Maybe I can step in and help Nancy, she really did a great job though
The problem with 100% grass fed beef, is that on that diet, it takes a steer 2 yrs to get to the right size. It won't be as tender because the stear is more aged and the fat "marbeling" won't be there because they aren't going to get fat on grass.
The traditional corn fed, lot steers are fed corn mix and usualy a hay pellet, major high protien and energy. They grow much faster than their grass fed siblings and get fat and we like the marbeling. Corn raised calves are burchared at around, 1 yr old. The meat is more tender because the meat is younger and and it has marbeling, tender and tastey.
Hay, calves will grow fine on just hay, but they won't grow fast. Hay is fiber, filler, it has nutrients but these past years its been really bad due to the weather. The calves don't need much hay as long as they are getting the protien, fiber and nutrients from another source. The farmer you spoke to is feeding a barley grain mix, its got corn, barley (hay), fillers (makes the calf full so he won't eat so much that he can't kill himslef) and vitamines and mineral supplaments, it could have oats for more carbs and it will have something for fat, many options there, peanut hulls is a poppular fat for a feed mix.
This mix, isn't much different than a the big time feeders, they feed a mix much like this, except they will do anything to cheapen it up and keep that calf growing fast.
Your farmer friend, probably has his calves in a larger, more confortable lot. He's got them on full hay (good for the belly) and they get to eat the grain mix as they please. They will be at butchar weight by the time they are 1 yr old.
My thoughts, go with the grain fed calf and be happy. It will be more tender, taste better and cheaper (because that calf isn't sitting around for 2 years).
Most likely theres no antibiotics or any growth hormones added to the feed, but its not illegal for a farmer to add it to his own calves but most don't.
I talked BF into holding back 6 calves, they were fall calves, we weren't going to get anything from them at the market so, he let me have my way. We raised them totally on hay and mineral lick in the winter and they did just fine, but, we kept 2 in the corn/feed lot and they did get bigger quicker. Over the summer the 6 calves ran with their mothers and kept growing, the other 2 calves were in the lot and they were bigger. The 2 lot calves were ready to butchar that fall, on their 1st birthday. The other 2 were brought up into the lot at the same time and they were considerably smaller especially in the meat department. They were leaner, less muscular and just looked lanky. We hayed those calves all winter again, out with the cows come summer and that fall, they were brought up for butchar. They were at butchar weight, but they didn't look at "bulky" as the other calves looked. Anyway, the taste test, there was very little marbeling in the meat. The meat seamed dryer, less tender and seemed to have a different flavor. We had to eat it, of course, we grew them. BUt now, all calves are on hay and grain rations when they are weaned till they are butchared in March.
Hope I wasn't overly wordy but I thought JoryTk wanted an full answer