A brief explanation of cattle feeding. This is not all inclusive and I clearly dont know everything about the entire cattle industry.
Most beef cattle come from small operations 100 head or less. Actually 50 or less make up a huge portion of the cow/calf operations.
The calves are on the teat for roughly 4-8 months until they are weaned. Some operations dont wean and just sell them. They are then instantly weaned, lose weight and can be sickly. Feedlots pay less for these calves usually. Their diet is mostly grass while on the teat and after that. (these can be steers or heifers)
These operations sell their calves at the sale barn, typically in the fall. It can be any month of the year though. (will be sold right off of pasture usually, no grain, no extras) They will usually be born between January 15th or so all the way until May.(even later for some operations) Most try to calve early. This way they have a larger calf to sell in October/November. They may weigh 500-650 pounds. Of course they can be smaller or heavier.
These calves are bought by feedlots. They typically raise them to about 14 months and then they go to slaughter.
While the calves are at the feedlot they are fed a ration that varies regionally and from feedlot to feedlot. The feed is roughly 62% roughage, 31% grain, 5% supplements (minerals and vitamins), and 2% premix. They can be fed sorghum, oats, alfalfa, soy, cotton seed, distillers grains, spent brewers grain, barley, wheat, poultry litter (look that up, you wont be happy) and more. They usually feed them to between 1100-1400 pounds.
When you take an animal straight off of pasture and slaughter it, clearly it will be leaner and lack the taste that we are used to with grain finished beef.
Data from the ag extensions at various colleges show that it takes 70-100 days of a high protein diet, such as corn, to change the yellow fat to white fat. The yellow fat is hard and gross. The white fat cooks down and flavors the meat.
The real difference is a grass fed animal is cheaper to produce, possibly healthier for you and much less tasty. A grain finished animal will be heavier, cost more to produce and be tastier.
As far as beef goes, remember mama cows, dairy cows, bulls, etc eventually get turned into meat. When you buy from the store you have no idea what age of animal you are eating. Although I would say steaks and roasts are more than likely younger animals. Straight hamburger could be any cow, bull, steer, heifer that is processed.
Buying local, if possible, at least gives you some choices in what you are buying and consuming.