Fall Weeds: Burndown and Control in Cover Crops

Fall burndown in fallow settings

In the fall, foliar-applied herbicides can be effective if the plants are green and appear healthy and actively growing. Fall is the best time to control dandelion, while both fall and early spring are good times to control winter annuals, including marestail. In most fallow fields (i.e., no cover crop planted), a basic tank mixture of 2,4-D ester (1 pint/A) plus glyphosate (1 quart) plus metribuzin (3-5 oz DF) tends to provide adequate control of marestail and other weed species such as common chickweed, henbit, mustards, and dandelion. It is relatively economical when sprayed in the fall. Application of 2,4-D alone controls many winter annual weeds, but 2,4-D will not control chickweed and is less effective on dandelion than when in a tank mixture with other herbicides. Killing marestail in the fall is usually easier since the seedlings and rosettes are small and more susceptible to herbicides. Once weeds like marestail get through the winter, their rosettes start to bolt upwards in the spring, making them much more difficult to control.

Contact herbicides (e.g., Sharpen, Gramoxone) and systemic products (glyphosate, 2,4-D, dicamba) are much less active at low temperatures. Relatively speaking, 2,4-D is slightly more active than glyphosate in cooler temperatures (<40°F); whereas dicamba tends to be more impacted by cold weather, therefore, tank mixing them improves overall control. As we move into November, since foliar herbicide effectiveness decreases, the inclusion of a residual herbicide may be desirable in corn or soybean rotations. If you include a residual herbicide, research over the last several years has shown that any chlorimuron-containing product (Canopy, Cloak, etc.) is at the top of the list if soybeans will be planted next spring, and simazine is one of the better products for fields going into corn. Other products that have had some success include Valor and Basis Blend. In general, 2,4-D should be tank-mixed with any residual product. Also, when applying systemic herbicides this late in the year, make sure to include an adjuvant such as ammonium sulfate (AMS) and/or crop oil concentrate or methylated seed oil to ensure adequate uptake of the herbicide. Also, try to keep the fall burndown herbicide costs at a minimum ($5-15/acre, if possible) since you will need to account for other herbicide costs next spring to control marestail and other weeds in the crop. Thus, glyphosate + 2,4-D (or dicamba) + metribuzin can be an initial low-cost option to consider that provides control of a relatively broad spectrum of weeds.

Fall weed control in cover crops

Cover crops can be very beneficial in managing winter annuals and overall minimizing weed pressure. However, cover crop blends that include clovers, radishes, peas, etc., will make it impossible to use any herbicides in the fall or early spring. Consider a cereal cover crop like rye or wheat on acres where you need to aggressively target marestail and other winter annuals because, in cereal crops, you have herbicide options. Products like 2,4-D, dicamba, and Audit 1:1 can be sprayed in the small grain cover crop this fall (or early spring) to control smaller weed seedlings.

Kill declining stands of sod now

This topic might seem somewhat haphazard for this article, but as a reminder, fall is the best time to kill declining sod stands (i.e., pure stand alfalfa, grass hay, or mixtures).  Although glyphosate is better at controlling alfalfa and perennial forage grasses in the fall than in the spring, an additional herbicide application (e.g., 2,4-D/dicamba) or tillage will be required to completely control the alfalfa/mixture. Unless you plan to get one last spring cutting, now is the time to control that old hay field; do not wait until spring when it is more difficult to get an effective burndown kill prior to planting.