if the wheat can be tasted, Chalah must be taken, even if the rice is the majority.

Suggestion: That is mid'Rabanan. Mid'Oraisa it is exempt.

Rejection (Seifa): One can be Yotzei (the Mitzvah of Matzah) with it. (This shows that mid'Oraisa, the wheat is not Batel, for it can be tasted!)

Avodah Zarah 67a (R. Avahu): If the Isur gives taste and is intact, if it has k'Zayis (the volume of an olive) of Isur bi'Chdei Achilas Pras (in the volume of a half loaf (three or four eggs)), it is forbidden, and one is lashed for eating the mixture.

Berachos 37a (Rav and Shmuel): One blesses Borei Minei Mezonos on anything from the five grains.

Rishonim

Rambam (Hilchos Berachos 3:11): Anything on which one blesses ha'Motzi beforehand, he blesses Birkas ha'Mazon afterwards. Anything on which one blesses Borei Minei Mezonos beforehand, he blesses me'Ein Shalosh (Al ha'Michyah) afterwards, except for rice.

Rambam (Hilchos Chametz u'Matzah 6:5): If made a dough from wheat and rice, if it has the taste of grain, he is Yotzei Matzah through it.

Ra'avad: This is only if it has k'Zayis of grain bi'Chdei Achilas Pras.

Ramban (Hilchos Chalah (amidst Hilchos Bechoros) p.31a): The Mishnah says that one is Yotzei Matzah with a dough of wheat and rice if it has the taste of grain. This shows that mid'Oraisa, it is like a dough of wheat, even if it has much rice. The Gemara (Zevachim 78b) explicitly says so. We hold that k'Zayis bi'Chdei Achilas Pras is mid'Oraisa when there is taste and substance, but if there is less than k'Zayis bi'Chdei Achilas Pras, it is not mid'Oraisa! How can one be Yotzei Matzah?! The Yerushalmi explains that it is due to Greirah (being dragged along). Only the five species of grain can become Chametz, and only wheat and barley are Gorer. (Bread of flour of) rice is not bread, but if it is mixed with grain, the entire mixture becomes bread. This applies only to wheat and barley, because people mix other species with them, but not to oats, spelt and rye. One opinion in the Yerushalmi says that Greirah is only with wheat and rice. If the wheat cannot be tasted, it cannot be Gorer! When it can be tasted, and can join, i.e. there is k'Zayis bi'Chdei Achilas Pras, it can be Gorer. The Halachah follows R. Shimon ben Gamliel, who exempts unless there is a Shi'ur of grain. Rav Heila always requires a majority and taste of grain. Rav Huna holds that wheat is Gorer rice even when the majority is rice. The Halachah follows him; the Bavli supports him.

Rosh (Hilchos Chalah (after Menachos) 15): What was the Ramban's difficulty? Even if there is less than k'Zayis of grain bi'Chdei Achilas Pras, it can be tasted and there is real substance. Ta'am k'Ikar is mid'Oraisa regarding Min b'Eino Mino. The taste of the rice becomes like wheat. R. Yochanan holds that one is not lashed for taste without substance, even though he holds like R. Akiva, that Ta'am k'Ikar is mid'Oraisa. R. Tam says that taste without substance is Min b'Mino. R. Chaim says that it is k'Zayis bi'Chdei Achilas Pras. We discuss when there is a k'Zayis of wheat bi'Chdei Achilas Pras of the (mixture with the) rice. Then, the rice becomes like wheat. The Yerushalmi discussed Greirah because even though we follow the taste and it is as if it is all grain, one cannot be Yotzei Matzah with something that cannot become Chametz. It answered that Greirah causes that the entire dough becomes Chametz.

Poskim

Shulchan Aruch (OC 208:9): If one mixed flour of millet or other legumes with flour of the five grains and cooked it in a pot, he blesses Borei Minei Mezonos beforehand and Al ha'Michyah afterwards. If he made bread from it, he blesses ha'Motzi and Birkas ha'Mazon. This is only if there is k'Zayis of the grain flour bi'Chdei Achilas Pras.

Gra (DH Kedei): There must be k'Zayis in Kdei Achilas Pras of the mixture, and he must eat k'Zayis in Kdei Achilas Pras of grain (in order to bless Birkas ha'Mazon afterwards).

Gra (DH bi'Tchilah): To bless ha'Motzi beforehand, we do not require k'Zayis in Kdei Achilas Pras. It suffices if it has from the five grains, as long as they are not (merely) to make it stick together.

Shulchan Aruch (ibid.): If not, for the bread he does not say Birkas ha'Mazon afterwards, but he says ha'Motzi beforehand since it has the taste of grain, even though there is not k'Zayis bi'Chdei Achilas Pras. Afterwards, he blesses Al ha'Michyah. For what was cooked it in a pot, he blesses Borei Minei Mezonos beforehand and Borei Nefashos afterwards.

Beis Yosef (DH v'Chasuv): Orchos Chayim and the Avudaraham says that if one mixed flour of millet with flour of the five grains and cooked it in a pot, he blesses Borei Minei Mezonos on it. If he made bread, he blesses ha'Motzi and Birkas ha'Mazon. This is like Rav and Shmuel, who say that one blesses Borei Minei Mezonos on anything from the five grains. R. Yonah says that this is only if one eats a k'Zayis of the grain flour bi'Chdei Achilas Pras. If not, on bread he says ha'Motzi beforehand and Al ha'Michyah afterwards. If it was cooked, he blesses Borei Minei Mezonos beforehand and Borei Nefashos afterwards.

Beis Yosef (OC 453 DH v'Im): the Ra'avad and Rashba hold that a Chalah must be taken from a dough and one can be Yotzei the Mitzvah of Matzah through it only if there is k'Zayis bi'Chdei Achilas Pras. The Magid Mishneh and Ramban hold that even if there is not k'Zayis bi'Chdei Achilas Pras, the wheat drags along the rice. This is only for rice, but not for other species. Also the Rambam and Rosh say so.

Magen Avraham (15): All agree that the laws of Chalah and ha'Motzi depend on each other. Regarding Chalah it says "Lechem". The Gemara (Berachos 37a) equates Matzah to ha'Motzi. Also the Rambam (Hilchos Chametz 6:7) says that one is Yotzei Matzah only with bread on which one blesses Birkas ha'Mazon. If so, according to the Magid Mishneh and those who agree with him, one blesses Birkas ha'Mazon on a mixture of rice and grain even if there is not k'Zayis grain bi'Chdei Achilas Pras. Regarding other species (than rice), they are not called bread regarding ha'Motzi or Birkas ha'Mazon.

Machatzis ha'Shekel: "Other species" refers to rice with grains other than wheat, or wheat with legumes other than rice, when there is not k'Zayis bi'Chdei Achilas Pras.

Magen Avraham (15): The Rambam says that whenever we bless ha'Motzi beforehand, we bless Birkas ha'Mazon afterwards. The Yerushalmi says so. This mixture is unlike the rule! We do not find anything like this in the Gemara! We must say that since one is liable for eating Isur slowly, i.e. he did not eat k'Zayis bi'Chdei Achilas Pras, the same applies to Birkas ha'Mazon. Here, he will not eat k'Zayis bi'Chdei Achilas Pras because there is not enough in the mixture. Likewise, he is not Yotzei. However, one blesses ha'Motzi on any amount.

Gra (DH uliv'Sof): The correct text of the Shulchan Aruch should say 'afterwards he blesses Borei Nefashos', like a cooked food without k'Zayis in Kdei Achilas Pras. Every Berachah Acharonah requires k'Zayis in Kdei Achilas Pras.

Mishnah Berurah (44,45): He blesses ha'Motzi beforehand even without k'Zayis in Kdei Achilas Pras because there is no Shi'ur for a Berachah Rishonah, and the grain is not Batel because one tastes it. If he does not taste it, it is Batel and he blesses sheha'Kol beforehand.

Mishnah Berurah (47): The Acharonim found the Shulchan Aruch very difficult. The Gra changed the text. One who fears Shamayim should eat such bread only inside a meal (of proper bread). However, we do not protest against one who follows the Shulchan Aruch. The same applies if the bread had k'Zayis of grain in Kdei Achilas Pras, but he ate less than a k'Zayis of grain in Kdei Achilas Pras. He says afterwards Al ha'Michyah according to the Shulchan Aruch, or Borei Nefashos according to the Gra.

Kaf ha'Chayim (54): When he did not eat a k'Zayis of grain, we descend one level. Therefore, after bread he says Al ha'Michyah, and after cake he says Borei Nefashos.

Mishnah Berurah (48): One can say Al ha'Michyah only if he ate k'Zayis of grain in Kdei Achilas Pras. This is not only when grain flour was mixed with other flour, and the grain is not so noticeable. It is also if he mixed grain flour with honey, spices or other ingredients, and there is not a k'Zayis of grain in Kdei Achilas Pras. This applies to potato Kenishes or Bourekos. However, the custom is to bless Al ha'Michyah on a k'Zayis of sweet cake, even if there is not a k'Zayis of grain. Perhaps because the spices are to improve the food, they join with the food to a Shi'ur, like the Magen Avraham said (210:1). L'Chatchilah one should ensure that there is a k'Zayis of flour.

Bi'ur Halachah (DH Eino): Sha'arei Teshuvah wrote that if one was satiated, he must bless Birkas ha'Mazon. He did not explain why. Perhaps he is concerned for the opinion that Ta'am k'Ikar is mid'Oraisa, and this applies also to Birkas ha'Mazon. Therefore, he is stringent if one ate to satiation and he is liable mid'Oraisa. However, the Gra and Mechaber connote that he never blesses on less than a k'Zayis bi'Chdei Achilas Pras.