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Understanding Metabolic Resistance

*This content was previously published by Corteva Agriscience.

When a weed scientist says a novel weed resistance issue is not well understood, it’s a little concerning.

Most farmers understand the need to apply multiple herbicide groups and rotate them to reduce the number of herbicide-resistant weeds going to seed. This practice reduces target-site weed resistance when a weed alters its genetic code so the chemical no longer fits the protein it was designed to attack.

However, some weeds are evolving to deploy suites of enzymes that work together to metabolize (detoxify) a chemical before it can kill a weed — known as non-target or metabolic resistance.

 

How big is this problem?

“Ten or 15 years ago, we weren’t seeing much metabolic resistance in waterhemp, as it was all target-site resistance,” says Pat Tranel, Professor and Associate Head of the Department of Crop Sciences in the College of Agricultural, Consumer and Environmental Sciences at the University of Illinois.

“Today, I’d estimate more than half of our waterhemp in Illinois has glyphosate resistance [target-site], and 50% have atrazine resistance [metabolic/non-target]. Approximately 10% of the waterhemp population has HPPD resistance which is all metabolic,” Tranel says. He suspects there’s more metabolic resistance than we realize in preemergence Group 15 herbicides because reduced residual control by a few weeks makes it difficult to pinpoint whether it’s resistance or weather-related.

At a molecular level, scientists can easily determine target-site resistance because they know the protein and can look directly at the genetic code of waterhemp to determine the responsible mutation. But for non-target metabolic resistance, it’s still a guessing game. Researchers can sometimes identify an enzyme class that detoxifies
a chemical but know little about which genes code for those enzymes.

Tranel and his colleagues have determined that waterhemp has evolved resistance to seven different herbicide groups, and all have some metabolic resistance. “Our research focuses on what enzymes or genes are involved, what is selecting for those enzymes, and why the same mechanism can confer resistance to other herbicide groups. Until
we understand all this, we’re at a loss to make herbicide recommendations for metabolic resistance,” he says.

 

Defining the resistance

Tranel uses Enlist® crops to explain metabolic resistance. The same gene used to confer 2,4-D resistance in Enlist corn or soybeans can also metabolize the Group 1
‘FOP’ herbicides like quizalofop. “The reason for this is an unpredictable cross-resistance that we talk about in weeds,” he says.

There’s some similarity around the chemicals of those herbicide molecules, allowing them to be recognized by the same metabolizing enzyme. This process is similar in weeds. “If a weed gets selected for an enzyme that can metabolize herbicide A that the farmer has used, it’s also possible that same enzyme can metabolize herbicide B,” Tranel says.

The metabolic resistance process gets further complicated over time. Weed scientists worry about weed populations that receive different herbicides over many seasons
will lead to numerous enzymes metabolizing numerous herbicide groups. In other words, the herbicides select for suites of enzymes that can collectively work together to metabolize different herbicides.

“It’s not an exaggeration that we are selecting weed populations that can metabolize herbicides that have not even been commercialized yet,” Tranel says.

Even when research narrows down the genes responsible for metabolic resistance, growers still need to worry about target-site resistance. Using multiple effective sites of action and rotating herbicides using a three-to-four-year plan is essential to manage target-site resistance.

“But just doing this alone will not prevent metabolic resistance,” stresses Tranel. “Farmers need to know they cannot beat weed resistance with herbicides. Non-chemical strategies are needed to manage weeds. The overriding goal should always be no weeds going to seed.”

Will farmers reach a point where mechanical weed seed destruction technology on combines becomes mainstream, like in Australia, where they’ve dealt with metabolic
weed resistance since the 1980s? Tranel thinks it’s certainly a possibility.

“Weed seed destruction technology has a fit, but with limitations as it works better in some crops than others,” he says. “Weeds will adapt, as I can predict waterhemp will start shattering seeds before combines roll. We preach diversity in strategies, as the more things you throw at weeds, it’s harder for them to adapt.”

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Tips for Summer Corn Scouting

*This content was previously published by Corteva Agriscience.

No matter when your customers were able to get into the field for planting this spring, regular scouting can help set the stage for a successful corn yield come harvest.

Getting into the field on a regular basis after planting will improve the timing on important crop protection decisions. Is there a new flush of weeds that needs to be managed? Will postemergence herbicides need to be applied sooner than anticipated? Are there any nutrient deficiencies? Are there any new insect or disease pressures? All these questions can be answered and addressed with regular scouting.

Scouting should occur multiple times throughout the growing season, but there are times when it’s more critical, says Joe Bolte, Market Development Specialist, Corteva Agriscience.

 

Pests to watch this year

With lengthy emergence periods, waterhemp and Palmer amaranth present an annual challenge — calling for a herbicide program approach that includes multiple modes of action for effective control. Bolte says, “Depending on your geography and planting date, waterhemp or Palmer amaranth may need to be controlled in every herbicide pass — not just the postemergence application.”

On the flip side, your customers in areas with heavy rainfall may not have had a chance to get their preemergence herbicides down in time. If this is the case, they may consider reallocating those preemergence herbicide dollars to create a more powerful postemergence pass.

Bolte also says that tar spot should be on everyone’s radar. Scouting the corn plant’s canopy will help determine if a fungicide application is warranted.

 

Scouting resources available

There are several free resources available to help customers with in-field corn scouting. “Many universities will put together scouting guides or calendars for common pests. Use these to determine when weeds, disease and insects are most likely to emerge in your geography,” Bolte says.

You also can contact your local Corteva Agriscience representative and download our Corn & Soybean Disease ID Guide and Corn Weed Scouting Checklist for more detailed information.

 

Key corn scouting timing

 


Corn & Soybean Disease ID Guide

 


Corn Weed Scouting Checklist

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© 2024 Corteva. Groundwork – June 2024

Spray Timely and Early for More Effective Weed Control

*This content was previously published by Corteva Agriscience

When looking at a weed, its overall height can be deceptively short. How is that possible? Because, to a herbicide, the weed’s height is not a linear measurement from the ground to the top of the plant. When a herbicide is fighting a weed, that fight takes into account every inch of the weed. It includes every point of growth that is on the plant, coming out of the main stem.

Corteva Agriscience Herbicide Trait Specialist Steve Snyder illustrates this in a video on application practices. (Use the QR code to check it out.)

In the video, Snyder measures off a single 8” waterhemp weed and pulls it from the ground. He then breaks off each branch growing from the main stem of the plant and lays them end to end on the ground and measures the length of all the sections of the plant as a whole. The total length comes out to 33″. Snyder explains how, to the herbicide, this is a 33″ weed, not an 8″ weed. He says that this is one reason why it is so important to apply herbicide early and in a timely manner. The herbicide has a much greater chance of being effective when the plant is 6” tall or less, because the weed has far fewer growth points than a larger, more mature weed.

Snyder’s demonstration makes a good point. Weeds that are left to grow taller than 6” make the job of the herbicide just that much tougher. Catching weeds early is an important aspect of good weed control and helps ensure the treatment is as effective as possible.

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Soybean Growth and Development

Soybean growth stages begin with the emergence of cotyledons from the soil surface (VE). When the unifoliate leaves unfold, the plant has reached the VC stage. After the first trifoliate leaves are fully expanded, numbers are used to signify each vegetative (V) and reproductive (R) stage of growth. As the plant begins to set flowers, the growth stages become reproductive, and the plant progresses through pod development, seed development, and plant maturity. Vegetative growth stages begin to overlap with reproductive stages at about R1. A new growth stage is established when 50% or more of the plants meet the requirements of the growth stage.

 

Determining Growth Stages in Soybeans


Figure 1. Soybean growth stages.

 

Soybeans are largely either indeterminate or determinate in growth habit. For indeterminate products, vegetative growth continues after flowering, and the rate of development is directly related to temperature. Determinate products generally complete vertical growth by the time flowering is completed.

 


Figure 2. Reproductive growth stages of soybeans.

 

Emergence (VE) Through First Trifoliate (V1)

After absorbing adequate moisture and depending on temperature, product, and planting depth, the primary root or radical emerges from a soybean seed. The hypocotyl pulls the cotyledons with it to the soil surface (VE, Figure 3). Cotyledons supply the plant’s nutrient needs for seven to 10 days after emergence. The loss of one cotyledon during this time has a limited effect on plant growth; however, if both cotyledons are removed at or soon after emergence, yield potential may be reduced by five to 10%.1,2 Soon after the cotyledons are fully exposed, unifoliate leaves emerge at the second node and begin creating energy through photosynthesis (the VC stage). Development and full extension of the first trifoliate leaflets (node 3) establishes the V1 stage of growth, and with each fully developed trifoliate on the main stem, another V stage is established.

 

Second Trifoliate (V2)

The V2 stage begins when the second trifoliate leaf is fully expanded. Root nodules begin to develop at this stage, and nitrogen (N) fixation in the nodules begins to occur when plants reach six to eight inches (15 to 20 cm) in height. As plants switch from soil-available N to fixed N, the plants may become yellowish. Lateral roots are developing rapidly in the top six inches (15 cm) of soil.

 

Third to Fifth Trifoliate (V3 to V5)

Axillary buds develop into flower clusters (racemes) in the top of the stem. Determinate varieties stop producing nodes on the main stem soon after the onset of flowering. For indeterminate varieties, the total number of nodes the plant can produce on the main stem is established at V5. Axillary buds that develop on an indeterminate soybean plant can help the plant recover from damage. This is typically the time that iron chlorosis deficiency symptoms become visible in impacted fields.

 


Figure 3. Emerging soybean (VE).

 

Sixth Trifoliate (V6)

Plants develop new growth stages about every three days, depending on environmental conditions. At this stage, lateral roots should overlap rows 30-inches wide or less. A 50% loss of leaves at this stage may reduce yield potential by about three percent.

 

Beginning Bloom (R1)

Flowering begins on the third to sixth node, continues up and down the main stem, and eventually moves to the branches. Nodes on the main stem usually have at least one flower. Vertical roots as well as secondary roots and root hairs continue to grow rapidly until R4 or R5.

 

Full Bloom (R2)

An open flower (Figure 4) develops at one of the top two nodes of the main stem. The plant has accumulated about 25% of its total dry weight and nutrients and about 50% of its mature height. Nitrogen fixation by root nodules is increasing rapidly. Loss of up to 50% of plant leaves from hail, insects, or disease at this stage may reduce yield potential by six percent.

 

Beginning Pod (R3)

A pod on at least one of the upper four nodes is 3/16-inch (5 mm) long or longer. Heat or moisture stress at this stage can reduce pod numbers, seed number per pod, or seed size, which may reduce yield potential. The ability of soybean plants to recover from temporary stress decreases from R1 to R5. Favorable growing conditions during this period may result in greater pod number and increased yield potential.

 

Full Pod (R4)

Pods are growing rapidly, and seeds are developing. At least one ¾-inch (19 mm) long pod has developed on at least one of the four upper-most nodes. Stress during this period (and through R6) can cause more reduction in yield potential than at any other growth stage. Timely rainfall or irrigation may help reduce the potential for yield loss.

 

Beginning Seed (R5)

At least one seed that is 1/8-inch (3 mm) long is present in a pod (Figure 5) at one of the four upper-most nodes. About half of the nutrients required for seed filling come from the plant’s vegetative parts and about half from N fixation and nutrient uptake by the roots. Nitrogen fixation peaks. Stress at this stage can reduce pod numbers, the number of seeds per pod, seed size, and yield potential. Plants attain maximum height, node number, and leaf area at this stage.

 

Full Seed (R6)

This “green bean” stage (Figure 6) marks the beginning of the full seed stage. At least one of the four upper nodes should have a pod with a green seed filling the pod cavity. The total pod weight peaks and leaves begin to yellow

 


Figure 4. A soybean plant flowering and forming pods.

 


Figure 5. Beginning seed (R5).

 


Figure 6. Green beans fill soybean pod.

 

Beginning Maturity (R7)

At least one normal pod on the main stem reaches its brown or tan mature color (Figure 8). Seed dry matter begins to peak. Seeds and pods begin to lose green color. Plants are safe from a killing frost. Yield potential may be reduced if pods are knocked from plants or if pods shatter, releasing seeds.

 

Full Maturity (R8)

When at least 95% of the pods on a plant have reached their mature color (Figure 9), the plant is fully mature. After the R8 stage has been reached typically five to 10 days of good drying weather are needed to obtain a harvest seed moisture content of less than 15%.

 


Figure 7. Soybeans at R7 growth stage.

 


Figure 8. Soybean pod and seeds changing to mature color (R7).

 


Figure 9. Fully mature soybean plant drying down.

 


Figure 10. Soybean pod and beans drying down.

 

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Sources:
Rees, J., Specht, J., Elmore, R., Nygren, A., and Mueller, N. 2019. Considerations after crusted soybean. University of Nebraska-Lincoln. https://cropwatch.unl.edu/2019/considerations-after-crusted-soybean
Conley, S. 2018. A Visual Guide to Soybean Growth Stages. University of Wisconsin-Madison. https://ipcm. wisc.edu/blog/2018/05/a-visual-guide-to-soybean-growth-stages-2/
Purcell, L.C., Montserrat, S., and Ashlock, L. 2014. Soybean growth and development. Arkansas Soybean Production Handbook, Chapter 2.
Kandel, H. and Endres, G. 2023. Soybean production field guide for North Dakota. North Dakota State University. A1172. https://www.ndsu.edu/agriculture/ag-hub/publications/soybean-production-field-guide-north-dakota
Web sources verified 5/7/2024.
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Performance may vary, from location to location and from year to year, as local growing, soil and environmental conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on their growing environment. The recommendations in this material are based upon trial observations and feedback received from a limited number of growers and growing environments. These recommendations should be consid-ered as one reference point and should not be substituted for the professional opinion of agronomists, entomologists or other relevant experts evaluating specific conditions. Bayer and Bayer Cross are registered trademarks of Bayer Group. ©2024 Bayer Group. All rights reserved. 1314_12400.

Optimize Weed Control by Avoiding Tank-Mix Mistakes

*This content was previously published by Corteva Agriscience.

While tank-mixing can bring you and your customers improved weed control, convenience and efficiencies, each product added to the tank also increases the potential for negative interactions. Physical incompatibilities can clump or gel an entire tank, and chemical incompatibilities often can result in reduced efficacy or crop injury.

Joe Bolte, Market Development Specialist, Corteva Agriscience, says incorrect product order, moving through steps too quickly and failure to agitate the mixture are all common mistakes that lead to incompatibilities and reduced product efficacy. The good news is that each of these mistakes can be avoided.

5 ways to avoid common tank-mix mistakes

  1. Do your research. Because there are so many types of herbicide formulations, knowing when to add each product to the tank can be confusing. Make sure you’re set up for success by reading all label directions and formulation information available. Many labels will tell you about the order in which you should tank-mix with other products, the agitation requirements and any product restrictions.
  2. Perform a jar test with your proposed tank-mix. Don’t waste time and money by going straight to mixing products in the tank. “Jar tests are a great way to test compatibility issues before heading out into the field,” Bolte says.
  3. Begin with a half-full tank of water carrier. Using low water volumes and high rates of crop protection products increases risk of incompatibility.
  4. Agitate chemical containers before each use. When liquid herbicides sit for long periods of time, they begin to separate. Therefore, it is important to shake the chemical jugs before use. This will help ensure the proper ratio of actives is getting mixed into the sprayer. Also, agitate the mixture itself throughout the process to keep things from settling.
  5. Take your time. Allow time for proper agitation, time for water conditioning and time between each new product added. If ammonium sulfate (AMS) is required, make sure the water conditioner has had enough time to circulate before adding any liquid herbicides. “Since many liquid herbicides can be tied up by hard water, we want to make sure the AMS has enough time to condition the water,” Bolte explains. “Failing to condition the water can reduce herbicide performance.”

The bottom line

There are many benefits to combining crop protection products, but a successful tank-mix requires research, testing and attention to detail. Work with your Corteva Agriscience representative to find products that offer tank-mix compatibility to fit your crop protection needs.

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An Inside Look at Reduced Stature Corn

*This content was previously published by Corteva Agriscience.

The adage “what’s old is new again” is applicable when plant breeders, researchers, agronomists and farmers talk about the possibilities reduced stature corn may bring. The hybrids may be new, but the breeding process to shorten plant stature is not. Corteva Agriscience researchers are striving to develop reduced stature corn with stronger stalks and an agronomic system that potentially allows for more yield per acre. Reduced stature plant breeding was developed with wheat genetics by world-renowned agronomist Norman Borlaug, in the mid-1950s. That successful work was later duplicated with rice, dramatically increasing yields in both crops.

Though this concept has been viable since Borlaug and his teams completed their work, it has been slow to gain broad application in corn hybrids. This doesn’t mean research has been lacking. Far from it.

Cory Christensen is the Corn Pipeline Development Leader at Corteva Agriscience. His job is to shape new product concepts in the corn pipeline from ideation or inception, and shepherd them through each stage of the development process until they’re ready to launch.

“Reduced stature corn is not a new idea, but interest in it has quickly picked up steam,” Christensen says. “Over the past 10 years, we have carefully analyzed a number of different genetic approaches to make corn plants shorter, and our researchers have identified one of those trait approaches that had the characteristics we desired.

“We were gaining confidence in this trait at the same time the big wind events hit the Midwest in 2020 and 2021,” he continues. “Since we have a native allele version of the trait, we are well-positioned to develop and launch a set of hybrid products to introduce the concept to the market. Hybrids demonstrated excellent resilience under high winds with our trait, and we are confident that growers will realize that benefit among others.”

Christensen is quick to point out other advantages of reduced stature corn include easier ground access for equipment to apply fertilizers and other inputs. Specific management recommendations are in development.

 

Delivering value to farmers is the reason for new hybrid development

The initial hybrid products containing the reduced stature trait are under evaluation and are expected to be on the market later this decade.

“Our goal is to give growers opportunities to experience these products and see if they perform like we expect they will,” Christensen says. “Most importantly, they must deliver value on customers’ operations just as any hybrid we release does.

“It’s important to understand we are following our normal product development process,” he adds. “We’re approaching this just like we do with other hybrids. We’ve taken some of our new, pre-commercial inbreds, the parents of future hybrid seed products, and converted them to reduced stature inbreds with our trait. These new potential hybrid products are working their way through our product characterization trials.”

Product characterization trials take place throughout corn-growing regions for at least two years before any new hybrid is launched. Product development teams want to observe genetic and environmental interactions so there is solid confidence in recommendations to growers. In addition, they’re evaluating performance at various densities and will have sound recommendations about planting densities upon launch.

 

Reduced stature corn offers beneficial new technology

Corteva researchers must address how to define the reduced stature corn concept, understand its genetics, ensure data is correct as well as how to characterize the product and launch it. They must also consider what the genetics and long-term plant breeding program looks like.

“When you think of all the mechanical advances, chemical advances and technological advances, I look at reduced stature corn as another piece of technology that happens
to be genetic technology,” Brandon Wardyn, Evaluation Zone Lead, Corteva Agriscience, says.

“It changes some physics of the plant, so this changes how we need to breed for different traits, namely traits associated with tolerating wind. Given this new level of standability, it can really open up the genetic space we can operate in. A basic principle in plant breeding is as you take pressure off one trait, you can apply it to a different trait.”

Looking at the history of corn breeding, researchers were able to get plants to silk and shed pollen in relative synchrony. This was a bigger issue 30 years ago than it is now, as breeding has solved it. Today, breeders will be doing similar things with reduced stature corn.

“Reduced stature corn is going to open what I call the ‘agronomic playbook,’” Wardyn says. “It is going to allow us to go back and reevaluate some basic agronomic principles and let us look at how we can improve them to get higher yields.

“The rate the industry and growers are successful at this will determine if or how fast reduced stature corn becomes the norm. The potential is there, but there are still a lot of ifs to consider,” he notes.

 

Testing in progress

There is considerable acreage of reduced stature corn in testing and it’s growing.

“I’m proud of the process we have,” Wardyn says. “Yes, we want to deliver reduced stature corn, but the overriding principle is that it has to be right for our customers. We can’t put any customer at risk and we’re not going to do that. When we launch a product, we’ll do it in a way that helps a customer become more profitable. Plus, we’ll promote it by sharing its strengths and weaknesses. This line of thinking is a purposeful part of every meeting we have across all team members.”

He notes that reduced stature corn will not require a significant “change of iron” by any customer, nor will it require a massive shift in how they operate equipment.

“As we start to open up the agronomic playbook, I think this is where it’ll get fun for some folks when we start to optimize practices that are going to be different depending on where a customer operates,” Wardyn says. “It may mean a different row configuration, a different fertility program or a lot of things. These practices will develop over time and it’s going to be amazing to watch.

“I consider the ag industry advanced and technologically savvy and when you see an industry get a technology that has the potential to change what has been thought of as basic agronomy and really change those principles, it’s going to be fun. It certainly doesn’t happen every day,” he adds.

Some things will remain the same. Reduced stature corn is still corn with genetics similar to conventional corn. There will be ways farmers can optimize it whether through fertility, fungicides or perhaps pesticides. Thus, expectations are now that standard practices will be adequate to grow a profitable crop.

“As a corn breeder, the most exciting thing for me with reduced stature corn is, in theory, we’ll be able to take some selection pressure off some agronomic traits that are important in tall corn, such as wind traits and stalk lodging,” Wardyn says.

“Historically those are the biggest yield limiters. So, when we launched a product before, all those boxes had to be checked. With reduced stature corn, we can’t ignore those traits but we can take some selection pressure off. It’s exciting for me to think about where yield can go when you unlock agronomic restrictions and unlock genetic potential. It’s like new tools showed up we never had before.”

 

What farmers can expect

The trait Corteva breeders selected to shorten the corn does so uniformly. Christensen says to expect plant height reduced by about 30% and ear height by about 25%.

There are two questions Christensen gets asked a lot when talking to growers about reduced stature corn. One is about ear height and whether it will get too low for harvest and the other is how the trait holds up against the variety of standability concerns — brittle snap, stalk lodging, root lodging and others. “In all the studies with different genetics and different environments we have done so far, our ears on hybrids are consistently above the 24-inch combine header threshold and we have observed excellent improvement in standability under artificial and natural wind treatments. We’re pleased with how the reduced stature hybrids address these challenges to date.”

“We’ll soon have reduced stature corn in demonstration plots throughout the country for growers to see,” Wardyn says. “They’ll be set up in ways to generate discussion, get a hands-on look and ask questions of local experts who’ll be on hand.

“Before we launch, we’ll have a full data set, just like we do on any new hybrid we have now. I tell folks it is going through the Corteva corn hybrid evaluation process just like every other Corteva hybrid has done. I’m proud of the robustness of characterization we put them through. It’s hard to make it through the evaluation.”

Wardyn sums up the best part about reduced stature corn.

“If you’re good at growing tall corn, you’re going to be good at growing reduced stature corn.”

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Why Preemergence Applications Always Pay Off

*This content was previously published by Corteva Agriscience.

Although it may be tempting to reduce herbicide costs by eliminating preemergence applications on soybean acres this season, doing so can lead to long-term consequences.

“Short-term savings on herbicide applications often result in increased costs and frustration in the long run,” says Scott Pringnitz, Market Development Specialist, Corteva Agriscience.

Here are three ways that incorporating preemergence applications as part of a full weed control program offer long-term benefits:

  1. Shifting weed populations. Weed spectrums are naturally shifting across the Midwest as populations escape herbicide program control. Pringnitz says waterhemp is one weed that will likely present more of a challenge to growers this season. “Waterhemp continues to increase in population density — germinating late into the growing season and affecting more acres over time,” Pringnitz says. A strong preemergence application with residual herbicides will help prevent challenging weeds like waterhemp from gaining a foothold in soybean fields.
  2. More modes of action. The chances of weeds developing resistance to herbicides that utilize a single mode of action are very high. Preemergence applications enable you to use more modes of action and residual herbicides to combat challenging weeds. “Multiple modes of action can control a broader spectrum of weeds and greatly extend the effectiveness of weed control programs,” Pringnitz says.
  3. Less pressure on postemergence applications. Preemergence applications are vital in reducing the yield effect from early season weed competition and extending the postemergence application window. Failure to incorporate preemergence herbicides in your weed control program will likely result in the need for multiple postemergence applications, more weed escapes and greater risk of weed resistance.

Weed pressure will vary by operation, so Corteva Agriscience offers a variety of strong and flexible solutions — including powerful preemergence herbicides for soybean fields.

View the full soybean herbicide portfolio to design a program that makes sense for your customers.

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© 2024 Corteva.
Groundwork – March 2024

Drought Effect on Pod Fill in Soybeans

Introduction

 


Figure 1. Aborted soybean seed pods. Shawn P. Conley Soybean and Wheat Extension Specialist Department of Agronomy University of Wisconsin, Madison.

 

Drought Loss Potential in Soybeans

The greatest risk for potential yield loss due to drought stress in soybean occurs during the R4 through R6 growth stages. When drought persists during these growth stages, soybean flowering stops and plants are unable to compensate for lost pods. Research has shown that from the second through the fourth week of seed fill, a 39 to 45 percent yield decrease can occur when there are four days of visible moisture stress. Drought stress during seed fill can also reduce the number of seeds per pod and seed weight. Understanding the differences between how indeterminate and determinate soybean plants react to dry growing conditions may help in the formulation of drought management decisions. (Note, indeterminate soybean plants continue to grow after flowering has begun, while determinate soybean plants essentially halt vegetative growth once flowering begins.)

 

How a Soybean’s Flowering Habit Can Affect Drought Stress Management

Drought stress can affect indeterminate and determinate soybeans differently.

Indeterminate soybean

Indeterminate soybeans continue growing upward at the tip of the stem for several weeks after flowering begins on the lower stem. Flowering will progress up the stem, and plants will continue to flower for six to eight weeks depending on plant maturity and growing conditions. Most northern commercial varieties are indeterminate. Indeterminate soybean varieties usually grow taller and do well in regions with short growing seasons. Indeterminate flowering-type soybean can recover from a short-term drought during the flowering growth stages by producing flowers and pods over a longer period. An indeterminate flowering growth habit should be advantageous to recovery from drought stress.

Determinate soybean

By contrast, determinate soybean plants complete their growth in height and then produce all the flowers at about the same time. These plants are also usually one-half to two-thirds as tall as indeterminate varieties. Most southern soybean varieties are determinate. Drought during this shorter flowering period can negatively affect a higher percentage of the blooms and ultimately yield potential. Severe drought can have a devastating effect on yield potential in determinate soybean varieties, because flowering stops and plants cannot compensate for lost pods.

 

How Heat and Drought Stress Effect the Soybean Plant

Drought Effects

Drought stress can cause flower and pod abortion, while reducing the number of seeds per pod and seed size. Moderate to severe drought stress can also substantially reduce or stop nitrogen fixation, disrupting seed development. If adequate rainfall occurs and photosynthates become available after R5, the plant may compensate for earlier losses by producing larger seeds (within its genetic capacity). Once the plant reaches R6, pods are not normally aborted.

If soybean plants are drought stressed to the point of losing leaves, it is time to decide whether to leave the plants in the field or cut them for hay. This decision depends on the stage of growth and condition of the plant, along with the producer’s need for soybean hay or the market for this type of hay. Plants with 30 percent of the leaves still attached may be considered for hay. These plants can produce 0.75 to 1.25 tons of dry matter per acre with 13 percent protein and 48 percent in-vitro dry matter digestibility.

Heat Effects

It can be difficult to distinguish the effects of high temperature from the effects of water stress on soybean plants. Often these stresses occur together and magnify each other’s effects. Temperatures above 95 °F (35 °C) can severely stress soybean plants, have been shown to substantially decrease pod set, and may potentially increase leaf loss. Very high soil temperatures of 90 °F (32 °C) or above can cause decreased nodulation and nitrogen fixation in soybean. High soil temperatures are most likely to occur on coarse textured soils and in later-planted bean (like those planted into double-crop production systems). While flower production of indeterminate soybeans can occur for 30 to 40 days or more under good conditions, moisture stress and high temperatures can shorten the flowering period.

 

Problems to Consider When Harvesting a Drought-Stricken Soybean Crop

  1.  Smaller seed size. Combines should be adjusted to keep harvest loss of small seeds to a minimum.
  2. Green seeds. Soybean plants that senesce before maturity due to stress may produce green seed that can result in dockage at the elevator.
  3. Green stems. Plants that die prematurely due to drought may have green stems at harvest that can slow harvest by wrapping on the header.
  4. Pod shattering. This is especially problematic if there are weather cycles of wetting and drying before harvest.
  5. Sprouted seed. Drought-weakened pods can allow moisture to infiltrate the pod and reach the seeds. This can result in seeds sprouting while still in the pod.
  6. Poor grain quality. Drought-stricken soybean pods are more susceptible to invasion by fungi, which can result in reduced grain quality.

 

Management Decisions

Managing stress from insect, disease, or nutrient sources can also help reduce the overall stress load on the plant and potentially limit drought stress-associated yield losses. When drought stress becomes severe, a management decision to keep the crop in the field or harvest the soybean for hay should be made when leaves start to curl and plants begin to defoliate. This decision should be based on the crop condition, growth stage, and on hay needs or hay markets.

Any producer who is considering using the production from a drought-stressed soybean crop for planting next year’s crop should understand that seed produced under severe drought stress, especially during the reproductive grow stages, may exhibit reduced germination and vigor.

 

Article Link

 

Sources
Lenssen, A. 2012. Soybean response to drought. Iowa State University Extension and Outreach, ICM News. https://crops.extension.iastate.edu/cropnews/2012/06/soybean-response-drought
Kness, A. 2020. Yield impact on heat and drought-stressed soybeans. University of Maryland Extension, Maryland Extension News. http://blog.umd.edu/agronomynews/2020/08/07/yield-impact-on-heat-and-drought-stressed-soybeans/
Wright, J., Hicks, D., and Naeve, S. 2006. Predicting the last irrigation for corn and soybeans in Central Minnesota. University of Minnesota Extension. Minnesota Crop eNews. https://blog-crop-news.extension.umn. edu/2018/08/predicting-last-irrigation-for-corn-and.html
Desclaux, D., Huynh, T.T., and Roumet, P. 2000. Identification of soybean plant characteristics that indicate the timing of drought stress. Crop Science. 40(3): 716–722. https://doi.org/10.2135/cropsci2000.403716x
Staton, M. 2020. Moisture stress and high temperature effects on soybean yields. Michigan State University Extension. https://www.canr.msu.edu/news/moisture_ stress_and_high_temperature_effects_on_soybean_yields.
Wiebold, B. 2018. Weather woes in the soybean field. University of Missouri. Integrated Pest Management. https://ipm.missouri.edu/croppest/2018/10/weatherWoes/
Dornbos, D.L. Jr., Mullen, R.E. and Shibles, R.M. 1989. Drought stress effects during seed fill on soybean seed germination and vigor. Crop Science. 29(2): 476–480.https://doi.org/10.2135/cropsci1989.0011183X002900020047x
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ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. Performance may vary, from location to location and from year to year, as local growing,
soil and environmental conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on their growing environment. The recommendations in this material are based upon trial observations and feedback received from a limited number of growers and growing environments. These recommendations should be considered as one reference point and should not be substituted for the professional opinion of agronomists, entomologists or other relevant experts evaluating specific conditions. Bayer and Bayer Cross are registered trademarks of Bayer Group. All other trademarks are the property of their respective owners. ©2024 Bayer Group. All rights reserved. 1322_61731

Conditions That Impact Soybean Germination and Emergence

KEY POINTS

 

The Germination Process

Once planted, a soybean seed begins to absorb water and swell. After the seed has absorbed approximately 50% of its weight in water and if temperature and oxygen conditions are favorable, the radicle breaks through the seedcoat (Figure 1) and rapidly develops into the primary root. Lateral roots emerge from the radicle as it elongates, and root hairs grow from the radicle and lateral roots. The root hairs become the main water- and nutrient-absorbing structures. Root hairs are barely visible and should not be confused with branch roots, which are easily seen and will develop later. Soon after the radicle appears, the hypocotyl, or stem tissue, starts growing and forms a hook shape which pushes upward toward the soil surface. When the hypocotyl emerges into the light it straightens and, in the process, pulls the cotyledons out of the soil (Figure 1). Once the cotyledons reach the surface, they turn green due to exposure to light and open to expose the epicotyl, which contains the main growing point of the soybean plant.

 


Figure 1. Germination and emergence sequence of a soybean seedling.

 

Early Growth

The epicotyl is the stem tissue just above the cotyledons. The epicotyl contains the first leaves, which are unifoliate (one leaf blade attached to opposite sides of the stem at the same node) (Figure 1). The first leaves open quickly to start the photosynthetic process. The first trifoliate leaves subsequently emerge from the growing point above the unifoliate leaves. After about a week, the cotyledons dry and drop off the plant because their stored energy has been depleted. Energy production in the soybean plant is now solely dependent on photosynthesis by the leaves.

 


Figure 2. Soybean hypocotyl hook and cotyledons after emerging through the
soil surface.

 

Challenges to Germination and Emergence

As mentioned above, soybean germination and emergence depend on the soil temperature, moisture, and oxygen levels within the seed zone. A soybean seed can germinate when soil temperatures are less than 55 °F (13 °C); however, germination is likely to be slow until soil temperatures warm to the upper 70s °F (mid 20s °C). When soil temperatures are between 70 °F (21 °C) and 90 °F (32 °C), seedling emergence should occur in less than a week. Lower soil temperatures can cause seeds to remain dormant, increasing their vulnerability to diseases and feeding damage from insects and wildlife, while soil temperatures above 95 °F (35 °C) can inhibit soybean germination and emergence, resulting in reduced stands. High soil temperatures often cause problems with double-cropped soybean, as the planting window will usually occur during the early summer when soil temperatures can often be above 95 °F (35 °C).

If irrigation is required to help fill the soil profile, it should be applied before planting soybean. Cold irrigation water used immediately after planting can cool the soil, slowing germination and early plant growth. Planting into a moist seedbed with good seed-to-soil contact is also necessary for the seed to absorb enough water to germinate. Though if surface moisture is low after planting, a light application of less than an inch of water can be provided by a center pivot irrigation system to help a crop germinate.

Note that more than one application may be required. Heavy rainfall or heavy irrigation can result in surface crusting if the soil surface dries out before emergence. A hard soil crust can delay or prevent seedling emergence and cause soybean hypocotyls to swell or break as they try to push through the crust. If a seedling’s hypocotyl breaks or its cotyledons do not reach the soil surface, it will die. Fields with fine-textured soils, low organic matter, and little surface residue can be vulnerable to crusting, especially where excessive tillage has taken place (Figure 3). A light, center pivot irrigation application of less than an inch of moisture can also be used to soften soil crusts that may be present when seedlings emerge, softening the crust as the cotyledons break through the soil surface.

 


Figure 3. Soybean seedling emerging through a soil crust.

 

Saturated, flooded, and compacted soils can reduce germination and slow emergence due to a lack of oxygen. Soil pore spaces filled with water have less oxygen available for seed respiration.

Compaction reduces the availability of water and oxygen required for germination, growth, and nutrient uptake. However, some management strategies can be used to help reduce the previously mentioned challenges caused by planting soybean into low soil temperatures. A seed treatment like Acceleron® Seed Applied Solutions can help protect seedlings from fungal diseases like Phytophthora, Pythium, Rhizoctonia, or Fusarium, and an insecticide can help control early season pests. Additionally, If plant-parasitic nematodes such as soybean cist nematode (SCN) are known to be a problem, a seed treatment like ILeVO® seed treatment can also be added to help protect the plants.

 

Pre-emergence Residual Herbicide Injury to Emerging Soybean

Pre-emergence, PPO-inhibiting herbicides are commonly applied as part of a pre-mix product used as additional modes-of-action to control a broad weed spectrum. The label states that these products are to be applied within three days after planting. If pre-emergence, PPO-inhibiting herbicides do cause damage to soybean plants, the signs of damage will often show up in cool, wet conditions that can slow emergence. Damage can also be observed if the growing season has been dry after planting, but the grower receives a light rainfall of 0.2 to 0.4 inches of moisture or applies a light irrigation just as the soybeans emerge through the soil surface. In this scenario, the dry post-planting conditions could have allowed the herbicide to remain on the soil surface and the following moisture could have moved the herbicide into the seed row and onto the emerging seedlings. The signs of PPO-inhibiting herbicide damage to the seedlings can vary, but often appear as a “burnt” reddish-colored spot to the side of the hypocotyl or on the already emerged leaves, which can often be confused with phytophthora root rot. Always read and follow the herbicide label directions.

 


Figure 4. Injury to soybeans seedlings caused by a pre-emergence application of flumioxazin.

 

Article Link

 

Sources:
Specht, J., Rees, J., Elmore, R., Mueller, N., and Glewen, K. 2019. Soybean germination/emergence with April planting dates relative to coincident air and soil temperatures in April and May. University of Nebraska – Lincoln. Institute of Agriculture and Natural Resources. Cropwatch.
https://cropwatch.unl.edu/2019/soybean-germination-tracking
Rees, J., Jhala, A., and Jakson-Ziems, T. 2020. Q & A: What is causing problems with soybean emergence? University of Nebraska – Lincoln. Institute of Agriculture and Natural Resources. Cropwatch. https://cropwatch.unl.edu/2020/q-what-causing-problems-soybean-emergence
Acceleron® soybean protection. Bayer Crop Science https://www.cropscience.bayer.us/seedgrowth/acceleron/soybean
Egli, D.B., Hatfield, J.L., Hill, J., and TeKrony, D.M. 1973. The influence of soil temperature on soybean seed emergence. University of Kentucky UKnowledge. Agronomy Notes. 5-1973
https://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1200&context=pss_notes
Legal Statements
ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. FOR SOYBEANS SEED TREATMENT PRODUCTS APPLIED DOWNSTREAM, EACH ACCELERON® SEED APPLIED SOLUTIONS OFFERING is a combination of separate individually registered products containing the active ingredients: BASIC Offering: metalaxyl, penflufen and prothioconazole. STANDARD Offering: metalaxyl, penflufen, prothioconazole and imidacloprid. FOR UPSTREAM TREATED SOYBEANS, EACH ACCELERON® SEED APPLIED SOLUTIONS OFFERING is a combination of separate individually registered products containing the active ingredients: BASIC Offering: metalaxyl, fluxapyroxad, and pyraclostrobin. STANDARD Offering: metalaxyl, fluxapyroxad, pyraclostrobin and imidacloprid. Not all products are registered in all states and may be subject to use restrictions. The distribution, sale, or use of an unregistered pesticide is a violation of federal and/or state law and is strictly prohibited. Acceleron® and Bayer are registered trademarks of Bayer Group. ILeVO® is a trademark of BASF Corporation. ©2024 Bayer Group. All rights reserved. Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenev-er possible and should consider the impacts of these conditions on the grower’s fields. The recommendations in this material are based upon trial observations and feedback received from a limited number of growers and growing environments. These recommendations should be consid-ered as one reference point and should not be substituted for the professional opinion of agronomists, entomologists or other relevant experts evaluating specific conditions. Bayer and Bayer Cross are registered trademarks of Bayer Group. All other trademarks are the property of their respective owners. ©2023 Bayer Group. All rights reserved. 1316_115191