Postemerge Apps: Are You Waiting Too Long?

It’s another POST soybean season, and we have largely been receiving questions similar to the past several years. These include how to manage giant ragweed, pokeweed and a few other perennials, vines, and marestail (good luck there). It is evident however that many fields were subject to a more comprehensive approach to marestail management, starting with herbicides applied last fall, and marestail problems are generally less frequent.

But this is possibly offset by the lack of rain to effectively activate residual herbicides in areas, which results in higher and more varied weed populations to manage with POST herbicides. Some things to consider relative to POST treatments in soybeans:

  • With regard to POST control of marestail, hiring a crew of people with hoes is probably the most effective option. Or at least cut the top of the plants off so that seed production is prevented. We are hearing mostly about difficulty controlling plants that overwintered in fields not treated last fall (there’s a lesson here somewhere).Plants that have survived one herbicide treatment already are not likely to be controlled by anything. In fields where the population is still sensitive to ALS inhibitors, a high rate of Classic or FirstRate may provide some suppression or control of small plants. Flexstar and other PPO inhibitors have essentially no activity on marestail, and low rates of 2,4-DB are not much better.
  • The most effective approach on giant ragweed is to make an initial POST application when plants are small — 4 to 8 inches — and then make a second application about 3 weeks later. Delaying the initial application until plants are larger in order to control late emerging plants usually results in less effective control. OSU research has shown that the single application approach on larger plants results in a higher frequency of plant survival, and higher numbers of seed returned to the soil seedbank. In populations that still mostly respond to glyphosate (at least 80% control/suppression with the first application), use a glyphosate rate of 1.5 lbs ae/A in the first POST application, and 0.75 lb/A in the second application. Populations resistant to glyphosate are usually also resistant to ALS inhibitors, and the most effective strategy for these will be to mix glyphosate with Flexstar (or use Flexstar GT). Use the highest labeled Flexstar or Flexstar GT rate for your geography, and include crop oil concentrate or a methylated seed oil. Flexstar is usually about an 80% herbicide for control of giant ragweed, so it’s possible that a later application of Cobra may be needed.
  • For pokeweed and many other perennials, a high rate of glyphosate is often more effective than anything else. Pokeweed control can be improved by increasing the spray volume, or slowing down, to improve coverage. Applying when plants have some size but are still completely underneath the spray boom can reduce the potential for regrowth, but be prepared to treat again later in the season when plants do regrow. The addition of Flexstar or Cobra can help with the dessication of perennial vines, and any of the products rated 8 or better on morningglory in the “Weed Control Guide for Ohio and Indiana” can help with annual morningglory control. The addition of Classic may improve control of dandelions.
  • We have heard often about the strategy of waiting until soybeans are about to canopy to apply POST herbicides. The principle here appears to be that the herbicide will control emerged weeds and then the shade from the crop canopy will prevent later-emerging weeds from competing with the soybeans. This can work where the residual herbicides applied prior to planting actually control most of the weeds up to this point, so that the POST treatment is primarily addressing a few small weeds. However, in fields where the residual herbicides are not this effective, or where certain tougher weeds are present (giant ragweed, waterhemp, etc), applying POST herbicides just prior to canopy closure is really too late. No-till soybeans may require 8 weeks to canopy, and delaying POST applications until this late inevitably means that at least some of the weeds are large. It’s almost always a more effective strategy in the end to make an earlier POST application to smaller weeds, and then a second application 3 weeks later if needed. Keep in mind also that as soybean planting is delayed later into the season, the first POST application occurs later in the overall weed emergence cycle, so the likelihood of needing a second POST application is reduced.
  • A final note about late-planted soybeans and POST application of herbicides. When soybeans are planted later in the season, the progression through growth stages occurs during a shorter period of time. Flowering can occur when plants are relatively small, and delaying POST applications until plants are a certain size may result in treatment of plants at a more advanced growth stage compared with early planting. Glyphosate labels specify that the herbicide can be applied through the R2 stage. The R3 stage starts when a pod of at least 3/16 inch long appears at one of the four uppermost nodes with a fully-developed leaf on the main stem. Applying after the R2 stage increases the risk of injury to soybeans. Injury is not necessarily likely for late applications, but can range from transient “yellow flash” to abortion of pods on the upper part of the plant. Labels for other herbicides may also specify application prior to flowering. Late application of some POST herbicides with substantial residual in soil, such as Flexstar, can also increase the risk of carryover especially if dry weather occurs afterward.

Leave a Reply

Herbicides Stories

glyphosate
Crop InputsWHO: Glyphosate ‘Unlikely’ To Cause Cancer
May 16, 2016
Via Reuters.com reporter Kate Kelland: The weed-killing pesticide glyphosate, made by Monsanto and widely used in agriculture and by gardeners, Read More
dicamba, Roundup Ready 2 Xtend
Crop InputsASA: No Dicamba On Roundup Ready 2 Xtend In 2016
May 16, 2016
While Roundup Ready 2 Xtend (RR2X) are tolerant to dicamba and glyphosate herbicides, no dicamba herbicides will be approved for Read More
CPP, glyphosate, dicamba, atrazine,
Crop InputsReport: Congress Pressing EPA On Glyphosate Review Handling
May 16, 2016
Via Reuters.com reporter P.J. Huffstutter: U.S. lawmakers have asked EPA to explain why it published – and then withdrew – Read More
Palmer amaranth in soybean stubble
HerbicidesWSSA: Billions In Potential Economic Losses From Uncontrolled Weeds
May 4, 2016
What losses would corn and soybean growers experience if they were forced to eliminate herbicides and other control techniques from Read More
Trending Articles
The Andersons Waterloo
ManagementFirst Indiana Facility Certified Under 4R Nutrient Stewardship Certification Program
June 27, 2016
The 4R Nutrient Stewardship Certification Program has announced that The Andersons, Inc.’s Waterloo, IN, facility has been added to its Read More
Food IT
Industry NewsCalifornia Event Will Mix Ag And Tech Professionals To Explore IT Solutions
June 20, 2016
Silicon Valley is hot on agriculture, and an upcoming event in California will bring together the food and tech industries Read More
Monsanto Luling Plant
Eric SfiligojWhat’s Next For Monsanto?
May 31, 2016
For the folks at Monsanto’s headquarters in St. Louis, MO, it has been an eventful few weeks. Back on May Read More
Soybean Plant closeup
Industry NewsMonsanto Rejects Bayer Bid; Open To More Talks
May 25, 2016
Monsanto Co, the world’s largest seed company, turned down Bayer AG’s $62 billion acquisition bid as “incomplete and financially inadequate” Read More
Eric SfiligojPity The Monsanto Haters
May 23, 2016
An old saying goes thus: “Be careful what you wish for. You just might get it.” Given current events, there Read More
Monsanto Luling Plant
Crop InputsReport: Bayer Eyeing Monsanto For Possible Mega-Merger
May 16, 2016
Bayer AG is exploring a potential bid for U.S. competitor Monsanto Co. in a deal that would create the world’s Read More
Latest News
Industry NewsCompass Minerals Names Protassium+ Sales Director
June 28, 2016
Compass Minerals, the producer of Protassium+ sulfate of potash, has named Scott Johnson director of sales for Protassium+ North America. Read More
Eric SfiligojThe British Leave Vote Impact On EU Agriculture
June 27, 2016
Like virtually everyone else in the world this past week, I’ve watched the global reaction to Britain’s vote to exit Read More
Valent David Nothmann
Crop InputsValent Names David Nothmann Vice President Of Marketing
June 27, 2016
Valent U.S.A. Corp. has announced the appointment of David Nothmann as vice president of marketing. Nothmann will be responsible for Read More
The Andersons Waterloo
ManagementFirst Indiana Facility Certified Under 4R Nutrient Stew…
June 27, 2016
The 4R Nutrient Stewardship Certification Program has announced that The Andersons, Inc.’s Waterloo, IN, facility has been added to its Read More
CropLife 100BRANDT Contributes To Mayo Clinic
June 27, 2016
BRANDT, a leading agriculture retailer and manufacturer of specialty agriculture products, has pledged a sizable donation to the Mayo Clinic Read More
ManagementReport: U.S. Farmland Values On Average Continue Gradua…
June 27, 2016
Land price movement has been in the forefront for farmers, landowners, lenders and land brokers for a decade now during Read More
AdjuvantsPrecision Laboratories Introduces Intact Xtra For Use W…
June 27, 2016
Precision Laboratories announces the launch of Intact Xtra, an adjuvant designed for applicators who want to maximize spray applications and Read More
Key Cooperative’s new liquid fertilizer, crop chemical, and seed facility in Grinnell, IA stores 1.5 million gallons of liquid fertilizer and 120,000 gallons of crop chemicals.
Retail FacilitiesKey Cooperative Facility Features The Latest In Murray …
June 27, 2016
When Key Cooperative wanted to improve the overall efficiency at its new state-of-the-art facility in Grinnell, IA, it turned to Read More
Allen Summers Asmark Institute PSM Training
EmployeesAsmark Institute Sponsors 25 ResponsibleAg Auditor Trai…
June 21, 2016
The Asmark Institute has announced plans to provide 25 scholarships through their affiliated State fertilizer and agrichemical associations to help encourage Read More
Kennebec Elevator
CropLife 100Wheat Growers Celebrates Kennebec Elevator Grand Openin…
June 21, 2016
Over 250 patrons and guests were on hand to celebrate Wheat Growers’ Kennebec Elevator grand opening. South Dakota Secretary of Read More
Land O Lakes
Industry NewsLand O’Lakes To Acquire Ceres
June 20, 2016
Land O’ Lakes, Inc. and Ceres, Inc. have announced that they have signed a definitive merger agreement under which Land O’Lakes Read More
Food IT
Industry NewsCalifornia Event Will Mix Ag And Tech Professionals To …
June 20, 2016
Silicon Valley is hot on agriculture, and an upcoming event in California will bring together the food and tech industries Read More
LegislationVilsack Christens WinField Crop Adventure, Talks Monsan…
June 17, 2016
The WinField Crop Adventure grand opening ceremonies at Fair Oaks Farms (near Winemac, IN) welcomed USDA Secretary of Agriculture Tom Read More
Crop InputsFields of the Future: WinField Doubles Down with Agri-E…
June 16, 2016
Land O’Lakes, Inc., joined by Agriculture Secretary Tom Vilsack, today will host Fields of the Future, a live broadcast about Read More
Bayer sign
Crop InputsMonsanto Turns Back Bayer Again
June 14, 2016
Bayer AG’s bid to buy Monsanto Co. for more than $60 billion has hit an impasse that could pose a Read More
Young Corn Field
Crop NutritionNext Generation Of Nitrogen Efficiency Technology Conve…
June 13, 2016
Commodity price projections indicate farmers will continue to face tight margins again this year. Delivering higher yields and more bushels Read More
Industry NewsLa Crosse Seed Acquires Cover Crop Solutions Brand
June 10, 2016
La Crosse Seed announces that it is the new and exclusive owner of the entire family of brands of Cover Read More