Strategic Farming: Let's Talk Crops! Discussed How to Make Herbicides Work Better

Feb 13, 2024

By Liz Stahl and Tom Hoverstad

On February 7, 2024, Dr. Tommy Butts, Extension Weed Scientist with the University of Arkansas, and Tom Hoverstad, Researcher at the University of Minnesota Southern Research and Outreach Center in Waseca, joined UMN Extension Educator-Crops Ryan Miller for a discussion on “Making Herbicides work better”. Specifically, this session focused on making choices and adjustments to ensure a successful herbicide application and was part one of a two part series titled “Making herbicides work better.” This was the fourth weekly episode of the 2024 “Strategic Farming: Let’s talk crops!” program which runs through March.

Watch this episode and other recordings: z.umn.edu/SFrecordings

Dicamba update

The program opened with an update on the status of dicamba products labelled for use on dicamba-tolerant soybeans. At the time of publication, a February 6 federal court ruling in Arizona vacated the labels for Engenia, Xtendimax and Tavium, three dicamba products previously labelled for over-the-top use on dicamba tolerant soybeans. At this time it is unsure if more guidance will be offered, but currently based on this ruling, there would be no dicamba products labelled for use on dicamba tolerant soybeans. Everyone is encouraged to stay tuned for any updates as more is learned about this.

Making herbicides work better

Nozzle selection

Of the many pieces that can affect weed control, nozzle selection is critically important. Understanding nozzle nomenclature is an important first step, since this tells us what type of nozzle we have, the spray angle, and the flow rate. For example, a nozzle labelled MR 110-04 is a mid-range flat-fan nozzle with a 110 degree spray angle and flow rate of 0.4 gallons per minute (GPM) at 40 psi (pounds per square inch).

Nozzles drastically influence spray coverage. Nozzles affect droplet size, which is the key factor influencing coverage, but if regulations or drift concerns mandate the use of a larger droplet size, you will likely need to increase your gallon per acre (GPA) rate to get the coverage needed.

With a contact herbicide like glufosinate (a.i. in Liberty), droplet size is even more important than spray volume, although both are key in achieving acceptable control. With systemic herbicides like dicamba or glyphosate (a.i. in Roundup), droplet size has less of an effect, but even here, if droplet size is too big and spray volume is too low, you will not have the coverage needed and control can be reduced. Dr. Butts emphasized that you can’t consider just one of these factors without considering the other.

It is important to keep in mind that although XR nozzles can result in greater coverage since they produce smaller droplets than a drift-reducing nozzle, like a TTI nozzle, flat fan nozzles also result in the highest potential for drift. Drift potential must also be considered when selecting the right nozzle for an application.

Compatibility and nozzle interactions with adjuvants

It can be hard to tell if products will be compatible in the tank without testing the mix on a small scale. If micronutrients are added to the mix, this adds another level of complexity. When products are not compatible, you can end up with precipitates in the tank, which in turn can clog sprayers and screens. It is highly recommended you conduct a jar test when planning to apply a tank mix (see details at: https://www.uaex.uada.edu/publications/PDF/FSA-2166.pdf), and that you watch what happens as you mix products. Dr. Butts commented that you can often tell if something isn’t looking right as it comes out of the spray boom. Some micronutrients, for example, can result in smaller droplet sizes and thus greater drift potential.

There can also be interactions between nozzles and spray adjuvants. In research trials, some adjuvants actually reduced droplet size, making the end product more drift prone even though the adjuvants were supposed to help reduce drift. Be sure to watch applications as they come out of the spray boom and make adjustments as needed.

Pulse width modulation (PWM) systems

A common question about PWM systems is whether or not they will produce more skips in the application. If everything is set up correctly, adjacent nozzles will be set on an alternating pulsing cycle (blended pulse), reducing the potential for skips. Driving at the proper speed and selecting the appropriate nozzle size are also important in reducing the potential for skips. Dr. Butts noted it is critical to not use air induction nozzles with a PWM system as these nozzles will not result in the proper spray pattern.

Nozzle sizing

The following equation is important to help ensure you have the proper nozzle size, spray pattern, and psi when making an application:
GPM = (GPA x W x MPH) / 5,940
Where: GPM = gallons per minute; GPA = gallons per acre; w = nozzle width/spacing in inches; mph = sprayer speed in miles per hour

Keep in mind that the speed used in this equation should take into account the range of speed used, not just the maximum. For example, you may have to slow down around the field perimeter. Select a nozzle that gives you flexibility for a range of speed.

Tips when using Liberty herbicide

Using glufosinate (the a.i. in Liberty) can add diversity to your herbicide program, but conditions at application can have a significant impact on efficacy. Dr. Butts recommends targeting applications during the day, from around 8:00 or 9:00 am to no later than 5:00 pm. Early afternoon is typically the prime time for applications so you can get the right temperature and humidity combination. Be sure to add AMS to the tank to enhance control.

Tom Hoverstad also noted that applying to smaller weeds is even more critical with glufosinate compared to glyphosate and other products. Proper nozzle selection is also critical when applying glufosinate, as it is a contact herbicide, versus a systemic herbicide like glyphosate. For example, TTI nozzles are NOT recommended when applying glufosinate due to the droplet size being too large to achieve the needed level of coverage.

Antagonism

Dr. Butts and Tom Hoverstad both discussed the issues you can have when mixing a grass herbicide with a growth regulator like Enlist or Xtendimax. Antagonism issues can result in significant reductions in control of weeds like volunteer corn in soybean. They have seen more issues with the “fops” (e.g., quizalofop, the a.i. in Assure II), than the “dims” (e.g., clethodim, the a.i. in Select), although antagonism can be seen with both when mixed with a growth regulator herbicide.

Splitting the application by applying a grass herbicide seven days after the growth regulator was applied resulted in greater control in trials conducted by the University of MN. See more details on this work at Managing volunteer corn in 2,4-D tolerant soybeans. Sequential applications where volunteer corn pressure was high resulted in greater yields in trials conducted at Waseca and Rochester, more than paying for the extra trip across the field.

The importance of temperature

Weed control can be impacted by temperature at the time of application as well. Some herbicides result in better control when applications are made at 80°F versus 90°F. Cold temperatures can affect control too, leaving weed survivors after application.

Recommendations for success and best ROI

The number one recommendation for weed control success is to use a residual herbicide in your weed management program. Residual herbicides are critical in helping control weeds with resistance to key postemergence herbicides and longer emergence patterns, like waterhemp and Palmer amaranth. Overlapping residuals is another tool, where you apply a residual herbicide at planting and then around 30 days after planting. It is important to mix up herbicide groups, however, to help delay the development of resistance to these products as well.

Strong residual programs with multiple modes of action preemergence gave the best weed control and best return on investment, in analysis conducted by Dr. Butts. It is imperative farmers also use integrated weed management strategies to control weeds and not just herbicides, to deal with increasing issues with herbicide-resistant weeds. These practices include strategies like increasing canopy development, narrow rows, earlier planting dates, cover crops, strategic tillage, cultivation, cleaning equipment, harvest weed seed control and more.