The first step in determining weed control needs in the fall is to do a critical assessment or evaluation of pasture fields and bordering fencerows. Not only take an inventory of the current weeds present, but scout fields looking for any developing new weed problems in their seedling growth stages. Identify areas of the field where potential cool-season weeds such as thistles, poison hemlock, and buttercup are emerging from seed and starting to develop. These seedling plants are often present in areas where mature plants had been observed earlier in the season when they were flowering during the spring and early summer months. They are also often found in fields with open bare areas where the desirable forage species are thin due to excessive grazing and other environmental conditions.
If you do see developing cool-season weed problems then you may want to take corrective action this fall and early next spring to address these problems. In general, herbicide products that contain 2, 4-D are usually effective on young biennial thistles, poison hemlock rosettes, and germinating buttercups. As long as daytime temperatures are near or above 60 F herbicide treatments can be applied when these weeds are in an active growing period during October and November. In some cases, herbicide treatments may not be the best solution or may not correct all weed problems observed.
Here are some additional points to consider as you make those decisions. When evaluating a pasture field with developing weed problems, you must decide whether or not to either 1) drill or overseed more forages into an existing pasture to improve the stand of desirable forage grasses to make them more competitive OR 2) spray herbicides to control emerged broadleaf weeds. You will not be able to do both practices at the same time since most pasture herbicides have the potential to injure newly emerging forage grasses or legumes. For pasture herbicides which contain only 2,4-D it is generally recommended to wait 3 to 4 weeks after spraying before reseeding forage grasses and at least 6 to 8 weeks before seeding clovers or other legumes. Other broadleaf herbicides may require a waiting period of 6 months or longer between time of application and seeding forage legumes (consult the label of specific herbicide products used). Therefore, as a general rule of thumb, if you decide to spray this fall you will need to wait until next spring before seeding additional forages. If you reseed in the fall, then it is recommended that you wait until the new forage grass seedlings have a well-established root systems with secondary roots before making a herbicide application. It is important to note that anytime broadleaf pasture herbicides are applied clovers or other desirable legumes are likely to be killed or severely injured within the areas treated.
If weeds are not prevalent and the existing stand of desirable forages appear to be competitive enough to minimize emergence of winter annual weeds, another course of action this fall is a “wait and see” approach. But, keep in mind that smaller weeds are easier to control than after they increase in size and become more mature. You may need to reevaluate these fields in late February and March to determine if corrective action is needed in early spring.
Source: JD Green, Extension Weed Scientist