ASK THE AGRONOMIST BLOG

By now, growers should have a pretty solid plan in place for their fields, including what crop will go where and the specific products and populations to plant. This plan should also include an established baseline for soil nutrients — how much you need and where you need it. If you haven’t mapped out your soil fertility goals yet, there’s still time, but it needs to start with soil sampling. Here are a few tips for establishing optimal soil conditions.  

Establish a Baseline
Your corn and soybean seed is more than a means to an end, but products alone will not yield the results you desire unless proper management practices are in place. I recommend growers work with their local Stine agronomist or extension specialist to determine the necessary nutrients for their selected corn hybrids and soybean varieties. Establish a baseline for each field based on those recommendations so that when you do take the soil sample, you’ll be able to determine which nutrients you need to add or if you need to add any at all.

Note Significant Changes from Season to Season
Your fields have a past, and as a grower, you should be responsible for journaling their history — the successes, the struggles, the quirks and everything in between, even soil fertility. Knowing the history of a field’s soil fertility and how it changes from season to season is equally as important as tracking the crops you’ve rotated in and out of the field. Some years, you may not have the time to soil sample, but if you’ve been diligent in taking notes each season, they should give you a good idea of where you need to start for soil fertility.

Keep it Consistent
How often should you take soil samples? Consistently. I recommend yearly soil samples. If that’s not an option, every other year or third year can be considered, depending on crop rotation. It’s especially important to check soil before each rotation of a corn crop and to be consistent when you take your samples. Take them at the same time each year (e.g., pre-planting or post-harvest) for the most accurate and consistent results.

Also, how you take the samples matters. Map out where you plan to take the soil samples each year. From there, we typically recommend that growers use a zig-zag or grid pattern for testing, and take 10 to 15 cores per samples for every 2.5 acres. For no-till, we recommend taking a sample at three to four inches. For tilled soil, samples should be taken at six to eight inches.

Know What the Soil Sample Information Means
A regular soil sample is able to test for available nutrients and soil pH. Nutrients such as phosphorus, potassium, magnesium and calcium can be detected in a normal soil sample test. Having too little of these nutrients can inhibit the growth and vitality of the plants, whereas having too much could be harmful to your crop. It’s also important to remember that certain crops use up more P and K than others, which can significantly impact yield the next season.

A normal soil sample also helps detect pH levels, which indicates the level of acid to alkaline in the soil and availability of each for crop production. If pH levels fall short, liming may be recommended. Your local extension office should be able to provide more localized recommendations for these levels.

You can also request other soil sample tests. Certain soil samples can detect nitrogen, soil salinity, organic carbon, insects (such as soybean cyst nematodes), aluminum and micronutrient levels. It’s important to note that sample measurements may vary for these additional tests, so it’s best practice to consult with your local extension office prior to sampling.

Last in our series of difficult-to-control driver weeds, we discuss kochia (a.k.a. tumbleweed). Kochia can be found in most regions throughout the United States and into Canada and can spread rapidly. From ditches to fences to cropland, kochia’s ability to transfer seed wherever the wind takes it and its broad adaptability mean this pesky weed can wreak havoc on growers’ yields. Learn more about the strengths and weaknesses of kochia and control tactics for this driver weed.

The Bad News (Strengths)

• Kochia has a high rate of pubescence, or “hair covered” leaves, which makes it difficult for herbicides to stay on the leaf long enough to be absorbed.
• It is also extremely bushy, with numerous branches that develop a heavy canopy, making coverage of leaf surface difficult.
• Kochia can reach up to six-feet high and have roots that can extend as much as 15- feet below the soil surface. This makes kochia extremely competitive with other plants in its environment. In fact, according to experts, kochia can reduce yields by up to 70 percent or more.
• Kochia is prone to cross pollination, allowing herbicide-resistant biotypes to pollinate herbicide-susceptible biotypes, leaving the prodigy resistant. This evolutionary advantage, as well as the “tumbleweed” aspect of the plant, allows these resistant biotypes to spread quickly and over long distances.
• In addition to blowing with the wind, as tumbleweeds, kochia can hitch rides on vehicles, travelling from the western Corn Belt areas to central and eastern Corn Belt areas while leaving seed along its path.

The Good News (Weaknesses)

• Kochia seed is viable in the soil for only a short time; however, initial germination rates are high and can range from 89 to 93 percent.
• Kochia seed typically emerges early for a summer annual, in soil temperatures ranging from 37–46° Fahrenheit. The early emergence makes kochia easier to control with cover crops and early spring pre-emerge herbicides.
• Germination is greater in shallow seed depths and decreases as soil depth increases.   

Management Strategies

*Note: Current known herbicide resistance includes Groups 5 (triazines), 9 (glyphosate), 2 (ALS) and 4 (synthetic auxins). There is some known cross resistance (single population) to both Groups 5 and 2.

• Know your resistance profile. Have your population tested to know what herbicides will be effective.
• Start clean. Consider tillage and/or an effective burndown control that eliminates actively growing weeds.
  • Tillage is the most effective but is not a viable alternative in all areas of the country because of erosion concerns and moisture loss.
  • Consult your local agronomist or university extension office for specific burndown recommendations.

• Utilize an effective soil-residual herbicide. Using these products in multiple sequential applications is encouraged where possible to delay and diminish the germination and emergence of kochia to allow soybeans a competitive advantage in row closure, canopy and plant density.
• Utilize an effective post-herbicide program. Apply an effective post-herbicide application(s) for pre-escapes.
• Narrow-row soybeans. In some cases, narrow-row soybeans may be warranted to increase photosynthetic competitiveness and decrease kochia growth habits.
• Cover crops. In some cases, cover crops have shown an increased ability to lessen weed seed bank populations and delay emergence.
• Manual eradication. In severe infestations, manual eradication may be necessary for a few years to get the population under control.
• Harvest eradication. In cases in the South, growers have turned to adding mechanical seed destroyers on their combines to control weed seed escapes.
• Conventional herbicide system. Using a conventional herbicide system that relies on multiple, timed passes with soil-applied residuals and effective post-applied herbicides with current broad spectrum herbicides can be an effective strategy. This system is generally costly; however, the increase in yield from reducing weed competition generally outweighs the cost of treatment.

Trait System Usage

Use trait systems that provide the ability to apply multiple effective modes of action simultaneously to combat herbicide resistance.  For instance, Enlist E3^® system allows the use of Enlist One^® with 2,4-D choline to be applied with both glyphosate and glufosinate and additional tank mix partners to eliminate and delay further kochia emergence. This system combines many of the strategies discussed into a flexible platform.

To learn more about kochia and how to manage this hard-to-control driver weed on your farm, contact your local Stine agronomist or university extension specialist.

Resources and Citations:

Kochia
https://iwilltakeaction.com/weed/kochia

Kochia Management in Soybeans
https://iwilltakeaction.com/uploads/files/57229-7-ta-hrm-factsheet-kochia-r2-lr-final.pdf

Kochia
https://ohiostate.pressbooks.pub/ohionoxiousweeds/chapter/kochia-section/

Next in our series of difficult-to-control driver weeds is a much-too-common foe for growers (and allergy sufferers) — giant ragweed. From farmland to roadside ditches, giant ragweed can be found in most areas of the United States. Its size and adaptive nature make it extremely problematic for corn and soybean growers, especially now that it’s developed resistance to glyphosate and ALS-inhibitor herbicides. Learn more about the strengths and weaknesses of giant ragweed and control tactics for this driver weed.

The Bad News (Strengths)

• Giant ragweed is extremely prolific. It’s not only capable of producing thousands of seeds per plant, but it also produces pollen at an alarming rate, making it a nasty allergen. According to university weed experts, a single plant can produce 10 million pollen grains per day and one billion during its life cycle. When compared to corn, which can produce around 4.5 million during its life cycle, you can clearly see the prolific nature of pollen production in giant ragweed.
• High levels of pollen mean increased chances of cross pollination in giant ragweed. When cross pollination transpires, more herbicide resistance can occur when those herbicide-resistant traits are carried over from plant to plant.
• New populations of giant ragweed are able to germinate and emerge in production fields from March through July.
• Giant ragweed is extremely competitive and can grow up to 16 feet tall, shading out neighboring crops and competing for the sunlight and nutrients necessary for plant vitality and yield.
• It is estimated that one plant for every 110 square feet can reduce yield by up to 50 percent in soybeans.
• Stem-boring insects can be detrimental to glyphosate control of glyphosate-susceptible populations of giant ragweed when applications are made after weeds are 6-inches tall.

The Good News (Weaknesses)

• A monoecious weed, giant ragweed grows separate male and female flowers on the same plant. This structure keeps genetic diversity limited to that plant so that it does not pass on that genetic diversity in the form of a mutation to surrounding giant ragweed plants. This slows the development of resistance and cross resistance to multiple herbicide modes of action, allowing for more time to eliminate that genetic mutation before it becomes something we can’t control.
• No-till systems may have a slight advantage in seed bank persistence because of predation and natural weathering of giant ragweed seeds.
• Controlling stem-boring insects can have a positive effect on giant ragweed control.
• Long-term no-till systems combined with adequate herbicide control programs can lead to improved control of giant ragweed populations and significant reduction of giant ragweed seed bank in the soil.

Management Strategies
*Note: Current known herbicide resistance includes Groups 2 (ALS) and 9 (glyphosate). There is some known cross resistance between the two groups.  

• Know your resistance profile. Have your population tested to know what herbicides will be effective.
• Start clean. Consider tillage and/or an effective burndown to control and eliminate actively growing weeds. Strategies should include multiple effective modes of action for chemical control in burndown. Consult your local agronomist or university extension office for specific burndown recommendations.
• Utilize an effective soil residual herbicide. Using these products in multiple sequential applications is encouraged, where possible, to delay and diminish the germination and emergence of giant ragweed to allow soybeans a competitive advantage in row closure, canopy and plant density.
• Utilize an effective post-herbicide program with layered residuals and multiple modes of action. Apply post-emerge applications to weeds less than six inches tall using multiple herbicides for adequate coverage. Follow early-post applications with a sequential application within 14–21 days to control late-emerging plants.
• Narrow rows. In some cases, narrow-row soybeans may be warranted to increase photosynthetic competitiveness and decrease giant ragweed growth habits.
• Cover crops. Cover crops have shown an increased ability to lessen seed bank populations and delay emergence.
• Manual eradication. In severe infestations, manual eradication may be necessary for several years to get the population under control.
• Conventional herbicide system. Using a conventional herbicide system that relies on multiple, timed passes with soil-applied residuals and effective post-applied herbicides with current broad spectrum herbicides can be an effective strategy. This system is generally costly; however, the increase in yield from reducing weed competition generally outweighs the cost of treatment.

Trait System Usage
Use trait systems that provide the ability to apply multiple effective modes of action simultaneously to combat herbicide resistance. For instance, the Enlist E3^® system allows Enlist One^® with 2,4-D choline to be applied with both glyphosate and glufosinate and additional tank mix partners to eliminate and delay further giant ragweed emergence. This system combines many of the strategies discussed into a flexible platform.

To learn more about giant ragweed and how to manage this hard-to-control driver weed on your farm, contact your local Stine agronomist or university extension specialist.

 Resources and Citations:

Biology and Management of Giant Ragweed
https://weedscience.missouri.edu/publications/gwc-12.pdf

Management of Herbicide-Resistant Giant Ragweed
https://weedscience.missouri.edu/publications/FactSheet_GiantRagweed.pdf

Arkansas Field Crop Weed Control Resources
https://www.uaex.edu/farm-ranch/pest-management/weed/field-crops.aspx

Giant Ragweed
https://iwilltakeaction.com/weed/giant-ragweed

Super Weeds
https://u.osu.edu/osuweeds/super-weeds/

Postemergence Control of Giant Ragweed in Soybeans
https://agcrops.osu.edu/newsletter/corn-newsletter/2014-20/postemergence-control-giant-ragweed-soybeans

Palmer amaranth: ID, biology and management
https://crops.extension.iastate.edu/palmer-amaranth-id-biology-and-management