There is nutrition in grass which is available to mammals like dogs (and humans). They are mostly cellulose, but plants use carbohydrates and proteins for their own structure and metabolism, and these are present in all plants in at least small amounts. Roots, shoots and fleshy stalks, are better than the leaves. Like an unpleasant, less nutritious lettuce or celery with too much fibre. Those are also hard to get meaningful calories from at like 50 - 100 calories per pound, but there's some calories, and there's other nutrients in them too.