Mont. Code § 81-6-201
A cattle protective district embracing all or parts of two or more counties may be formed upon the filing of petitions by the cattle growers of such counties with the boards of county commissioners of each county to be wholly or partially included in the district. Such petitions must be signed by at least 51% of the cattle owners owning 55% of cattle for the protection of which the district is to be formed residing within the area designated as part of the district in each of the counties affected. Upon receipt of such a petition, each board of county commissioners must within 30 days declare the designated portion of its county a part of such cattle protective district, and the district shall be formed immediately upon the action of the last board of county commissioners to act.
§ 81-6-201, MCA