BILLINGS, Mont. (AP) -- The Bureau of Land Management says it is considering giving fertility control vaccines to more wild horses in the Pryor Mountains.

BLM spokesman Greg Albright said Wednesday that about 70 percent of mares in the approximately 150-horse herd have been given fertility vaccines over the last two years.

Agency officials say expanding that program could reduce the need for periodic roundups that are held to keep the herd's size under control.

Horse advocates have fiercely oppose previous roundups, including one last year that resulted in 57 animals being put up for adoption.

The horses in the Pryor Mountains, about 40 miles south of Billings, are descended from the mounts used by Spanish Conquistadors.

Copyright 2010 The AP.

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