Grass fed cows are cattle that are allowed to graze and forage for their own food although standards aren't set for meat to be labelled grass fed. Grass fed cows eat mostly grass or hay but the majority of these cows are still fed grain before slaughter.
how to feed
Look for grass fed options from your local butcher or check for sourcing details from pet food manufacturers
Grass fed may help some dogs with allergies to specific proteins because if the proteins they are eating is grain fed, they could be actually allergic to the grain.