Soybean South April 2021

ONE GROWER PUBLISHING, LLC

APRIL 2021

Don’t ignore sulfur deficiencies

An ominous sign UArk confirms pigweed resistant to glufosinate

An ominous discovery Confirmation of glufosinate-resistant pigweed prompts call to scout fields, overlap residuals and rotate effective herbicide MOAs.

By Tom Barber, Jason Norsworthy and Tommy Butts

L

PHOTOS BY TOM BARBER, UNIVERSITY OF ARKANSAS

ast summer, an article was written about two fields of concern in Mississippi County, Arkansas, where three applications of glufosinate failed to control Palmer amaranth. Upon first observation of these fields, a familiar sinking feeling started to appear. These left-over pigweed plants were in only a few spots across each field and appeared to be the result of seed from a surviving mother plant the year before. Usually, if resistance is occurring, it will be in small spots across the field or in a combined pattern from harvest the year before. These two fields had that “look” and were very suspicious, to say the least.

Areas of live plants were flagged in each field, and plants were soaked with a 2% solution of Liberty — a brand of glufosinate ­­— and evaluated 10 days later. At the time of evaluation, few plants were dead in the area that had been sprayed earlier.

Seed was collected from both field sites from mature plants for screening in the greenhouse over the winter in Fayetteville, Arkansas.

Greenhouse screening

Dr. Jason Norsworthy’s weed science program at Fayetteville has evaluated both samples from Mississippi County along with another sample collected the previous year from Crittenden County. Initial greenhouse experiments involved a rate response study to evaluate glufosinate effectiveness on these samples.

Initial results indicated that 32 fl oz/A Liberty (glufosinate) was not providing control in the greenhouse, and survivors were present following applications up to 256 oz/A. The “official” screening process to confirm resistance involved a glufosinate rate titration where several rates (up to

Figure 1. Palmer amaranth survivors following three applications of glufosinate in Mississippi County, Arkansas.

16X the labeled rate of 29 oz/A) of glufosinate were sprayed on the populations in question.

Based on a LD50 curve, which is the lethal dose needed to kill 50% of the population, and compared to a susceptible standard, the biotype from Crittenden County is 3.5 times less sensitive to glufosinate.

Rate titrations are continuing on the Mississippi County pop-

Figure 2: Palmer amaranth survivors following five applications of glufosinate in Crittenden County, Arkansas.

2

SOYBEAN SOUTH

|

APRIL 2021

SOYBEANSOUTH.COM

Figure 3: Glufosinate rate response on a Mississippi County Palmer amaranth population.

ulations, but at this time they appear to be at least 15 times more resistant than the susceptible standard used for evaluation in the screening.

According to Ian Heap’s International Resistant Weed Database, only four weeds in the world have developed resistance to glufosinate. They are perennial ryegrass (New Zealand), rigid ryegrass (Greece), Italian ryegrass (New Zealand, Oregon and California) and goosegrass (Malaysia).

This finding will represent the first documented case of broadleaf resistance to glufosinate herbicide in the world. It is not a big surprise that Palmer amaranth has developed resistance to glufosinate considering the history.

Cotton producers have heavily relied on glufosinate since 2007, where widespread occurrence of glyphosate (Roundup) resistance was present in pigweed populations across this geography. Twelve or more years of selection pressure due to glyphosate resistance in Northeast Arkansas has placed tremendous stress on several key herbicide modes of action including glufosinate.

Further testing is currently underway to determine if there is multiple resistance to other herbicide modes of action in these three populations. Currently, this problem does not appear to be widespread across Mississippi and Crittenden county or the Northeast Arkansas area based on extensive screenings that have occurred in this geography.

But it stands to reason that several other fields likely contain small segments of the Palmer amaranth population that has reduced sensitivity to glufosinate.

Be alert for escaped pigweed

Special care should be taken in the future to remove any escaped pigweed populations prior to seed set, regardless of the crop grown in the field. In addition, we are recommending that these fields be rotated to corn if possible due to the herbicide alternatives available in a corn production system.

These populations have also been screened with dicamba (Engenia). At this time, it appears dicamba has good activity on two of the three glufosinate-resistant populations. Growers who will not rotate from cotton or soybeans should strongly consider the Enlist system as a technology rotation because Enlist One plus glufosinate continues to provide the most consistent control of emerged Palmer amaranth following a single application.

TWITTER: @SOYBEANSOUTH

Figure 4: Number of Palmer amaranth survivors per 100 plants evaluated following multiple rates of glufosinate.

Applying multiple residuals at planting such as Brake + Cotoran in cotton or metribuzin + Group 15 (Anthem/Zidua, Dual Magnum, Outlook etc.) in soybeans is HIGHLY recommended, regardless of the technology planted, as well as overlapping Group 15 residuals post to reduce Palmer amaranth emergence throughout the season.

Unfortunately, all of these suggestions are short-term solutions. A long-term solution for pigweed control in Arkansas will require multiple cultural tactics including deep tillage, cover crops, crop rotation, hand weeding and harvest weed seed control to reduce selection pressure from current herbicides and increase focus on pigweed seed bank reduction. Our neighbors in Tennessee have identified dicamba-resistant pigweed, and it stands to reason scattered populations also exist in Arkansas. There is currently now more than ever a dire need for development of a new herbicide mode of action for pigweed control in Arkansas.

Dr. Tom Barber is a University of Arkansas Extension weed scientist. He may be reached at tbarber@uada.edu. Dr. Jason Norsworthy is a University of Arkansas weed science professor.

Dr. Tommy Butts is a University of Arkansas Extension weed scientist.

On the cover: To slow herbicide resistance, take an integrated approach to control Palmer pigweed in your fields.

PHOTO BY VICKY BOYD

APRIL 2021

|

SOYBEAN SOUTH

3

The quiet nutrient COURTESY NORTH CAROLINA STATE UNIVERSITY

Don’t ignore sulfur deficiencies, which have become more common as atmospheric emissions grow cleaner.

By Larry Oldham

S

ulfur is essential for growing plants. It is a component of two of the amino acids that make up proteins. According to The Fertilizer Institute, 200 bushels of corn per acre remove 16 pounds and 60 bushels of soybeans per acre remove 11 pounds S per acre. From North Carolina State: cotton seed and lint remove about 5 pounds S per acre.

Practically all S used by plants is absorbed as the sulfate ion (SO42-) from soil.

Most S already present in the soil or added by manures such as poultry litter is in organic forms that become plant available when mineralized to the form used by plants.

Rates of these microbially mediated conversions depend on temperature and moisture in the soil For many years, soil S was supplemented by atmospheric deposition — it literally fell out of the sky. However, with improved smokestack scrubbing technology and widespread use of automobile catalytic converters following implementation of the Clean Air Act, S deposition decreased over the decades.

Prone to leaching

Another similarity with nitrate nitrogen is because the plant-available sulfate ion is negatively charged, it is prone to leaching downward through the profile with water movement. Leaching loss of either ion is more likely in coarse-textured, sandy soils.

Sulfur also was once co-applied through some pesticides and phosphorus fertilizers of which it was a component. Over time, these sources have been refined to have less or no S content or are no longer used.

Soil testing for phosphorus and potassium provides insight into plant-available levels of each in the soil and provides the basis for relevant recommendations.

4

SOYBEAN SOUTH

|

APRIL 2021

Normal plants (left) and S-deficient plants (right).

With S, however, similarities with nitrate (e. g. mineralization from organic matter, leaching vulnerability) also hamper developing soil tests that can provide interpretable results in our warm, humid southern climate.

Note: Public laboratories in Tennessee, South Carolina and North Carolina provide some S soil testing services. The Mississippi State University Extension Soil Testing Laboratory provides estimated soil S if organic matter is requested.

The estimation of soil S based on organic matter is from past research completed in Mississippi.

Symptoms and solutions

Determining whether S deficiency exists depends on observation and using what you know. Based on the issues above, sulfur deficiencies will occur first on sandy, low CEC soils and/or low organic matter soils that may be higher in the landscape, such as a ridge.

Young leaves that are pale green to yellow in color are a symptom of S deficiency. Older leaves on the same plant may be darker as S does not relocate within growing plants.

S-deficient plants will be spindly and

small with slow growth and delayed fruiting. Early season deficiencies do not always carry forward to reduced yields because soil organic S, the chief source of “natural” S, mineralizes more rapidly to the plant-available sulfate form as it warms up.

Deficiency can be addressed early in the growing season with a fertilizer containing readily available S, i.e. sulfate, but note the location and adjust nutrient planning for the next crop.

Several sources of fertilizer sulfur are available (see https://bit.ly/2LXrW9T).

Elemental S must be oxidized to the sulfate form before it is available to plants, which takes several months.

Readily available (in the plant nutrition sense of available) S sources include ammonium sulfate, potassium sulfate, gypsum, and zinc sulfate. The fertilizer industry has developed various nitrogen-phosphorus-sulfur products that are being evaluated by Extension across the country.

Larry Oldham is a Mississippi State University Extension soil fertility and nutrient management specialist. He may be reached at larry.oldham@msstate.edu.

Crop nutrition with sulfur is challenged by: } Amount of S in soil depends on soil organic matter levels. } Less land application of S-containing soil amendments, including manures. } Less atmospheric deposition is occurring. } More leaching potential in coarse-textured, sandy soils. } Increasing crop removal through yield increases. } Inconsistent soil test results on which to base recommendations.

SOYBEANSOUTH.COM