Tillage and herbicide strategies to control winter annuals

Winter annuals are fairly easily controlled by either fall or spring tillage (or both), but in no-till or reduced-till fields, a weed control strategy that includes a burndown treatment in fall, spring, or both is likely necessary. Winter annuals present several windows of opportunity for control, each involving important considerations to maximize your return on your weed control investment.

📸: BASF Contributing Writer/Featuring fall tillage, followed by final seedbed preparation in the spring, which alleviates the need for herbicide application to control winter annuals.

After harvesting your summer annual crop, you may have winter annual weed seedlings as well as ungerminated seeds of winter annuals in your field. Treat too early in the fall, and you may miss weeds germinating later in the fall. Treat with a burndown application too late in the fall, and you may have inconsistent control based on slowed plant metabolism and poor translocation of systemic herbicides. Waiting until spring for a burndown treatment (that will likely include a soil-residual herbicide) might result in reduced weed control if wet soils delay getting the treatment on the field after problematic weeds are bolting.

📸: BASF Contributing Writer/Featuring winter annuals, which pose different risks to next year's intended crops. Pictured here shepherd's purse (upper left), carpetweed (lower left), and marestail (right).

Specific weeds or scenarios that demand attention

• Consider a fall burndown where you have marestail emerged and growing. As mentioned in my last article, nearly all marestail in the readership regions is glyphosate-resistant, and much of it is resistant to ALS chemistry. The addition of growth regulator herbicides or triazines to the tank is essential for consistent control.

• Know our soil drainage characteristics and err on the side of fall control where there is a history of standing water in the spring (particularly on claypan soils). If your spring burndown is delayed and winter annuals enter the reproductive stage (bolting and flowering), they are much more difficult to control.

• Use a fall burndown in the western Corn Belt where soil moisture in the spring can be maximized by removing competing vegetation.

• Use a fall burndown where the probability of wet soils in the spring is high, particularly when low sprawling, prostrate weeds are the main weed species and can leave dense mats that reduce soil surface evaporation.

• Perennials like dandelion that can germinate in late summer and follow similar growth patterns to winter annuals are more easily controlled with a fall burndown application.

📸: BASF Contributing Writer/Featuring fall application of postemergence herbicides to emerged winter annuals.

Guidelines for successful Fall control of winter annuals

The sweet spot for application timing is after most of the fall germination is complete and the weeds are still actively growing. A frost or early freeze does not necessarily "close the window", provided the temperature rises back above 55 F and green vegetation still exists. When the temperature stays below 40 F, the application window may be closed until spring. Make sure to use appropriate adjuvants for the herbicides you are using. As temperatures drop, the small winter annuals may exhibit thickened cuticles as they prepare to survive the winter in a dormant state. Adjuvants should improve cuticle penetration and make herbicides more effective. With almost all marestail being glyphosate-resistant, plan to use a growth regulator herbicide in your application.

📸: BASF Contributing Writer/Featuring an untreated field (Left) vs. fall treatment with Distinct^® herbicide, metribuzin, rimsulfuron and thifensulfuron-methyl.

Should a residual herbicide be part of the Fall burndown?

In general, the answer is no. Let's think of the fall burndown as a treatment directed at emerged weeds whose removal before spring is economically and agronomically preferable. A residual herbicide would need to persist until spring to control late-emerging winter annuals and a second burndown with residual herbicides may already be a planned treatment. There is some limited use of simazine (before corn) and chlorimuron-containing products (before soybean) as their half-life in the soil allows them to persist until spring planting.

Coming next: Targeting weed control based on weed life cycle.

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BASF provides the information in this article as a service to its customers; however, the views expressed by guest writers are their own and do not necessarily reflect the views of BASF.