Gemara

(Rav Masnah): If one made a roof from Zera'im (planted crops), the Zera'im (become Tahor and) is considered like a roof;

After making a roof from it, it is not considered a food.

Sukah 11a (Mishnah): If one draped vines over a Sukah, it is Pasul. If there was more Schach than vines or if he cut them, it is Kosher.

Question: If he did not do Chavatah (beat the branches down), why it is Kosher? Pasul Schach joins with Kosher Schach!

Answer: We must say that he did Chavatah.

13b (R. Aba): If one harvests grapes in order to make wine, there are no Yados (the vine twigs are not considered handles for the grapes);

(Rav Menasiya): If one harvests for the sake of Schach, there are no Yados;

R. Aba holds that if one harvests for Schach, there are Yados, for they are useful for Schach, so it will not scatter.

Suggestion: Tana'im argue about Rav Menasiya's law;

(Beraisa): If there are fig branches with figs, vine twigs with grapes, straw with ears, Machbedos (broom-shaped date branches) with dates, if there is more Pesoles (waste) than food, it is Kosher. If not, it is Pasul;

Others say, the straw must be more than the Yados and food.

Rejection: All agree to his law. The case is, he cut them to eat, and reconsidered and decided to use them for Schach.

Question: If so, what is Rabanan's reason?

Answer: The case is, he threshed the Yados through legs of animals or a hammer. They receive Tum'ah through intent, but become Tahor only through an action to change them.

Rishonim

Rambam (Hilchos Sukah 5:3): If one used food for Schach it is Pasul, for it is Mekabel Tum'ah. If there are fig branches with figs, or vine twigs with grapes, or Machbedos with dates, or all similar cases, if there is more Pesoles than food, one may use it for Schach. If not, one may not use it for Schach.

Rambam (12): If one draped leaves and branches of a tree over a Sukah and put Schach on them and then cut them (the leaves and branches), if there was more Schach than them, it is Kosher. If the Schach that was initially Kosher is not more than them, one must move them after cutting them, so it will be made l'Shem Sukah.

Rosh (Sukah 1:27): If there are fig branches with figs, or vine twigs with grapes, if there is more Pesoles than food, one may use it for Schach. If not, one may not use it for Schach. If one harvested them for Schach, they have no Yados, for he does not want them. They help to be Mevatel the food. If one cut them to eat, and reconsidered and decided to use them for Schach, his intent is not Metaher the Yados. He needs enough Pesoles to be Mevatel the food and Yados, until he does an action to show that he wants it for Schach, e.g. he threshes them in the granary.

Rashi (129a DH ha'Mesachech): If one covered his house with ears containing Zera'im, since he made them Schach, the Zera'im are Batel and it becomes an important Ohel.

Tosfos (Sukah 13b DH Im): The Gemara says that if there was more Pesoles than food, it is Kosher. If there was a majority of food, it is Pasul. We say in Chulin that if one made a roof from Zera'im, the Zera'im become Tahor, because he was Mevatel them. We can distinguish between a house and a Sukah. One does not Mevatel them to a Sukah, for he is prone to destroy it after Sukos. Alternatively, we require that the Schach is not Mekabel Tum'ah at the time he covers the Sukah with it. If not, the Sukah is Pasul due to Ta'aseh v'Lo Min ha'Asuy (the Schach was not Kosher when it was placed on the Sukah). Alternatively, there he changed it through an action. It does not lose Tum'ah through mere thought. Here, he did not do an action. Putting them on the Sukah is not an action. We say similarly in Bava Basra (19b) that putting a wafer in a window is not considered Bitul without an action.

Or Some'ach (12): If one banged down the branches and mixed them with the Kosher Schach, it is Kosher even if the tree gives more shade than sun. Since the Kosher Schach has more shade than sun, it is Kosher and important by itself. The tree is Batel in Schach of the Sukah. The same applies if both the tree and the Kosher Schach have more sun than shade. This is why the Tana Stam discussed draping a gourd over a Sukah, for the law applies as long as both are equal (both are mostly shade, or mostly sun). The Riva and Ritva hold that beating it down never helps if the tree is more shade than sun. On 13b we say that if fig branches have more Pesoles than food, it is Kosher. Tosfos holds that this is even if the food is mostly shade. The Ritva holds that the food must be mostly sun. How can we answer for the Rambam, who disqualifies mixed Schach even when the Pasul Schach is the minority?

Or Some'ach: The Rambam's son answered that he intends only for the Pesoles to be Schach. This is unlike one who uses also attached Schach (e.g. a tree), for there he intends that it be Schach. Here he does not intend for it, so it is not Schach at all. This is a great reasoning. It is like one who spread a sheet for decoration. Since it is not for Schach, it is not considered Pasul Schach. Therefore, even if the food is mostly shade, it is Kosher. This is why the Poskim did not distinguish.

Or Some'ach: In one answer, Tosfos said that if Schach was Pasul when he covered the Sukah with it, it is Ta'aseh v'Lo Min ha'Asuy even when he finishes. If so, when there is more Pesoles than food, surely the food is Tahor and Batel to become Schach, and it is not Mekabel Tum'ah any more. (Rashi in Chulin says that we say so only if he used ears, but it is not normal to be Mevatel food (without Pesoles attached) and leave it to be Schach. If one did so, we are not concerned for his opinion.) It is Pasul due to Ta'aseh v'Lo Min ha'Asuy. It is like cutting the gourd; if the Kosher Schach is the majority, it helps, for there is no Pasul Schach. The majority is proper Schach. The minority was Pasul at the time it was put up, therefore the Sukah is Kosher.

Poskim

Shulchan Aruch (OC 629:9): All kinds of food are Mekabel Tum'ah. One may not use them for Schach.

Mishnah Berurah (27): This refers to food for people. Food for animals is not Mekabel Tum'ah.

Mishnah Berurah (28): It is Pasul even if it was not Huchshar.

Mishnah Berurah (29): Any spice not eaten to enjoy the spice itself, rather, just for the smell or appearance, one may use it for Schach. The Pri Megadim is lenient about cinnamon, but Birkei Yosef proved that it is considered a food, for we bless Borei Peri ha'Adamah on it (202:17).

Shulchan Aruch (10): If there are fig branches with figs, or vine twigs with grapes, if there is more Pesoles than food, one may use it for Schach. If not, one may not use it for Schach. If one cut them to eat, the Yados are considered food and are Mekabel Tum'ah, so the Pesoles must have enough to be Mevatel the food and the Yad. If one cut them for Schach, the Yados are not considered food. They join with the Pesoles to be Mevatel the food and the Yad. If one cut them to eat, and reconsidered and decided to use them for Schach, his intent does not make the Yados cease to be considered food until he does an action to show that he wants it for Schach, e.g. he threshes them.

Beis Yosef (DH Kol): The Rambam and Rosh rule like the first Tana. The Rosh wrote that the case is, he reconsidered and threshed. The Rambam did not mention Rav Menasiya. Do not say that he rules like R. Aba, for in Hilchos Tum'as Ochlim (5:7) he rules like Rav Menasiya! It seems that in Hilchos Sukah he relied on what he wrote in Hilchos Tum'as Ochlim.

Magen Avraham (12): The Rosh and Beis Yosef say that Yados are only until three Tefachim. However, in Uktzin 1:3 we say that Yados join, even if they are very long. Therefore, one should be stringent.

Gra: The Halachah follows Rav Menasiya, for the Gemara discusses his opinion.

Mishnah Berurah (31): Based on this, one must be careful not to use for Schach straw of grain before the grain was knocked out. The Pri Megadim was unsure even after this. The Birkei Yosef agrees with the Taz (21), who permits. One must be careful, for often a little grain remains in the ears even after threshing, and the straw is still a Yad. Therefore, if one wants to use straw, it is good to remove the ears. For straw of a kind of ears harvested with a scythe, since letter of the law only three Tefachim is considered like food, if the rest is more than three Tefachim, one need not be stringent after beating it.

Kaf ha'Chayim (62): Even if there is mostly shade only through joining the food, it is Kosher, for it is Batel to the majority of Pesoles.