Soybean South February 2022

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K-State researchers hope to crush weed seeds

Stamp out ‘hidden hunger’ New tool evaluates in-season tissue K levels

Stamp out ‘hidden hunger’ New tool evaluates in-season tissue potassium levels, maximizing profitability.

By Trent Roberts

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These thresholds also can help producers decide when in-season potassium applications may be profitable and ensure that potassium is not limiting yield.

The work

The results

NICK KORDSMEIER, UNIVERSITY OF ARKANSAS

otassium deficiency is the primary limitation affecting soybean profitability. Producers must carefully manage potassium levels in their soil by knowing when, where and how much fertilizer to apply.

Soybean yield and profitability suffer when potassium levels are too low, but producers have few tools until now to guide potassium management decisions. Current potassium fertilizer recommendations are based solely on soil test results, which may not provide the precision required to maximize profitability.

But a new tool that looks at in-season potassium tissue levels can help growers ensure Dr. Trent that the macronutrient is not limiting soybean Roberts production potential or profitability. It also can help protect their crop from “hidden hunger,” where plants experience a yield-robbing nutrient deficiency but otherwise look healthy.

These soybeans show symptoms of potassium deficiency in the upper canopy.

Over 20 years, soil fertility researchers with the Arkansas Agricultural Experiment Station conducted a series of experimental trials to evaluate the response of soybeans to potassium fertilizer rates. Among the researchers was Dr. Nathan Slaton, an agronomist and associate vice president for agriculture and assistant director of the Arkansas Agricultural Experiment Station.

The data was correlated and calibrated with soil test data, allowing researchers to make more targeted estimates about when and where potassium deficiency might occur. Where potassium deficiency occurs, there is a risk of limiting soybean yield. With the help of agricultural economist Mike Popp, the data from these trials was used to develop a tool that producers can use to estimate profit-maximizing potassium fertilizer application rates based on their input data.

More recently, the soil fertility team began looking at soybean tissue potassium concentrations over a series of growth stages. Previous research had shown that tissue potassium concentration decreased as soybean crops continued to grow. Thus, the timing of sample collection is crucial to interpreting tissue potassium concentrations. This work resulted in the development of dynamic critical potassium concentration thresholds.

Using the dynamic thresholds allowed researchers to use soybean tissue potassium levels to evaluate in-season soybean crops for “hidden hunger” or to corroborate visual potassium deficiency symptoms.

The two tools developed as a result of these experiments — the profit-maximizing potassium rate estimation tool and the dynamic critical tissue potassium concentration thresholds — can help producers maximize soybean yield potential and profitability. They can use this information to closely monitor the potassium status of the soil in their fields and the tissue concentration of a growing soybean crop.

ON THE COVER: Potassium deficiency symptoms are visible during the early vegetative growth stages of soybean. The leaves are yellowing, starting from the outside and working its way in. Photo by Dr. Rasel Parvej, LSU AgCenter.

Dr. Trent Roberts is University of Arkansas’ Endowed Chair in Soil Fertility Research. This research was sponsored in part by the Arkansas Soybean Promotion Board and fertilizer tonnage fees.

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The value

The profit-maximizing potassium rate estimation tool can provide a data-based potassium fertilizer rate that balances fertilizer and application costs with Visit the Potassium Rate yield potential, existing potasCalculators at https://bit. sium concentrations in the soil ly/3z2VJSt. and soybean prices. At typical soybean prices and input costs, producers can potentially increase profits by over $100 per acre compared to applying no fertilizer, depending on field-specific parameters. This tool can also help producers determine when applying fertilizer may hurt their profits.

By using the dynamic critical tissue potassium concentration thresholds, producers could see a net return of about $25 per acre for a field suffering from hidden hunger and about $75 per acre for a field suffering from extreme potassium deficiency, assuming typical soybean prices and potassium fertilizer and application costs.

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HERBICIDE HERBICIDE

12/20/21 12/20/21 9:16 9:16 AM AM

Herbicide-resistant weeds? K-State researcher hopes to crush them

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COURTESY KANSAS STATE UNIVERSITY

Kansas State University researcher in western Kansas says he may have an innovative way to kill weeds commonly found in farmers’ fields: simply put, he wants to crush ‘em.

That’s right, says Vipan Kumar, a weed scientist at the Agricultural Research Center in Hays. He’s studying a method known as harvest weed seed control (HWSC) in which weed seed is literally pulverized by a rolling cage mill that is fitted to the back of a combine during harvest.

“This system is designed to destroy weed seed so that it becomes non-viable for the next year,” Kumar said.

System developed in Australia

HWSC is a method first advanced in western Australia, known as a region where weeds have developed widespread resistance to common herbicides. A version of the rolling cage mill was actually developed by an Australian grower who had become frustrated with other weed control methods.

Current versions are now being manufactured in Canada. Kumar is cooperating with weed scientists at the University of Arkansas and Iowa State University to find out just how effective a system that crushes weed seeds will work in Midwest, High Plains and Mid-South fields.

“This system is designed to destroy weed seed so that it becomes non-viable for the next year.”

“We don’t know much yet; not much data exists in the United States,” Kumar said. “We conducted a preliminary test this fall in a grower’s field, and the seed crusher did a decent job of crushing Palmer amaranth seeds in a milo field that was heavily infested with pigweed.

“I’m hoping that this will be a good fit for wheat and soybean folks, as well, and that they can make good use of it in terms of integrated weed management.”

Another mode of action

Kansas farmers routinely face such nemeses as Palmer amaranth and other pigweeds, feral ryegrass and kochia, which have developed varying degrees of herbicide resistance. Because of that — plus the rising cost of chemicals and other inputs — farmers need options for managing weeds.

“Longer term from a weed science standpoint, HWSC is one type of technique farmers need to bring into their management system if they want to mitigate the resistant weeds on the farm,” Kumar said. “There is no silver bullet; you won’t resolve all your problems with this method, but it can play a critical role in integrated weed management if you combine it with other meth-

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Researchers in Kansas, Arkansas and Iowa are studying the effectiveness of a roller cage mill, which can be attached to the back of a combine during harvest to — literally — crush weed seeds.

ods of weed control, and you are making weed management plans for the next four to five years rather than an annual basis.”

A look at the economics

To date, the roller cage system is likely cost-prohibitive for farmers — manufacturers currently are producing the system for about $75,000. And using the system will require a combine to pull at 80 to 100 horsepower. But Kumar anticipates the price for the system will come down over time as the technology is perfected and use becomes more widespread.

“As a researcher, we can show growers if it’s going to work or not,” Kumar said. “Over the next couple of years, we will create data from Kansas fields; we already have data from Iowa where we found that this system destroys more than 90% of waterhemp seed, using the same unit from the same manufacturer that we are using.”

He anticipates that K-State’s research also will dig deeper into the economics, factoring in the cost of equipment, potential yield increases and decreased use of herbicides.

Kumar, who is part of a national core of scientists that go by the acronym GROW (Getting Rid Of Weeds), has also studied the benefits of another HWSC known as chaff lining. With this system, crop chaff and weeds are funneled into narrow rows behind a combine. The residue is left to over-winter, causing the weed seed to decay, or the thin row can be destroyed with fire.

He has teamed with weed scientists from Colorado, Oklahoma, Iowa and Arkansas on two multi-state grants funded the by U.S. Department of Agriculture’s National Institute of Food and Agriculture. The researchers will explore the effectiveness and relevance of chaff lining and the seed crusher as part of integrated weed management in the High Plains’ no-till dryland crop production systems.

Kansas State University contributed this article.

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