The prolonged dry period experienced during planting will affect both weed emergence and performance of preemergence residual herbicides. In many areas of Michigan, there was little-to-no rainfall for two to three weeks after corn and soybean planting.

Even with the potential for future rainfall events, the dry conditions that occurred soon after planting, as well as shortages of several herbicides, will likely alter weed management plans for the remainder of the season.

So how will these dry conditions likely affect early-season weed control and what weed management strategies should be considered for the remainder of the season?

I applied a soil-applied preemergence residual herbicide, was this application wasted given the dry weather? The short answer is not completely. Many preemergence residual herbicides currently used will stay on the soil surface until precipitation is received. A majority of these herbicides are formulated to be fairly resilient to photodegradation and volatilization.

However, for these preemergence residual herbicides to be effective they need to be in the soil solution, so they can be in contact and absorbed by germinating weed seeds. When there is no rainfall for herbicide incorporation, these herbicides are not in contact with germinating weeds, therefore weed control during dry conditions is reduced.

Given the dry conditions soon after planting, many weeds germinating during this period will not be effectively controlled. However, these herbicides will likely provide some control of later germinating weeds after a significant rainfall. Typically, 0.5 to 0.75 of an inch of rain is needed to move the herbicide into the weed germination zone.

If a field is not yet planted and dry conditions are still in the forecast, one other option is to shallowly incorporate the residual preemergence herbicide. These preplant incorporated herbicide applications will move the herbicide off the soil surface and into the weed germination zone. Check herbicide labels or the Michigan State University Extension Weed Control Guide for Field Crops to see which preemergence herbicides can be incorporated. 

Will dry conditions slow down weed emergence? Once again, the answer is not completely. Like crops, weed seeds need to imbibe water to germinate. Small-seeded weeds, such as common lambsquarters, germinate from shallow depths and will likely be affected by the dry weather if there is not enough moisture in the upper soil profile to support sufficient germination.

However, large-seeded weeds, like giant ragweed and velvetleaf, are able to germinate from deeper depths in the soil profile where moisture is not as limiting. Although weed emergence may be reduced because of this dry weather, it is highly likely that most fields will have some weeds that emerged during this dry period. Additionally, when precipitation is received weeds will germinate soon after.

Weed control considerations

Here are some weed control considerations:

Scouting. Scouting should start immediately. Because of the dry conditions, weed and crop emergence can be highly variable in a field. When scouting it will be important to not only determine what weeds are present, but also to note both the minimum and maximum weed and crop growth stages in the field.

This information will be important in determining not only what, but when postemergence herbicides should be applied. Also keep in mind we typically start to see both herbicide-resistant waterhemp and Palmer amaranth emergence around this time in Michigan, and without effective preemergence herbicides, postemergence control strategies will likely need to be implemented earlier. 

Early postemergence herbicide applications. Generally, early postemergence herbicide programs should be applied when weeds are less than 2 inches tall. In corn, many of these programs will not only control emerged weeds, but also contain residual herbicides to control future emerging weeds that also will require rainfall.

Many of these programs are extremely effective for season-long weed control and results evaluating several of these early-postemergence herbicide programs can be found on the MSU Weed Science website under the Annual Results tab.

Early postemergence herbicide programs are generally used when a preemergence herbicide is not applied. However, because of the reduced weed control from preemergence herbicides this year, if a grower chooses to use one of these programs, it will be important not to exceed the maximum herbicide active ingredient amounts per season.

For example, if Acuron was applied at a full-rate preemergence, Halex GT at a full rate should not be applied early postemergence, since the total in-season rates of mesotrione (Callisto) and s-metolachor (Dual II Magnum) would exceed the total allowable active ingredient amounts per year. Consult the label for maximum yearly active ingredient amounts.

Unlike corn, in soybeans there are very few one-shot early postemergence herbicide programs that will provide season-long weed control. With the shortages of glufosinate (i.e., Liberty, etc.) and glyphosate (Roundup, etc.) this year that are often relied upon for weed control in conjunction with several of the herbicide-resistant soybean traits available, the reliance on traditional herbicides used in non-GMO soybean may be more common.

Additionally, using Enlist (2,4-D choline) products in Enlist E3 soybean or XtendiMax and Engenia (registered dicamba herbicides) in Xtend or XtendFlex soybean may be more widely used.

It is important to remember if using these herbicides to consult the Guidelines and Precautions for 2,4-D and Dicamba Use on pages 102 and 107, respectively, in the 2021 MSU Weed Control Guide for Field Crops and the herbicide labels. Also, keep in mind there are cutoff dates and maximum allowable amounts per season for several soybean products.

Minimum and maximum crop and weed heights and stages

As mentioned previously, the dry conditions have led to uneven emergence of crops throughout a field. This situation will affect both what and when certain herbicides are applied. For example, currently it is not uncommon to see fields with newly emerged corn, corn with two collars (V2), and corn seed that has not even germinated and emerged all in the same field.

In choosing a herbicide, it is important to realize certain products have minimum crop heights or stages before application. For example, to use the herbicide Status, corn needs to be 4 inches tall and at the V2 growth stage before application. If certain areas in the field are not at this minimum size, this product should not be applied until the entire field reaches this minimum crop stage.

Additionally, it is important to make sure herbicide applications are made when the weeds are small enough to get adequate control. In the 2021 Weed Control Guide for Field Crops, Tables 1I and 2K provide some guidance on both maximum and minimum crop and weed heights for herbicides used in corn and soybean.

Ultimately, the extended dry period during planting will alter both weed emergence and performance of preemergence residual herbicides. Deciding whether plans for postemergence weed control will need to be altered is going to rely on regularly scouting fields to identify what weed species are present and the optimal time to make postemergence applications.

Burns and Sprague write for the Michigan State University Department of Plant, Soil, and Microbial Sciences.