For musk thistle, you need to use 3 to 5 oz of Milestone Specialty Herbicide per Acre, which breaks down to about 0.07
Milestone Specialty Herbicide can be used in grass pastures as directed on the product label; however, it is not safe to use on desirable alfalfa or other desirable broadleaf crops or plants. Milestone is labeled to control rush skeletonweed, but it would also harm the alfalfa. This article is from Washington and not Idaho, but it has great information about this plant. This site has local information for you about this and other weeds. We also recommend you contact your local Cooperative Extension for reporting and for assistance with this invasive plant.
Per the label