Hmm! Well the truth of the matter is that I will not like to recommend muscle building drugs for your animals because it also has a lot of side effects.
Rather, to fatten up your ruminant animals, they need well concentrated feeds, regular deworming and adequate disease prevention and treatment. Am sure that if all of these are in place, your ruminant animals will be free of diseases and the feed being consumed by them will be converted to muscle on time.
There are also certain vitamins to aid muscle build up without exposing the animals to certain side effects. These vitamins can be gotten and given to your animals at the recommendation of your consultant.
See also: How to treat Ruminant Animal Diseases