It’s All About the Weed Seedbank – Part 1: Where Has All the Marestail Gone?

Source: Mark Loux

For the second year in a row, we are scrounging to find enough marestail at the OARDC Western Ag Station to conduct the research we had planned on this weed.  After years of having plenty of marestail, we have had to look around for off-site fields where there is still a high enough population.  Which, since we are scientists after all, or at least make our best attempts, left us thinking about reasons for the lack of marestail, and our overall marestail situation, and seedbanks.

While the short game in weed management is about getting good enough control to prevent weeds from being a yield-limiting factor and interfering with harvest, the long game is about preventing seed production and managing the soil seedbank.  One of the characteristics shared by marestail, giant ragweed, and the nasty pigweeds, waterhemp and Palmer amaranth, is a rapid decline in seed viability in the soil within the first year, and an overall decline to 5% or less viable seed within 3 to 4 years.  Another characteristic of marestail and pigweed seed is a relative lack of dormancy, which results in the potential for an almost immediate increase in population the year following a year of substantial escapes and seed production.  How big that increase is depends upon how many plants go to seed and how many seeds are produced per plant, with the potential of up to about 200,000 seeds per marestail plant and one million per waterhemp or Palmer amaranth plant.  The net result of these two characteristics, though, is that these weeds can ramp up population fast following a year of poor control, but populations can also decline rapidly with good control that prevents seed.

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Adapting Burndown Herbicide Programs to Wet Weather Delays

Source:  Dr Mark Loux, OSU

While it’s not terribly late yet, the wet soils and wet forecast could keep most of us out of the fields for a while.  The questions about how to deal with burndown herbicide treatments in delayed planting situations are rolling in.  One of the most common ones, predictably, is how to kill glyphosate-resistant marestail and giant ragweed and generally big weeds in soybeans when it’s not possible to delay planting long enough to use 2,4-D ester (Enlist soybeans excluded).  While we wrote last week about marestail populations being on the decline, this does not mean it’s gone by any means.  Overwintered marestail plants become tougher to kill in May, and the fact that fall weather was not conducive for herbicide applications makes the situation worse in some fields.  The good news is that we have some additional herbicide/trait options for help with burndown since the last time we wrote an article covering this in 2016, although our experience is that nothing we suggest here is infallible on large marestail.

A burndown of glyphosate and 2,4-D struggles to control marestail in the spring anyway, especially in the absence of fall herbicide treatments.  Our standard recommendation, regardless of when spring treatments are applied, is to either replace the 2,4-D with something more effective, or to add another herbicide to supplement the 2,4-D.  Sharpen has been the frequent replacement/supplement, and we now have the option to use dicamba in the Xtend soybean system instead of 2,4-D.  While it’s possible to use higher 2,4-D rates in the Enlist soybean without waiting to plant, higher rates do not necessarily solve this issue based on our research, although a follow up POST treatment that includes glufosinate or 2,4-D usually finishes off plants that survive burndown.  We also would not expect the addition of Elevore to consistently solve this issue either, and it requires a 14 day wait to plant any soybean.  There’s a list of suitable soybean burndown treatments in our marestail fact sheet, and also below – these are for fields not treated the prior fall.

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Spring Herbicide Applications on Winter Wheat – Part 2 Labeled Herbicides

Source: Purdue University (Edited)

If weed infestations are severe enough to require a herbicide application, the use of liquid nitrogen fertilizer solution as a carrier is a popular option for applying herbicides and topdressing the wheat crop in a single pass over the field.  Caution should be taken when using a liquid fertilizer as a herbicide carrier as moderate to severe crop injury can result, especially in saturated conditions.  Many post applied wheat herbicide labels allow for liquid nitrogen carriers, but require different rates and types of surfactants than if the herbicide was applied with water as the carrier.  Table 1 includes precautions to be taken when applying wheat herbicides using liquid fertilizer as a carrier; further details and directions can be acquired from the herbicide label.

Another consideration growers should take into account when planning early spring herbicide applications is the plant back restrictions to double crop soybeans.  A large percentage of the herbicides listed in Table 1, especially those with activity on Ryegrass and Brome, have soybean plant back restrictions greater than the typical three month time period between spring applications and double crop soybean planting.  The soybean plant back restrictions greatly reduce the number of options available to wheat producers who double crop soybeans after wheat.  Refer to Table 1 for more specific plant back timing restrictions.Click Here For Complete Table

Spring Herbicide Applications on Winter Wheat – Part 1 Growth Stages

Source: Purdue University (Edited)

The winter is finally winding down and we are bound to have warmer days and spring in the near future. As we look towards the warmer weather there a few field activities that are going to start quickly, including winter wheat greenup herbicide applications and winter annual weed burndown applications in no-till fields. There are few things to keep in mind as these activities are added to the calendar. Many wheat producers, especially in the southern regions of Indiana will soon be or already are topdressing their wheat.  Those looking into topdressing need to also be scouting for weeds and determining if a herbicide application is necessary on any existing winter annual weeds.  The following information will outline winter annual weeds to look out for, weed scouting tips, crop stage restrictions, and herbicide recommendations.

Some common broadleaf weeds to scout for in your winter wheat are dandelion, purple deadnettle, henbit, chickweed, Canada thistle, and wild garlic.  These winter annual species that emerge in the fall can remain relatively inconspicuous through the winter and become competitive and troublesome during the spring if not controlled early in the spring.  Summer annual weeds such as ragweed will be of less concern in the early spring and will be outcompeted by the wheat crop if managed properly.  Grass weeds to be aware of and scouting for are: annual bluegrass, annual ryegrass, cheat, and downy brome.

Determining the severity of weed infestations in your wheat fields is key in determining the necessity of a herbicide application.  As with all agronomic crops, you should scout your entire field to determine what weed management practices need to be implemented and determine any areas of severe weed infestations.  Wheat fields that contain uniform infestations of at least one broadleaf weed and/or three grass weeds per square foot should be taken into consideration for a herbicide application to avoid yield loss and harvest interference problems.  Some fields that have less uniform infestations, but rather pockets of severe infestation should be managed to reduce weed seed production and future infestations.

When determining your herbicide program for spring applications, the stage of the wheat crop should be considered.  The majority of wheat herbicides are labeled for application at certain wheat growth stages and some commonly used herbicides have very short windows in which they can be applied.  The popular broadleaf weed herbicides 2,4-D and MCPA are efficient and economical, but can only be applied for a short period of time between tillering and prior to jointing in the early spring.  Wheat growth stages and herbicide timing restriction are outlined in Figure 1 above.

The LL-GT27 soybean – what’s legal?

Source: Dr. Mark Loux, OSU Extension

We are starting to see the availability of soybean varieties with more than two herbicide resistance traits, which can expand the herbicide options, improve control, and allow multiple site of action tank mixes that reduce the rate of selection for resistance.  One of these is the Enlist soybean, with resistance to glyphosate, glufosinate, and 2,4-D.  As of this writing, full approval for the Enlist soybean is still being held up by the Philippines (because they can apparently).  The other is the LL-GT27 soybean, which has resistance to glyphosate, glufosinate, and isoxaflutole (Balance).  There is no label for use of isoxaflutole on this soybean yet, but it is legal to apply both glyphosate and glufosinate.  In Ohio, as long as neither label prohibits applying a mixture of two herbicides labeled for a specific use, it’s legal to apply the mixture.  So, it’s also legal to apply a mixture of glyphosate and glufosinate to the LL-GT27 soybean.  There is no label that actually mentions or provides guidance for this mixture, which does not affect legality, but could affect who assumes liability for the recommendation to apply a mixture if that matters to you.  Some seed companies are making the recommendation for POST application of the mix of glyphosate and glufosinate to the LL-GT27 soybean in printed materials.  Our interpretation after discussion with ODA, is that these materials are essentially supplements to labels, and so the seed company would assume some liability for the recommendation.

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Achieving Full-season Waterhemp Control in Soybeans

Dr. Bob Hartzler and Meaghan Anderson, Integrated Crop Management News, and Iowa State University Extension and Outreach.

Although there are many ways weeds escape control in crop fields, one of the leading causes of waterhemp control failures is emergence of plants following postemergence herbicide (POST) treatments.

Waterhemp requires more than twice as many growing degree days to reach 50% emergence as giant foxtail or velvetleaf (Figure 1), resulting in much of the population emerging after mid-June.

The layered residual system is one of the best ways to reduce late-season waterhemp escapes in soybean. It involves a split application of herbicides with residual activity – the first application is made at or near planting, and then additional residual is included with the POST application (Figure 2). The additional residual herbicide extends activity later into the season than a single application, and is especially beneficial in years with heavy rains following planting.

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Developing a long-term comprehensive weed management system – Part 2

Source: Iowa State University, (Edited)

Post 2 of a 4 post series.

As the end of the year approaches and we reflect on the 2018 growing season we need to look at what changes or improvements we need to make in our production plans for 2019.  Herbicide resistant weeds are continuing to create problems.  New, very invasive and harmful weed species (Palmer Amaranth and Waterhemp) are now prevalent in Knox County.  Therefore, a review of the effectiveness of your herbicides program is definitely in order.

To effectively battle these new weed problems, creating a comprehensive, all-encompassing weed control strategy is essential in today production agriculture.  Over the next 4 weeks I will share information developed by Meaghan Anderson and Dr. Bob Hartzler at Iowa State University on developing a long-term weed management system.

Last week’s post:   Herbicide program development: Using multiple sites of action

This week’s post: Herbicide program development: Using effective herbicide groups

After you’ve started working on a program that contains multiple herbicide groups (sites of action), you need to make sure you’re using multiple herbicide groups that will be effective against your target weeds. For most people, the target weed will be waterhemp. Others may have problems with giant ragweed, horseweed/marestail, or other weeds. Waterhemp is the target weed in my example, but consider what your most problematic weeds are to run through this exercise for yourself.

Things to consider when determining whether a herbicide is effective against your target weed include (1) whether the herbicide is labeled to control the weed and (2) whether your target weed is resistant to the herbicide group.

Let’s look at herbicides as if waterhemp is the weed that causes us the most issues. Here’s a table of herbicide groups used in Iowa crops.

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Developing a long-term comprehensive weed management system.

Source: Iowa State University, (Edited)

As the end of the year approaches and we reflect on the 2018 growing season we need to look at what changes or improvements we need to make in our production plans for 2019.  Herbicide resistant weeds are continuing to create problems.  New, very invasive and harmful weed species (Palmer Amaranth and Waterhemp) are now prevalent in Knox County.  Therefore, a review of the effectiveness of your herbicides program is definitely in order.

To effectively battle these new weed problems, creating a comprehensive, all-encompassing weed control strategy is essential in today production agriculture.  Over the next 4 weeks I will share information developed by Meaghan Anderson and Dr. Bob Hartzler at Iowa State University on developing a long-term weed management system.

This week’s post:   Herbicide program development: Using multiple sites of action

With the stagnant development of new herbicides and weeds seemingly evolving herbicide resistance faster than ever before, it’s important to maximize the usefulness of every herbicide application. A new herbicide site of action (or herbicide group number) for use in corn and soybean production has not been discovered since the early 1980s. According to Dr. Ian Heap with www.weedscience.org, since the 1980s, the confirmed number of unique cases of herbicide resistance globally is increasing at a rate of about 12 discoveries per year.

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Inversion and Drift Mitigation – Workshop on December 14

Inversion and Drift Mitigation Workshop
Dec. 14, 2018 • 10 a.m. – noon

10 – 11 a.m. Weather Conditions and Potential Inversions
Speaker: Aaron Wilson, Weather Specialist & Atmospheric Scientist, OSU Extension, Byrd Polar & Climate Research Center

11 – noon Using FieldWatch to Communicate
Speaker: Jared Shaffer, Plant Health Inspector, Ohio Department of Agriculture

Two choices for attending the workshop:

Attend in person (pre-registration required, limited to the first 75 people registered)
Ohio Dept. of Agriculture • Bromfield Administration Building
8995 E. Main St., Reynoldsburg, OH 43068
Click here to register

Attend virtually:
Link coming soon – no pre-registration required

No cost to attend. Core commercial and private pesticide recertification credits available only athe Reynoldsburg in-person location. Limited to the first 75 registered. No recertification credits given for virtual/internet attendees.

For more information about the workshop, contact:
Cindy Folck, folck.2@osu.edu
614-247-7898

Event sponsored by OSU Extension IPM program and the USDA NIFA Crop Protection and Pest Management Competitive Grants Program (Grant number 2017-70006-27174).