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Make fall weed control effective

Fall is a favorite season for many of us in agriculture. It brings relief from the heat and humidity of summer; another school year kicks off; we can enjoy fall activities like sports and pep bands; and we get to finally open up the crop fields and see if the hard work put in all spring and summer brings yields that match expectations. For those of us who dislike weeds, fall offers a last cha

Make fall weed control effective

Fall is a favorite season for many of us in agriculture. It brings relief from the heat and humidity of summer; another school year kicks off; we can enjoy fall activities like sports and pep bands; and we get to finally open up the crop fields and see if the hard work put in all spring and summer brings yields that match expectations. For those of us who dislike weeds, fall offers a last chance for vengeance and a strategic way to get ahead of them for next year.

Interest in fall weed control has really grown the last few years. Here are some common queries from growers and agronomists about fall spraying.

Should I get a jump on fieldwork next spring by applying some of my residual herbicide this fall?

If you are targeting typical spring and summer annual weeds like foxtails, waterhemp, ragweed, sunflowers and others that need seasonlong control, then fall application may not be the best use of residual herbicides. Most experts agree fall applications should target the weeds that are present and that we should hold off with residual herbicides until planting season in the spring. They make some good points as to why:

Fall, winter and early-spring weather is unpredictable. If the weather is warmer or wetter than average, the fall-applied residual herbicides could break down too soon and lead to weed escapes. If we apply later in fall to dodge some risk, we could be facing frozen soil surfaces that could lead to direct herbicide loss from runoff.

Unless the residual you are using is labeled for corn and soybeans, you may be in a jam if you want to change your planting intentions next spring.

Occasionally the opposite of rapid breakdown occurs; we see residuals persist too long and crop injury can occur from the additive effects of a fall-applied residual herbicide that hung on better than expected, combined with activity from a spring residual that was applied. I see this more often in soybeans than corn. The crops usually outgrow it, but it is a tangible risk with fall-applied residuals.

There are exceptions to about every rule, and this axiom may be true of fall-applied residuals. The recent uptick in the use of layered residuals (applying roughly one-half to two-thirds of your residual herbicide ahead of planting and the balance of it early postemergence to extend control longer into the season) has seen some success in areas with the early application taking place in the fall rather than spring.

Other examples are areas where perennially wet spring soils leave growers with very small spraying and planting windows. They point out that any work they can get done in the fall including residual herbicide applications gives them a better shot at success come planting time.

While there can be environmental and economic risks with these strategies, they might be minimized with good management. If you are considering fall application of residual herbicides, work with your agronomists and chemical company reps to focus on field-by-field product selections, application rate structure and application timing.

If I do have some fields I want to try some fall-applied residuals on, how can I increase my odds of success?

Here’s my chance to get in a favorite line by agronomists: “Always read and follow label directions.” Especially with fall applications, following label recommendations can mean the difference between success and wasting money along with risking the environment.

Here are some things I’ve learned about fall-applied herbicides from the very beginning — impregnating dry fertilizer with Dual or Treflan in the mid-1990s for the next season’s annual weed control and now fighting today’s nearly bulletproof marestail in no-till and minimum-till production systems:

Scout first and know what’s out there, especially if you are in a minimum or no-till system.

Don’t skimp on fall-applied rates, especially if part of the mixture is designed to knock out winter annuals.

No matter how good it looks next spring, plan on adding another shot of residual around planting time or early post. Fall applications buy time but don’t consistently hold back weeds seasonlong as well as spring applications.

If at all possible, plant the fall-applied fields on the early end of your spring window; the crop canopy can help hold back weeds as well or better than about any herbicide program.

Watch weather forecasts to target fall application timing. Apply too early and you may start burning up residual activity or miss later-emerging fall and winter weeds. Too late and you may not have good activity on those fall and winter weeds, and you can run a greater risk of herbicide runoff if the ground is frozen.

McGrath is the On-Farm Research and Extension coordinator for the Iowa Soybean Research Center at Iowa State University. Contact him at

Best fit for fall-applied programs

What’s the best fit for fall herbicide programs? We see the most success when minimum-till and no-till growers are targeting weeds like kochia, pennycress, shepherds purse, chickweed, henbit, purple deadnettle, mustards and dandelions. These weeds run a spectrum of fall, winter and early-spring germination and growth, so fall applications may not knock them all out. But if they are bad enough, fall applications can make a huge difference.

What about marestail? Fall applications are one of the best tools to either bring marestail back under control or keep it from becoming a bigger problem, especially ahead of soybeans. I’ve seen marestail survive some nearly nuclear spring burndown herbicide tankmixes; of course, it typically has no trouble surviving any subsequent cleanup that follows the failed burndown.

I’ve also seen marestail torn to shreds with minimum tillage tools only to come back to life and shake off about any herbicide used as a backup. So over time, clients and I have learned that to contain marestail, or to rein this weed in before it becomes a problem, fall applications done right are the ticket.

I like this statement from Purdue University: “The use of a fall application, regardless of whether or not it includes a residual, is a must if you are trying to control marestail in no-till soybean.” It goes on to explain that the emergence pattern of marestail (it comes in fall, spring and summer) means multiple herbicide applications are needed, and to start in the fall to be successful.

While fall application targets recently emerged marestail rosettes (so often the application consists of growth regulators like 2,4-D and dicamba), I’m a fan of several weed scientists’ recommendation to consider adding a low-cost residual component to the tankmix. There are a lot of choices for the residual, so talk that over with your local agronomist. While adding a residual herbicide in fall won’t eliminate the need for a spring treatment, it can prevent some germination in the fall and provide a couple weeks residual in early spring as well.

That short amount of residual can make spring burndowns more effective. Fewer marestail plants to control is helpful, and the plants you’ll burn down will be more of the smaller spring-emerged rosettes. You should see fewer of the large, bolting fall-emerged plants that often survive spring burndowns or light tillage attempts.


WEED ESCAPES: Seeing weeds in crops before harvest gets farmers thinking about applying a fall herbicide after harvest. Fall application is a practice more farmers have used the past few years to provide help for next spring’s herbicide application.

This article published in the August, 2015 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2015.

Weed Control

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