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Strategic Farming: Field Notes discusses early season weed control

By Angie Peltier, UMN Extension crops educator, and Phyllis Bongard, UMN Extension educational content development and communications specialist

Early season weed flush. Photo: Tom Peters
On May 18, 2022, Dr. Debalin Sarangi, UMN Extension weeds specialist, joined UMN Extension educators Ryan Miller, Dave Nicolai and Jared Goplen for a discussion of how best to employ early-season weed management tactics for season-long weed management success. This was the second episode of the 2022 Strategic Farming: Field Notes program in this series.

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Pre-emergence herbicides provide value

Pre-emergence herbicides (PRE) are applied to the soil after planting, but before crop emergence. PREs need to be dissolved in soil water - ‘activate’ - before weed seeds can take them up as they imbibe water. PRE’s target weed seeds in the process of germination and if they work as intended, sensitive weed seedlings will not emerge until the soil concentration of the PRE declines.

With the increasing frequency of herbicide-resistant weed species, PREs have become a reliable foundation for season-long weed management success. In broadleaf crops grown in fields with herbicide resistant weeds, there may be few remaining effective, post-emergence herbicide options. In these situations, PREs will be called upon to do the “heavy lifting” to manage weeds that would otherwise be unmanageable post-emergence (POST).

It is generally recommended that folks do what they can to plant into a “clean field”, or a field free from already emerged weeds. This year’s rainy spring has led farmers to quickly get their crops planted in between rain showers. Unless a planter is quickly followed by a sprayer, application of PREs can get delayed by more rain. If the weeds that subsequently emerge are resistant to many of the POST herbicide options or get too big as more rain continues to delay application, this delay can prove quite costly due to impacts on both cash crop yield potential and additions to the weed seed bank.

Weed seedlings already emerging, some already outgrew chemical control

Giant ragweed plants have already been observed in Minnesota this year that have outgrown the ability to be managed chemically with a POST application. With soils already having warmed throughout much of the state, even short delays between tillage to prepare a weed-free seed bed and planting or planting and PRE herbicide application can result in a difficult to control lawn of emerging weeds. One positive way to look at what has been a challenging spring so far is that in much of the state the bulk of giant and common ragweed and lambsquarters seedlings have emerged. If one plants into a weed-free field there will not be as many of these weed species to manage post-emergence.

There have already been grower and crop advisor reports of waterhemp emergence this spring. Waterhemp is difficult to manage in broadleaf crops for several reasons, including its season-long emergence pattern, its prolific seed production and its ability to develop resistance to multiple herbicide site of action families. Since waterhemp seedlings can begin emerging in May and continue to emerge into August, it needs to be managed all season long. Female waterhemp plants can produce as many as 500,000 viable seeds, meaning that a ‘management miss’ may cause headaches for several years to come. Waterhemp populations have also proven themselves to be a capable adversary, having evolved resistance to one or more herbicide groups. Resistance to up to five herbicide groups has been found in a single Illinois waterhemp population.

Recent Minnesota research has found that there are many waterhemp populations that are resistant to Group 14 herbicides (ex. Flexstar, UltraBlazer, Cobra) applied POST. Further study of these populations found that while waterhemp may no longer be managed by foliar Group 14 herbicide applications, Group 14 herbicides applied PRE were still effective at keeping seedlings from emerging.

Environmental concerns

The discussion focused briefly on the potential for atrazine and metribuzin carryover after the issues that some experienced during the dry 2021 growing season. Fields with higher soil pH are at greater risk for carryover due to herbicide adsorption to soil particles.

Metribuzin was also described as a great tank mix partner, working synergistically with herbicides from other site-of-action families and reducing the chance of selecting out herbicide-resistant weed populations. However, both metribuzin and atrazine have the potential to leach into groundwater in areas with permeable soils or shallow groundwater. In southeastern Minnesota where karst topography is prevalent, Miller noted that laws and regulations require atrazine set-backs from waterways and limit total atrazine use per year to 0.8 lb a.i. Mixing a lower label rate of atrazine with a Group 27 herbicide (ex. Laudis, Callisto), for example, diversifies herbicide sites of action and helps to retain effective chemical weed management strategies. This is particularly important, as select Minnesota waterhemp populations have recently been confirmed as resistant to both atrazine (Group 5) and Group 27 herbicides.

Layered residual herbicides

Another recommended weed management strategy is to layer residual herbicides, with the goal of having an effective concentration of herbicide in the soil solution for as long as possible. One would apply a PRE herbicide that has some residual activity; then three to four weeks later, as the PRE concentration begins to decline, apply another herbicide with residual activity. This strategy is one way to try and manage around the fact that in many fields, there are few effective post-emergence herbicide options.

A couple of tips for those that didn’t get their PREs on in time

Reading herbicide labels this time of year is time-consuming but worthwhile, as there are some herbicides (ex. Valor and Verdict) that make excellent PRE options but shouldn’t be sprayed if the crop has already emerged or one would risk burning their crop.

Miller shared that some of the work that he and others have done in southeast Minnesota has really pointed to the importance of properly timing management efforts, by using a PRE followed by layering on a residual herbicide 20 to 30 days later. While an earlier application may not be warranted as the PRE may still be at an effective soil concentration, an application later than 30 days would risk multiple weeds germinating as the concentration of the PRE decreased with time.

Herbicide efficacy can also be improved by targeting weeds that are shorter than 4 inches tall and by improving coverage by increasing carrier volume and (if the label allows) changing spray nozzles.

What about flame-killing weeds?

An audience member asked for some pointers to successfully manage weeds by flame weed management in organic crops. Flame weeding is similar to your non-selective, burndown type herbicide treatment, where the weed seedlings managed are those that have already emerged. Ideally, the weed seedlings should be shorter than 3 inches in height. Managing grass weeds in this way may require two passes with the flame weeder and may also require cultivation. In corn, there is no yield penalty when just the first leaf collar has emerged, but as the crop grows, it can become much more difficult to avoid injuring the crop while flame weeding. Soybean cotyledons are quite resistant to flame, but true leaves are not, so one should try to limit flame weeding to newly emerged seedlings.

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Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!

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