Pasture Weed Management
by Robert C. Bellm, Extension Educator, Crop Systems
Late summer and early fall is an excellent time to implement a broadleaf weed control program in your pastures. Both perennial and biennial weeds are sending food reserves to their root structures at this time of the year, making it easier to move systemic herbicides into these roots to permanently kill the weeds. For best effect, these systemic herbicides need to be applied at least two weeks prior to a killing frost.
Some of the more problematic weeds in pastures have a biennial life cycle, meaning that they live for two years. These types of weeds propagate only by seed, and are most effectively controlled in the early vegetative stage. The seeds sprout during the spring and early summer of the first season, forming a low-growing rosette of leaves. They live through the first winter in the rosette form, and then the following spring they bolt a flower stalk, set seed, and die. Pasture weed species that fall into this category include, bull thistle, musk thistle, teasel, mullen, Queen Anne’s lace, and poison hemlock. Herbicide applications made during the late summer or early fall will kill these weeds while they are still in the rosette stage, thereby preventing seed production. If a fall herbicide application is not possible, then there is a second opportunity for control in the early spring, before the weeds begin to bolt a flower stalk. Once the weed has begun to flower, herbicide applications are much less effective. Even though the plant can be killed it will often still produce viable seeds, though total seed production should be reduced. One option to minimize this possibility would be to mow the bolting flower stalks off before they bloom, and then spray with an herbicide to kill the plants before they rebolt.
Perennial weeds originate from seeds, but once established maintain themselves indefinitely through vegetative root structures. Perennial weeds may be woody (blackberry, locust, autumn olive, multiflora rose) or herbaceous (ironweed, goldenrod, Jerusalem artichoke). Once perennial weeds are established, successful control involves the use of a systemic herbicide that will kill the root system. For best effect, the herbicides need to be applied at a point in the plant’s life cycle when it is actively transporting food reserves into its roots. Early spring applications are typically less effective, since the nutrient flow at that time is from the root upward into the plant. Better control will be obtained by applying herbicides at the late bud to early flower stage of growth. Best control is often obtained by a late summer or early fall application as the weed is rebuilding root reserves prior to entering winter dormancy.
Keep in mind that any herbicide that will kill a broadleaf weed will also kill any legumes that you want to keep in your pastures. If broadleaf weeds are a problem, however, choosing a herbicide with low residual activity may allow you to reintroduce legumes through frost seeding later on during the winter.
Robert C. Bellm, Extension Educator, Crop Systems, 618-692-9434