The USDA is currently developing guidelines to define the term “grass-fed”, and it is expected to call for an all-grass diet of at least 95%. Strictly grass-fed cattle tend to be leaner than grain-fed.
The United States Department of Agriculture is presently formulating regulations to clarify the meaning of "grass-fed", which will likely require cattle to consume no less than 95% grass. As a result, grass-fed animals are generally thinner than their grain-fed counterparts.