Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.034.

It is permissible for the owner to eat from the grain, and whatever he consumes will not be included in the quantity subject to zakah, for this is a long-standing custom. In any case, only a small amount is actually eaten. It is the same as an owner of a fruit-bearing tree eating some of its produce. Therefore, the zakah will be estimated on the actual amount after he harvests the crop and husks the seeds. Ahmad was asked about the eating of farik (rubbed green wheat) by the owner, and he answered that there is no harm if the owner eats what he needs. This is also the opinion of ash-Shaf’i, al-Layth and Ibn Hazm. However, Malik and Abu Hanifah hold that the owner will have to account for what he eats.