It is my understanding that there are no known poisonous grasses in the world. Of course we are used to eating the fruit of the grain (in breads and pastas), but the blade is also consumable in time of need. They may taste bad, but the blades contain most of the elements that compose the human body, which is something to keep in mind.
In a survival situation, the best way to extract the nutrients is by locating a healthy patch of young grass, growing in shade or partial shade. You can cut the grass and bring it back home to share with family (it will keep in a plastic bag very well) or consume it right on the spot. You should think of it as your vitamin pill: chew it for several minutes so that the cell walls in the blades are crushed, and the green juices flow into the mouth where it will be swallowed. Spit out the pulp. Only very small quantities of the juice are needed to recieve significant benefits, so you will only need to chew a few mouthfulls each day.
The modern wheat grasses have the best flavors, though the taste is incredibly strong, and it is hard to say which of the grasses have the "best" flavors. I have sampled many of the native grasses, too, which have even stronger flavors. But, you should remember that young green grass is a very valuable survival tool which can help your family through the hard times.
Graphic by Michel.