My family has raised cattle for commercial & show ring since 1947. We all showed cattle in Texas & have had many steers win at the county & major show levels. There are to factors on beef quality. Type of breed of cattle & marbling. The British & European breeds like Angus, Maine Anjou, Limousin, Charolais & so on have a high yield of fat then the other breeds. Grass feed beef can't delivery the marbling required to yield quality beef that would score Choice & Prime grades. Grass fed beef also has to be cooked different than grain fed beef due to the lack of marbling but also the flavor that grass fed beef has.
Grass-fed beef will usually be leaner with less fat in the edible portion than grain-fed beef, and this is due to less marbling, or the intramuscular flecks of fat measured in the ribeye steak. The conflict for beef customers and producers is that consumer studies indicate the desirable factors of tenderness, juiciness, and flavor-generally described as "quality" by consumers-are highly related to marbling content. One has to be careful what is described as "lean" because leanness will be relative to marbling content in the edible portion of the meat. Consumers generally describe a steak as "fat" when it has a large amount of exterior or subcutaneous fat left on it. Since the consumer seldom eats this fat or it is cut off in the processing phase, little attention is given to the real source of fat in beef steaks-marbling. As the marbling content is increased, we increase the amount of saturated and other fats in the edible portion. Studies & personal experience also show steaks can be too lean because it will not be as desirable to consumers. We walk a fine line between keeping the product lean and making it a desirable eating experience. Premium grain-fed beef such as Certified Angus Beef must meet a high standard of marbling content, and few grass-fed cattle can meet this standard. We have no idea if the higher levels of marbling-resulting in high Choice and Prime quality grades-in grass-fed meat have a positive relationship to eating satisfaction. One study showed it may actually be negative because of the influence on meat flavor (Steinberg, 2009).
If it was me first make sure you buy a breed of cattle that gives you a chance to yield a well marbled steer like a Angus or Maine Anjou or Maine Angus cross. Second I would feed the steer a quality grain a minimum 90 days before slaughter, some cattle take longer to finish. Grain feed needs to have a high fat content around 3 to 5% with a Crude Fiber no more than 8%, & a crude protein not less than 11.5%. The way we feed our show steers & steers we eat they eat grain full time starting them on a mid level grower ration grain like Lyssy & Eckel show calf #135. We provide coastal hay twice a day, and the cattle are turned out a night to graze in costal fields then brought back inside to their pens each morning. Steers are feed twice a day morning & evening then turned out to graze at night. Feed consumption needs to be increased as steer grows. Usually start around 14 pounds per day then increase a pound of feed every two weeks. About 90 days before slaughter or 90 days before a major show we switch to a finisher feed like Lyssy & Eckel #160. I also add additional steam rolled corn & barley which is soaked in beer. They are feed around 20 plus pounds a day which is split in two feedings morning & evening still getting fresh coastal hay with each feeding and they continue to get turned out at night to graze on coastal pasture.