Tallcool, you are correct in that all 3 of these compounds are classified as non-steroidal stimulants and beta-2 agonists. Unbridled Stallion is also right in that Clenbuterol has a longer half life in the body than the other two. Optaflexx actually has the shortest half life at approx. 4 hours and Zilmax is in between. Clenbuterol is only approved for respiratory therapy in horses since it is a bronchodilator. I don't believe it is approved for any other uses in livestock in the U.S. I'm also fairly certain that in the past Clenbuterol has been associated with significant damage to heart tissue. It was likely given to show steers years ago for the same reason many desire to use Zilmax and Optaflexx in that all of them increase lean muscle tissue at the expense of fat cells. I'm a big proponent of integrity in the show ring and in life, and if folks use an illegal or banned substance to artificially improve the appearance of an animal then they need to be willing to face whatever consequences come their way. None of these products should be used in young, growing animals which are still depositing accelerated amounts of protein/muscle. Even many of the feedyards I know that used Zilmax or currently use Optaflexx still base use of the beta agonist on body condition of the calf in that a thin or calf lacking cover will not receive the product.
Unbridled Stallion, all I can tell you is that Zilmax had a more obvious effect on both live animals and carcasses in enhancing musculature. It's also known as a more aggressive re-partitioning agent than Optaflexx anyways, so this is not surprising. In industry circles, it is also common knowledge that it was very easy to "screw" a calf up as well if he was not a good candidate for Zilmax to start with.
