Wow,
Let me help y'all out a bit with grassfed beef.
If you take an animal to slaughter that is not gaining on pasture, you will not likely enjoy the result. Without the proper genetics, in other words, cows/steers that are breed to gain on grass only, you will not end up with a good product. Start with the right genetics before you want to "try" grassfed. [By the way, I tell my kids not to "try" anything, either "do" or "don't do" because trying rarely works.] I would not recommend trying to finish an animal that you have said won't finish, without grain. I am 100% for grassfed beef, but you don't throw the baby out with the bath water.
Work with what you got wildsawmill and fatten that animal up before you take her to slaughter. The genetics are first, then as backhoeboogie said, the forage you have is key. These two components are often missing and why so much grassfed beef is not as good as it should be, not to mention slaughtering before they are fat. Grassfed beef is not just marketing Texas, it is the only growing segment of the beef industry for a reason, it is healthier and produced right, it is better unless of course you prefer flavorless beef.
The problem is too many people are not doing it correctly, you will not finish a 1500, 1600 lb cow on grass, she requires too much feed and is extremely inefficient at converting that feed. You aren't able to take a conventional herd and convert it to grassfed because you will have the wrong genetics. Come out to our conference and learn more.
http://www.grassfedexchange.com