This article is from the Llamas & Alpacas FAQ, by Michael Shealy and William Bagwell with numerous contributions by others.
It is not generally recommended to ride llamas, except for small children. An adult male will reach a weight of between 300 and 450 lbs. and stand 5 to 6 feet tall. They are expected to carry approximately 1/4 of their body weight, so a rider or load of between 75-115 lbs. may be carried. This weight may be increased to a maximum of 1/3 of their body weight as they reach top physical conditioning.
 
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