Menahot, Chapter Ten, Mishnah Seven
There are five species of grain: wheat, barley, spelt, oats and rye. All other species, such as rice, do not count as grain, and laws that apply to grain do not apply to them. Our mishnah teaches a few general halakhot that are applicable to these five species.
1) Wheat, barley, spelt, oats and rye are subject to hallah.
2) And they are reckoned together.
3) They are forbidden [to be eaten] as new grain before the omer.
4) And they may not be harvested before Pesah.
5) If they had taken root before the omer, the omer permits them;
a) And if not, they are forbidden until the next year’s omer.
Section one: These five species of grain are subject to the laws of hallah (when one kneads a certain amount of them, he must separate hallah and give it to the kohen, if the hallah is pure).
Section two: These five species are reckoned together for all prohibitions and obligations. Thus if one kneaded together dough made from wheat and dough from barley and when reckoned together there was enough dough to require one to separate hallah, he would be obligated to take out hallah (see Hallah 1:1). The same would be true for prohibitions such as hametz on Pesah and other obligations, such as eating matzah on Pesah.
Section three: Before the omer is offered on the 16th of Nissan, these five species are prohibited from the new crop.
Section four: Before Pesah, one is not supposed to even harvest any of the new grain. The Torah states that the omer should be the first of your harvest (Leviticus 23:10), implying that this grain should be the first to be harvested.
Section five: This section determines when grain is considered is to be part of the new crop. If it took root before the omer was offered, then it is part of the pre-omer crop and can be harvested once the omer was harvested. However, if it had just been planted before the omer and had not yet taken root, then one would have to wait for the following years omer before it can be harvested.