The boards of supervisors in said counties are hereby authorized and empowered in their discretion to levy a tax on the taxable property in said counties lying outside municipalities not exceeding two mills per annum thereon for the purpose of paying the cost of construction and maintenance of such fences and cattlegaps. In order to raise additional revenue for the purpose of paying the cost of construction and maintaining such fences and cattlegaps, the board of supervisors in those counties which have heretofore elected not to come under the statewide livestock law may in their discretion levy a tax against the owners of cattle permitted to roam at large in the amount of twenty-five cents (25¢) per head for cattle located in the respective counties in which an election has carried.