Mont. Code Ann. § 80-7-505

Each owner of cropland benefited under a cropland spraying program is liable for the portion of the landowner's share of the program costs as the owner's cropland is a portion of all cropland within the boundaries of the program. The county treasurer shall compute each landowner's liability and mail a special assessment to each owner or occupier of land within the boundaries of the program. Unless otherwise mutually agreed upon, this assessment is due and payable within 32 days of mailing or 30 days of receipt if receipt is shown to be more than 2 days after mailing. A delinquent assessment is a lien upon the land assessed.