By Mark Loux

There is plenty of information on fall herbicide treatments in the C.O.R.N. newsletter archive and on other university websites. Our philosophy on this has not changed much over the past decade. A few brief reminders follow:

1. When to spray?  

Anytime between now and Thanksgiving will work, and possibly later. We have applied into late December and still eventually controlled the weeds present at time of application. Once hard freezes start to occur, there is usually a substantial change in the condition of certain weeds, such as dandelion and thistle, that renders them less sensitive to herbicides. We discourage applications during periods of very cold weather, which can occur starting about Thanksgiving, and also (obviously) when the ground is snow-covered.

2. What about all of that crop residue on the ground after — won’t that cause problems?

We have not worried about this, and the herbicides seem to work regardless. Most dealers seem to have the same impression. On the other hand, it probably wouldn’t hurt to wait awhile after harvest to let the residue settle down, and the weeds to poke through. Dense crop residue usually prevents marestail from emerging anyway.

3. Don’t make it too complicated or pricey.

Keep in mind that the primary goal is control of weeds that have already emerged. This is hard to accomplish with a single herbicide, but there are a number of relatively low cost two-way mixtures that easily achieve this goal. Our philosophy has generally been to start with 2,4-D, and then add another herbicide that results in more comprehensive control.  

Herbicides that make the most sense to add to 2,4-D based on our research:  glyphosate, dicamba, metribuzin, simazine, Basis (and generic equivalents), Express (and generic equivalents), Canopy/Cloak DF or EX, or Autumn Super. These allow either corn or soybeans to be planted the following year with these exceptions: simazine — corn next year; Canopy/Cloak — soybeans next year; Basis — possibly restricted to corn based on rate and geography.  

We do not see the need for three-way mixtures, although a case can be made to add a low rate of glyphosate to a two-way mix to control grass or improve activity on perennials. A two-way mixture of glyphosate and Sharpen could also be used, but we believe Sharpen has more utility in marestail control programs when used in the spring.

4. Is there an advantage to including residual herbicides?

No, because almost all of them peter out over the winter and fail to provide any control of spring-emerging weeds. Want a more scientific description of the process than “peter out?"  Okay — a combination of herbicide degradation and dilution and off-site movement occurs that results in inadequate concentrations of herbicide remaining in spring to control emerging weeds.  

The primary exception to this is chlorimuron (Canopy/Cloak), which, for whatever reason, does persist at high enough concentrations to provide some control in spring. Our research has repeatedly shown that applying other residual herbicides in the fall to get control in spring is a waste of money. 

The good news here is that any effective fall herbicide treatment with or without residual will result in a weed-free seedbed in spring, usually into April, so that the spring-applied burndown/residual treatment just has to control small weeds that emerge in the few weeks prior to planting. That is the goal.

5. It doesn’t take a lot of herbicide to control weeds in fall, just the right ones. 

There is a tendency for some manufacturers to promote an expensive mixture of too many herbicides that just isn’t necessary. Avoid most residual herbicides, and also those that mainly “speed up the kill." Consider that fall treatments should comprise no more than about 25% of your total herbicide budget for a crop, and it can be accomplished for even less than that.