Barley grass can be considered a “green food.” When we talk about "green foods," we’re referring to a group of foods that includes young cereal grasses but also spirulina and blue green algae. Nutritionally, barley grass is a close cousin to dark green leafy vegetables, but offers far greater levels of nutrient density. In other words, an ounce of these concentrated green foods contains much more of the beneficial phytonutrients found in an ounce of green vegetables.
The nutrient profiles of barley grass changes quickly as it grows. With maturity the chlorophyll, protein, and vitamin content of cereal grasses declines sharply and the level of cellulose (indigestible fiber) increases. So young barley grass may be best from a nutritional standpoint.