Corn and Soybean Weed Control in a Wet Year

Mike Hunter, CCE – North Country Regional Agriculture Team

small common lambsquarters
Small common lambsquarters that emerged before the soybean planted in this field. Photo taken in Jefferson County June 2019

The cool, wet month of May and start of June has created some challenging weed management situations for both corn and soybean.  Unfortunately, delayed planting seasons force growers to focus so much on getting the corn and soybean planted they may not have had the opportunity to make a timely planned preemergence (PRE) herbicide application.

Here is a common situation that we are already encountering this season.  We have a field with corn or soybeans planted and cool conditions have delayed crop emergence but the weeds have already emerged before the PRE herbicide treatment was made.  Do we stick to our original plan and apply a PRE herbicide to this field or do we need to make adjustments to the herbicide program?

If your planned PRE herbicide application has been delayed, it is very important to carefully consider your herbicide choices and make necessary adjustments if any weeds are emerged at the time of application.  With adequate rainfall, PRE herbicides can provide excellent weed control; however, once the weeds are emerged they will generally need some additional product to the tank mix.  The additional product could be another herbicide to add to the tank mix or just an adjuvant such as non-ionic surfactant (NIS), crop oil concentrate (COC) or methylated seed oil (MSO).  There will be many more options in corn than soybeans.

Corn fields not treated with an herbicide prior to crop emergence need to be looked at carefully.  If very small weeds are emerged at the time of the PRE application the answer may be as simple as adding adjuvant to the PRE herbicide.  Consult the herbicide label and follow the adjuvant recommendations based on the products in the tank mix.

If the corn has emerged and the annual grasses are over 1 inch tall and the broadleaf weeds are 2 to 3 inches tall, it may be necessary to add another herbicide to the PRE herbicide.  If the corn is glyphosate tolerant, you may only need to add glyphosate to the preemergence herbicide program.  Using this same scenario with conventional corn, you will likely need to include a postemergence (POST) herbicide to the PRE herbicide.  Examples of POST tank mix herbicides to consider for control of both emerged annual grasses and broadleaf weeds include: Revulin Q, Realm Q, Resolve Q, Capreno, Laudis, Armezon.  The effectiveness of these POST herbicides varies with the control of different annual grasses making proper weed identification critical.  Again, check the herbicide label prior to making any herbicide applications.

If you are using a PRE soybean herbicide it will likely be an Herbicide Group 2 (Pursuit, Python, Firstrate), 3 (Prowl, Treflan, Sonalan), 5 (TriCor, Dimetric, metribuzin), 7 (Lorox, Linex), 14 (Valor, Sharpen) or 15 (Dual, Warrant, Outlook).  Soon after soybeans are planted, there is a narrow window to make certain PRE herbicide applications.  Valor (flumioxazin), Sharpen (saflufenacil), metribuzin and any premixes containing these active ingredients must be applied prior to crop emergence.  Lorox (linuron) is another PRE soybean herbicide that must also be applied prior to crop emergence.  Prowl, Treflan and Sonalan are applied prior to planting soybeans.

Soybean fields not treated with a PRE herbicide after crop emergence where very small weeds have emerged can be more difficult to deal with, especially if a population herbicide resistant tall waterhemp is present.  Recently, Dr. Bryan Brown, NYS Integrated Pest Management Program, conducted tall waterhemp herbicide resistance screening trials at Cornell University.  Using tall waterhemp seeds collected from three different fields in New York, preliminary results indicate that two populations were resistant to glyphosate (i.e. Roundup, Group 9), three populations resistant to atrazine (i.e. Aatrex, Group 5) and two populations resistant to imazethapyr (i.e. Pursuit, Group 2).  Fortunately, none of the tall waterhemp screened were found to be resistant to lactofen (i.e. Cobra, Group 14).

If a population of multiple resistant tall waterhemp is present, our effective herbicide options are limited.  The PRE herbicides that will provide control of multiple resistant (Group 2, 5, 9) tall waterhemp include Dual, Warrant, Outlook (S-metolachlor, acetolchlor, dimethenamid-P), Prowl, Treflan, Sonalan (pendimethalin, trifluralin, ethafluranlin) Valor SX (flumioxazin) and Lorox, Linex (linuron).  If both the soybeans and multiple resistant tall waterhemp have emerged, our effective herbicide options are very limited.  Dual, Warrant and Outlook are the only PRE herbicides listed that can be applied POST; however, these products will not control emerged weeds.  In this situation it would be necessary to include either Reflex or Cobra (Group 14) to the tank mix to provide control of the emerged tall waterhemp.

Soybeans with the herbicide resistant technologies such as Liberty Link (glufosinate tolerant i.e. Liberty), Xtend (dicamba tolerant i.e.Xtendimax, Engenia, FeXapan) and Enlist E3 (2,4-D i.e. Enlist, glufosinate and glyphosate tolerant) provide additional options for POST control of resistant tall waterhemp.

This spring has provided very limited opportunities to plant corn and soybeans due to frequent rainfall and wet field conditions.  This challenging spring has also made it difficult to apply planned PRE herbicides in a timely manner.  It is important to carefully scout your fields before making any herbicide application to make sure the right products are included in the tank mix. And as always, check the herbicide label prior to making any herbicide applications.


Statewide Corn Trials Underway: Do We Need Neonicotinoid Seed Treatments in NY?

By Jaime Cummings, NYS Integrated Pest Management

No-till corn field
(No-till corn field, photo by Ken Wise, NYS IPM)

Most corn and soybean growers in New York plant seed treated with insecticides.  But are those treatments really needed in every field?  The recent scrutiny on neonicotinoids (aka: neonics) causing harm to pollinators has brought this question to the forefront.  Given all the negative attention that neonicotinoid have received in the media in recent years regarding pollinator health, it’s no surprise that they are on the chopping block in some NY legislative bills.  These bills follow similar bans on neonics that have already taken place in Europe and Canada.  Neonics have a bad reputation as having negative effects on bee health.  Think about all the news you’ve seen or heard about the “bee-pocalypse”.  And, it’s true that they can be lethal to bees and other beneficial pollinators, especially if applied to crops at incorrect timings or under the wrong conditions.  They are insecticides, after all, and they are effective at killing insects, the good ones and the bad ones.  But, let’s not forget about the bad ones they are intended to target to help farmers raise healthy and productive crops to feed livestock and all of us.  It’s important that we consider both the positive and negative effects of these seed treatments when determining an overall need and value in our agricultural systems.

Hundreds of dead bees near a bee colony, believed to have been exposed to neonics
Hundreds of dead bees near a bee colony, believed to have been exposed to neonics (Photo courtesy of D. Schuit)

Neonic insecticides are used in many cropping systems, from fruits and vegetables to ornamentals to corn and soybeans to protect against some troubling pests.  Neonics are available as seed treatments, soil drenches or foliar sprays to various crops, depending on the target pests and formulation of the products.  And farmers everywhere have been using them for years, often as part of an integrated pest management program, to help minimize losses to some destructive pests.  They were so widely adopted because they are effective pest management tools and because they were perceived as having a reduced overall negative impact on the environment and human health compared to many of the older insecticide chemistries, such as the organophosphates.  But then it became apparent that these neonics, though safer for human health than many other insecticides, were taking a toll on beneficial insects, especially our bees that we rely on for pollinating many of our important food crops.  And we need to pay attention to that issue and determine ways to mitigate those risks.

Seedcorn maggots feeding on a corn seed.
Seedcorn maggots feeding on a corn seed. (Photo courtesy of J. Kalisch, University of Nebraska)

As mentioned above, neonics are used in various ways in many cropping systems.  But their use as corn and soybean seed treatments has received a lot of attention because of the known potential for the neonic seed treatments to drift off-target as dust during planting.  Because corn and soybeans cover such vast acreage in NY and across the country, mitigating these risks could potentially have a large impact on reducing bee mortality.  Therefore, these seed treatments seem a likely, and potentially easy target for negative attention when folks start looking to reduce overall neonic usage.  In response, the seed and seed treatment industries working with the EPA, USDA and other organizations started looking into ways that they could reduce the negative impacts that neonic treated corn and soybean seed could have on bees.  In 2013, the EPA held a Pollinator Summit to focus on reducing exposure to dust from treated seed.  Much research went into this focus group with collaborations between industry, academia and governmental regulatory agencies to address the off-target movement of dust from planting neonic treated seeds.  I encourage you to read all the details for yourself regarding what they did and what they discovered in the Dust Focus Group and the Seed Treatment Group.  For the sake of this article, I will briefly summarize their findings, conclusions and recommendations:

  1. Start with clean, quality seed with minimal dust to begin with – many major seed companies have high standards for starting with clean seed which allows seed coating to better adhere, resulting in less dust.
  2. Use improved polymers for adhering treatments to seed coats: seed coating technologies have improved dramatically over the years, meaning that less active ingredient ‘falls off’ the seed coat.
  3. Use bee-safe seed lubricants:  this minimizes the dust moving to off-target plants. New seed coating polymers and polyethylene wax lubricants for talc replacement can result in 90% total dust reduction and 65% active ingredient reduction in the dust.
  4. Eliminate flowering weeds in and around fields prior to planting – this reduces the number of flowers that may accidentally have neonic dust that bees may forage on.  The goal of many field crop growers is to ‘start clean’ meaning that they often try to minimize or eliminate competition from weeds prior to planting.
  5. Be aware of wind speed and wind direction at time of planting – be a good neighbor. Avoid planting on the windiest days (when possible) and be aware of nearby bee colonies and wind direction.  There are actually apps available to connect bee keepers with farmers to raise awareness of pesticide applications and locations of bee colonies.
  6. Re-direct the exhaust flow of vacuum planters downward to minimize off-target movement of dust.  Simple modifications can be made to some planters to re-direct the flow of the exhaust down toward the soil, which can significantly reduce dust movement.  (Many planters don’t exhaust, and most air seeders already exhaust downward).
  7. Follow labeling instructions for handing, storage, planting and disposal of treated seed and containers.
EPA Website Screenshot on Pollinator Protection

The findings, improvements and recommendations from this summit can greatly decrease the risk of negative effects of dust from neonic seed treatments on pollinators.  However, some people are also concerned about neonics that might move through soil or water showing up in water and non-target plants. Because of these issues, neonic seed treatments might still be banned on corn and soybean in NY, so growers are concerned.

Neonic seed treatments come standard in just about every bag of corn and soybean seed planted.  Some say we do need them, others say we don’t.  If we look back in history a few decades, before the neonic seed treatments became available, farmers in NY struggled with early season pests like the seedcorn maggot, which can significantly reduce stand counts (and yields) under high pest pressure.  The seedcorn maggot is favored by situations with actively decomposing organic matter, such as manure applications and terminated cover crops.  Both of these situations are quite common in NY corn and soybean production systems.  But, since the neonic seed treatments became so widely used, the seedcorn maggot issues have decreased dramatically, and possibly faded from memory.

Skips in a corn row caused by seedcorn maggots eating the corn seed.
Skips in a corn row caused by seedcorn maggots eating the corn seed. (Image courtesy of T. Baute, OMAFRA)

But it still begs the question:  Do we have pest pressure that warrants neonic seed treatments on the vast majority of our acres of corn and soybean every year?  That is a good question.  From an integrated management perspective, it’s never recommended to rely so heavily on one tool in the tool box for as long as we have with these neonics.  We know that our insect pests can develop resistance to insecticides, just like weeds develop resistance to herbicides.  There is also some evidence from various academic studies showing that the neonic seed treatments can negatively impact predatory insects, such as ground beetles, and other beneficial insects that serve as natural biological control of some of our other pests, such as slugs.

Given those reasons, it seems like it might be a good idea to reduce our dependence on neonic seed treatments, and only use them in situations where we know we need them, right?  But, on the other hand, it’s important to keep in mind that, for now at least, it’s not easy to buy corn or soybean seed without a neonic as part of the seed treatment package.  It’s very efficient and economical for the seed industry to include these standard treatments on their seed, and it’s also important for their liability for the guarantees they may offer farmers who purchase their seed.  Not to mention that it’s not easy to anticipate when and where we need the neonic seed treatments each year.  Sure, we know that fields with a history of manure and/or cover crops may be more likely to have seedcorn maggot issues, but there’s no guarantee they won’t also show up in a clean, tilled field.  And, once the crop is planted, it’s too late to scout and determine whether or not the seedcorn maggots are going to be a problem.  This unpredictability is why the neonics are used as an ‘insurance plan’ against these sort of pests.

This conundrum is why this is such a highly polarizing debate, and why the proposed legislative bans have some farmers riled up.  It’s not that farmers want to use or pay for any more pesticides than they need, but the risk can be too high for their bottom line to just stop using them altogether.  We need local research to back up any claims for or against the use of insecticidal seed treatments in corn and soybean production, and specifically the need for neonics.  Much of the research from other states shows that they may not be necessary after all, because they don’t seem to cause a yield benefit.  But, upon closer inspection of some of those results, many of those trials didn’t have measurable insect pest pressure to produce meaningful comparisons of treatments.  And, many of the areas where those trials were conducted do not have the same practices with high organic inputs from manure and cover crops that many NY farms do.

Seedcorn maggot damage to soybean
Seedcorn maggot damage to soybean (Photo courtesy of University of Minnesota)

So, it’s complicated.  A great deal of pollinator risk research is being conducted at Cornell, including a risk assessment of neonic use.  The results of that assessment could influence the future of legislation on the availability or restriction of these products.  We know the dust from the neonic seed treatments poses a potential threat to pollinators (especially bees).  But we also know from the EPA pollinator summit that there are ways to mitigate those risks through improved seed coating and seed lubricant technologies, which have been widely implemented.  However, we still don’t know if there are other risks from these seed treatments, and we need more concrete evidence to know whether or not these neonic seed treatments are really necessary in such a large percentage of our NY corn and soybean acreage.

That’s why we decided earlier this spring to coordinate five statewide large-scale, on-farm trials with NYS IPM and Cornell Cooperative Extension specialists to try to evaluate the effect of these neonic seed treatments on 1) plant populations, 2) yield, and 3) how they compare to the anthranilic diamide seed treatments which could potentially replace them if the neonics are banned.  These corn silage trials will be located in eastern, western, northern and central NY, all on farms that have typical NY cropping practices that incorporate manure and/or cover crops.  Stand counts will be taken to measure plant populations and to determine insect pest pressure, and yields will be measured to compare the three seed treatments:  1) neonic + fungicide, 2) diamide + fungicide, and 3) fungicide only.

Regardless of our results, we will need multiple years of study to better characterize the pest pressure.  Ideally, we would determine methods to predict and monitor seedcorn maggot pressure in individual fields and years for exploring biological, cultural and genetic means for suppressing these insects.  However, this has proven challenging, as evidenced by the current situation in Canada.

Stay tuned for the results of these studies later this fall.  And, I want to extend my sincere gratitude to all parties involved for coming together to conduct this research on extremely short notice.  Thank you to Seedway for donating the seed, to Syngenta for donating the seed treatment products, to the participating farmers who are taking time out of their busy schedules to plant and harvest these trials, and to our CCE and PRO-Dairy collaborators (Joe Lawrence, Mike Hunter, Mike Stanyard, Aaron Gabriel and Janice Degni) for volunteering their time to conduct this important research.  THANK YOU!!!