Western whorled milkweed removal
Western whorled milkweed is a perennial and reproduces by seeds and underground roots known as rhizomes. It stores energy from season to season in the rhizomes. In order to get rid of this weed you must exhaust or kill the root system. The best way to do this is with a systemic herbicide, such as roundup. There are a couple of strategies for getting rid of this weed but mainly depend on the amount of acreage of the weed that is in your pasture because in order to get rid of this milkweed you also might have to also kill the pasture around it. The first strategy is to spray it with roundup once it gets to 4 or 5 inches, this will likely not kill it depending on how long its been establishing. Depending on how bad the weed problem is will help determine a course of action. This is one of our worse weed in western Colorado. Roundup will not kill it but will set it back. Keep the milkweed cut back and keeping it from going to seed until the fall and then hitting it with roundup is the best general strategy. Please give me a call and we can discuss further as its tough to put everything in an email response.
Agronomist - CSU Extension - Delta
Western whorled milkweed is one of our most challenging weeds to control in pastures. If there is alfalfa mixed in with the grasses, you will lose that when you apply herbicides because all labeled products are relatively non-selective. They will damage all broadleaf plants.
Some broadleaf herbicides (listed by active ingedient) that have been used with varying degrees of success are picloram, dicamba, dicamba + 2,4-D, chlorsulfuron, metsulfuron, or metsulfuron + chlorsulfuron.
For more information on pasture management, take a look at the Intermountain Grass and Forage Legume Production Manual at http://wci.colostate.edu/shtml/ForageManual.shtml