Many factors influence the quality of grass.
We see a considerable variation in the nutrient composition of grasses. You may think that these variations in species may be only a percent or two, but this adds up to a considerable nutritional value over the grazing season. Another variation we see is the season of the year and variation within a season. If we consider wheat or oats compared to brome or orchard grass in the spring we see a definite increase in the feed value of the wheat or oats compared to the mentioned cool season grasses. Keep in mind that some forage species just reach their peak of nutritional value at different seasons or times within a season when they are compared to other forages.