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.

To listen to a recording of this episode subscribe to Strategic Farming: Field Notes on your favorite podcasting platform or visit this website: www.strategicfarming.transistor.fm/#. Pre-emergence herbicides provide value 



(PRE) are applied to the soil after planting, but before crop emergence. PREs need water to ‘activate’ because they need to be dissolved in soil water in order for weed seeds to take them up as the seeds 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. 

According to Dr. Debalin Sarangi, “PREs are the foundation of waterhemp and lambsquarters management,” particularly with the increasing frequency of herbicide-resistant weed species. In broadleaf crops grown in fields with herbicide resistant weeds, there may be few remaining effective, post-emergence herbicide options. Ryan Miller shared what has been a successful 

weed management strategy for him in southeast Minnesota, “While some herbicide labels do allow for an early post-emergence (POST) application, weather delays are certainly possible and so I would continue to prioritize your pre-emergence application.” In these situations, PREs will be called upon to do the “heavy lifting” to manage weeds that would otherwise be unmanageable post-emergence. 

It is generally recommended that folks do what they can to plant into a “clean field,” or a fieldfree 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. “PREs will not manage those weeds that have already emerged as seedlings,” reminds Dr. Sarangi. 

Weed seedlings already emerging, some already outgrow 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 weeds emerging. 

Sharing a positive outlook of the 2022 growing season with the audience, Jared Goplen said, “the one thing that you can feel a little bit better about in a year like this with late planting is that we do get “free weed control” with tillage. With the bulk of giant and common ragweed and lambsquarters seedlings have emerged (throughout much of the state), if one plants into a weed-free field there will not be as many of those seedlings to have to manage postemergence.” 

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, it is a prolific seed producer and has proven itself capable of evolving 

resistance to multiple herbicide sites of action families. Waterhemp seedlings can begin emerging in May and can continue to emerge into August and so 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 (ex. Valor) were still effective at 

keeping seedlings from emerging. 

The discussion focused briefly on the potential for atrazine and metribuzin carryover after the 

carryover issues that some experienced during the 2021 growing season. Fields with higher soil 

pH are at greater risk for carryover due to herbicide adsorption to soil particles. 

Metribuzin was also discussed 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. Ryan Miller mentioned the karst topography that is prevalent in 

southeast Minnesota and the laws and regulations regarding using set-backs from waterways and diversifying the sites of action while reducing the atrazine component by mixing a lower label rate of atrazine with a Group 27 herbicide (ex. Laudis, Callisto). Diversifying our chemical weed management strategies is important, particularly 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 

(https://extension.umn.edu/herbicide-resistance-management/managing-waterhemp-layeredresidual-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, another 

herbicide with residual activity would be applied. 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. 

Ryan 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 


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 non-selective, burndown type 

herbicide treatments, 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 and so one should try to limit flame weeding to newly emerged seedlings. 

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