http://fyi.uwex.edu/wbic/files/2010/11/Beta-Agonists-Factsheet.pdf The gains are substantial and the real savings is in the amount of feed the animal needs to grow. The most popular brand name is zilpaterol hydrochloride (Zilmax) is produced by Merck.
Here is a bit of text from a beef grower about the use of a beta-agonist...
Beta agonists, like ractopamine, work by activating the beta 2 receptor on the muscles of my cattle. This binds specific beta receptors in the muscle cell membranes and increases protein synthesis. What does this mean exactly?
1.As animals grow larger and get close to the time of harvest, their bodies tend to turn nutrients into fat instead of lean muscle. Ractopamine encourages or repartitions those nutrients into muscle growth through protein synthesis rather than fat deposition.