What's Cropping Up? Blog

Articles from the bi-monthly Cornell Field Crops newsletter

April 5, 2019
by Cornell Field Crops
Comments Off on What’s Cropping Up? Volume 29, Number 2 – March/April 2019

What’s Cropping Up? Volume 29, Number 2 – March/April 2019

February 21, 2019
by Cornell Field Crops
Comments Off on New problem weeds in NY – waterhemp and Palmer amaranth

New problem weeds in NY – waterhemp and Palmer amaranth

Bryan Brown, New York State Integrated Pest Management, Cornell University
Mike Hunter, Regional Field Crops Specialist, Cornell Cooperative Extension

Several populations of waterhemp have been found on farms in the central and western parts of our state. These populations have established over the past couple years by plants that have escaped control, likely due to resistance to certain herbicide sites of action. One waterhemp population survived several herbicides and reduced soybean yields by around 50 percent.

Additionally, Palmer amaranth was found growing near one farmer’s barn. He believes the seed arrived on some used equipment from the Midwest. The plants were collected and burned, but we’ll continue to monitor the site for future emergence.

Both of these weeds are likely resistant to glyphosate and PPO inhibitors, among others. We’re hoping to run some spray chamber trials this winter to determine their resistance to certain sites of action.

Fig. 1. Palmer amaranth leaf and leaf-stem. ID tip: if the leaf-stem is longer than the leaf, as seen here, it is Palmer amaranth, not waterhemp. Photo: W. Curran and D. Lingenfelter, Penn State

While other pigweed species have short hairs on their stems, waterhemp and Palmer amaranth have smooth stems. The best way to distinguish waterhemp and Palmer amaranth is to rip off one of the lower leaves. As shown in Figure 1, if the leaf-stem (petiole) is longer than the leaf, it’s Palmer amaranth.

Unlike other pigweeds, waterhemp and Palmer amaranth have separate male (pollen producing) and female (seed producing) plants. Herbicide resistance traits can transfer by pollen, which has allowed these weeds to develop resistance faster.

To prevent these weeds from taking hold, growers are also recommended to start weed-free with tillage, followed by a 2-pass program of residual and post-emergence herbicides that utilizes several effective sites of action. Foliar applied herbicides should be used when these weeds are less than four inches tall. Since these weeds emerge over a broader timeframe than most weeds, mid-season residual herbicide applications should be considered, along with increased planting density or tighter row spacing to help close the canopy earlier.

If you do find yourself with escapes of these weeds, it makes economic sense to go hand-rogue those weeds out of your fields rather than deal with 200,000 to one million seeds in your soil from each weed. If there are too many to bag up by hand, consider sacrificing that patch of your crop by mowing and tilling the area before the weeds produce seed. Avoid harvesting these areas. Combines are especially good at spreading weed seeds. If you must harvest these areas, know that combines can carry 150 pounds of plant material even you think it’s empty, so check out some of the great online videos on how to clean them out after going through weedy fields.

The weakness of waterhemp and Palmer amaranth is the short lifespan of their seeds in the soil. Of those that don’t germinate, very few will survive in the soil for more than four years. So, if you can keep it under control for four years, you won’t have much of it after that. But as one Pennsylvania grower put it, “the cheapest way to control Palmer amaranth is to never get it in the first place.” So, it’s important to make sure that your seed, feed, bedding, and equipment are clean from the start.


Ohio State University has been dealing with these weeds for a while, and has a helpful listing of resources: https://u.osu.edu/osuweeds/super-weeds/palmer-amaranth/

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February 12, 2019
by Cornell Field Crops
Comments Off on What’s Cropping Up? Volume 29, Number 1 – January/February 2019

What’s Cropping Up? Volume 29, Number 1 – January/February 2019

January 30, 2019
by Cornell Field Crops
Comments Off on Herbicide Resistance Management: Get to know herbicide sites of action

Herbicide Resistance Management: Get to know herbicide sites of action

Mike Hunter, Cornell Cooperative Extension-North Country Regional Ag Team

Glyphosate resistant marestail in soybean field in Western New York

According to the International Survey of Herbicide Resistant Weeds, there are 497 unique cases (site of action x species) of herbicide resistant weeds globally.  This organization also has reported that weeds have evolved resistance to 23 of the 26 known herbicide sites of action.

Herbicide resistance management strategies must be included in all weed control recommendations.  Herbicide resistant weeds are not new for growers in New York State.  In fact, we have four officially confirmed herbicide resistant weeds which include common lambsquarter, smooth pigweed, common groundsel and common ragweed all of these cases are resistant to triazine herbicides.  Common ragweed was the last herbicide resistant weed case reported back in 1993.

This list will soon grow to include at least two, if not, three new herbicide resistant weed cases in NYS.  Added to the list will be horseweed (marestail) and tall waterhemp.  Many growers in Central and Western New York are now dealing with herbicide resistant marestail and a much smaller number of growers are now finding resistant populations of tall waterhemp in their fields.  The third suspected herbicide resistant case is the recent discovery of palmer amaranth on a NY farm in October.  This is the first report of this weed growing in NYS.  For those unfamiliar with palmer amaranth, the Weed Science Society of America ranks it as the most troublesome or difficult to control weed in the United States.

Remembering back to pesticide applicator training classes, you may remember the term Mode of Action when herbicides were discussed.  The mode of action can be used to describe the process or how the herbicide controls the targeted weed.  When we talk about herbicide resistance management we need to think about Site of Action (SOA).  The SOA is the location in the plant where the herbicide acts or has its effect on the plant.  The SOA is sometimes referred to as the Mechanism of Action.

When selecting herbicides to include in the tank mix we must now pay close attention to not only the mode of action, but also the site of action.  Resistance management strategies include using herbicides with different sites of action.  It is challenging enough to come up with an effective herbicide weed control program and now we are being asked to include herbicide sites of action in the decision making process.  Fortunately, there has been a numbering system developed to make this an easier task.  There are now herbicide group numbers assigned to each different SOA.  The group numbers are found on the first page of almost all herbicide labels that we currently use in field crop production.  Multiple numbers in the box indicate the herbicide or herbicide premix has more than one SOA (see example below).


It has been said many times before that there is an app for just about everything.  This is true about an app used to look up the specific SOA(s) for herbicides.  Take Action on Weeds has a very handy herbicide lookup tool app that can be used on Android and Apple smartphones, tablets, as well as, desktops.  It can be found at www.IWillTakeAction.com/app

When we use herbicides with the same SOA over and over again it fosters the development of herbicide resistant weed populations.  To prevent or delay the development of herbicide resistant weeds we must include herbicides with different sites of action in the tank mix.  In order for this resistance management strategy to work you must have at least two different SOAs that are effective on the targeted weed.

Here are some scenarios that demonstrate how we can best use the herbicide group numbering system when making herbicide application recommendations.

Let’s examine a no-till soybean burndown program for control of multiple resistant marestail (glyphosate and ALS resistant (or Group 9 and 2)) using a tank mix of glyphosate (group 9) + Valor XLT (a premix of Classic (group 2) + Valor (group 14)) + 2, 4-D LVE (group 4).  This herbicide program contains herbicides with 4 different SOAs, a group 9, 14, 2, 4.  The marestail in our example is resistant to both group 9 and 2 so these products will do nothing to control the marestail; however, the Valor (group 14) component in Valor XLT and 2,4-D LVE (group 4) will provide two different effective SOAs on our targeted weed, multiple resistant marestail.

Here is another example using glyphosate resistant (GR) tall waterhemp in Roundup Ready soybeans.  A Flexstar GT (a premix of Flexstar (group 14) + glyphosate (group 9)) application applied postemergence to soybeans for the control of emerged GR tall waterhemp will provide control.  It will provide control of GR resistant waterhemp because the Flexstar (group 14) in the Flexstar GT is providing the control.  However, from a resistance management strategy this may not be the best program because the only effective SOA in this program is from the Flexstar component.  This will put greater selection pressure on our population of tall waterhemp and it could eventually become resistant to the Group 14 herbicides.  To improve this program you could elect to apply a soil applied herbicide preemergence such as Dual II Magnum, Outlook or Warrant (all group 15 herbicides) followed by a postemergent application of Flexstar GT.  This will provide two different SOAs (Group 15 and 14) that are effective on our targeted weed, GR tall waterhemp.

Utilizing effective herbicide resistant management strategies goes beyond just using herbicides with different effective sites of action.  This is just one part of the resistance management puzzle that we need to piece together so that we can delay the development of resistant weeds from showing up on our farms.  Herbicide names used in this article are for illustrative purposes only and do not constitute and endorsement of the product.

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October 4, 2018
by Cornell Field Crops
Comments Off on What’s Cropping Up? Volume 28, Number 4 – September/October 2018

What’s Cropping Up? Volume 28, Number 4 – September/October 2018

September 24, 2018
by Cornell Field Crops
Comments Off on Extremely Low Weed Densities in Conventional Soybean and Relatively Low Weed Densities in Organic Soybean (especially in the Corn-Soybean-Wheat/Red Clover Rotation) in 2018

Extremely Low Weed Densities in Conventional Soybean and Relatively Low Weed Densities in Organic Soybean (especially in the Corn-Soybean-Wheat/Red Clover Rotation) in 2018

Bill Cox and Eric Sandsted

We initiated a 4-year study at the Aurora Research Farm in 2015 to compare different sequences of the corn, soybean, and wheat/red clover rotation in conventional and organic cropping systems under recommended and high input management during the transition from conventional to an organic cropping system. We provided a detailed discussion of the various treatments and objectives of the study in a previous news article (http://blogs.cornell.edu/whatscroppingup/2015/07/23/emergence-early-v4-stage-and-final-plant-populations-v10-psnt-values-v4-and-weed-densities-v12-in-corn-under-conventional-and-organic-cropping-systems/). Unfortunately, we were unable to plant wheat after soybean in the fall of 2016 because green stem in soybean, compounded with very wet conditions in October and early November, delayed soybean harvest until November 9, too late for wheat planting. Consequently, corn followed soybean as well as wheat/red cover in 2017 so we are now comparing different sequences of the corn-soybean-wheat/red clover rotation with a corn-soybean rotation (Table 1). This article will focus on weed densities in soybean in 2018 (highlighted in red in Table 1) at the full pod stage (R4), the end of the critical weed-free period for soybean.

The fields were plowed on May 17 and then cultimulched on the morning of May 18, the day of planting. We used the White Air Seeder to plant the treated (insecticide/fungicide) GMO soybean variety, P22T41R2, and the non-treated, non-GMO variety, P21A20, at two seeding rates, ~150,000 (recommended input) and ~200,000 seeds/acre (high input). We also treated the non-GMO, P21A20, in the seed hopper with the organic seed treatment, Sabrex, in the high input treatment (high seeding rate). We used the typical 15” row spacing in conventional soybean and the typical 30” row spacing (for cultivation of weeds) in organic soybean. We rotary hoed the organic soybeans on May 29, followed by a close cultivation on June 14, and then three in-row cultivations (June 19, July 10, and July 26). We applied a single application of Roundup to conventional soybeans on June 20.

Conditions were very dry for the 2 months following planting (3.12 inches from May 17 until July 16). Consequently, weed densities were quite low through late July. Over the next 10-day period (July 17-27), however, 4.89 inches of precipitation were recorded at the Aurora Research Farm. Consequently, very robust weeds (velvet leaf, foxtail, and ragweed in particular) were visible in the organic plots when we took our weed counts on August 10 at the full pod stage (R4 stage), the end of the critical weed-free period in soybeans. Conditions remained relatively moist with 3.53 inches of rain in August and another 2.0 inches of rain during the first 2 weeks of September.

Photo 1: Weed free conventional soybeans (soybeans in the corn-soybean-wheat/red clover on the left and in the corn-soybean rotation on the right) at the R 8.0 stage.

Weeds were almost non-existent in the conventional plots that received only a single application of Roundup (Table 2). This is the 4th consecutive year in soybeans where we applied a single application of Roundup for weed control and had almost complete control. Rotation and management inputs did not affect weed densities in conventional soybean (Table 2). The use of the moldboard plow in conjunction with a Roundup application about 5 weeks after planting has certainly been an excellent weed control combination for conventional soybean in this study (Photo 1).

Photo 2: Organic soybean had fewer weeds in the corn-soybean-wheat/red clover rotation (on the left) compared with the corn-soybean rotation (on the right) at the R 8.0 stage.

Although weed densities were relatively low in organic soybeans (mostly less than 1.0 weed/m2, Table 2), the weeds were very robust (Photo 2). Undoubtedly, the very wet conditions from mid-July through mid-September provided excellent growing conditions for the late-emerging velvet leaf and ragweed. Unlike conventional soybean, rotation did affect weed densities in organic soybeans with higher weed densities in the corn-soybean rotation compared with the corn-soybean-wheat/red clover rotation in all three fields (spring grain, corn, and soybean fields in 2014). We also observed a rotation effect for weed densities in organic corn in 2017 (but not in conventional corn) with far fewer weeds in organic corn in the corn-soybean-wheat/red clover rotation compared to the corn-soybean rotation (http://blogs.cornell.edu/whatscroppingup/2017/08/10/wheatred-clover-provides-n-and-may-help-with-weed-control-in-the-organic-corn-soybean-wheatred-clover-rotation/). High seeding rates did not affect weed densities in organic soybean in 2018.

In conclusion, conventional soybean had virtually no weeds in 2018 for the 4th consecutive year when combing moldboard plowing with a single application of Roundup. In contrast, organic soybean had very robust weeds in 2018, which resulted in a somewhat trashy looking field, but weed densities were relatively low for the 4th consecutive year. The corn-soybean- wheat/red clover rotation had lower weed densities when compared to the corn-soybean rotation in organic soybean so the inclusion of wheat/red clover in the rotation appears essential to maintain weed densities at a manageable level in organic soybeans. The very wet conditions from about mid-July (R3 stage) through mid-September (R7 stage), however, may mitigate any potential yield losses in the corn-soybean compared to the corn-soybean-wheat/red clover rotation, despite ~ 2x higher weed density. High (~200,000 seeds/acre) compared to recommended seeding rates (~150,000 seeds/acre) did not reduce weed densities in organic soybean. Perhaps more emphasis should be placed on identifying the best crop rotations rather than high seeding rates for reducing weed densities in organic soybean in New York.

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