Posts Tagged ‘herbicide resistant weeds’

Cleaning Equipment to Prevent Spreading Weed Problems Around

Thursday, August 2nd, 2012

Mark VanGessel, Extension Weed Specialist;

This summer has been very challenging for weed control so I want to remind you to not spread the problems around the farm or from farm to farm. I believe that a lot of our new weed infestations are due to transporting seed on equipment, whether the equipment is mowers, combines, or vegetable harvesters. I have seen a number of fields with heavy weed pressure due to escapes. Some of these are suspected to be resistant biotypes, others just hard to control weeds. If a particular weed is giving you headaches, wouldn’t you rather deal with it in only one field rather than all of your fields? Ask yourself, what you would do if you could no longer use the best herbicide for a problem weed. In vegetables, where we only have one or two broadleaf herbicides, what are your options when they are no longer effective?

Granted weeds that get blown around (like marestail or thistle) or spread (by birds like pokeweed) are difficult to prevent. Nevertheless, many of our problems are due to moving seeds from field to field on equipment; pigweed and lambsquarters are two that come to mind. Take the time to clean the equipment in the field before it gets moved and isolate where those infestations are located. This is true for all fields. A new weed or a resistant biotype does not just take over a field in one year. A few plants get started and they produce seeds which next year leads to more plants and more seeds (see where this is going?). Prevent the problems from developing and spreading. Clean the equipment thoroughly, before it leaves the field, and leave the weed seed where you found it.

Glyphosate Resistant Palmer Amaranth on Delmarva

Friday, June 29th, 2012

Barbara Scott, Research Associate; & Mark VanGessel, Extension Weed Specialist;

Glyphosate-resistant Palmer amaranth has been confirmed on Delmarva. There has been suspicion of glyphosate-resistance in the region, but this is the first year of confirmed resistance. This discovery is not surprising since Palmer amaranth has a tendency to develop resistance. Fields with Palmer amaranth (or suspected Palmer amaranth) should not be treated with glyphosate alone. In soybeans glyphosate should be tank-mixed with an ALS inhibiting herbicide (group 2) or PPO herbicide (group 9).

Postemergence Control of Glyphosate-Resistant Horseweed

Friday, June 15th, 2012

Mark VanGessel, Extension Weed Specialist;

Options for controlling horseweed resistant to glyphosate after the soybeans have emerged are very limited. Liberty Link soybeans are an exception, because Liberty 280 is fairly effective on horseweed (be sure to keep your rates up).

For non-Liberty Link soybeans the options are very limited. Liberty Link soybeans can be treated with Liberty (or Ignite) for fair control of horseweed. For non-Liberty Link soybeans, FirstRate or Classic are effective on small, newly emerged seedlings. However, neither will consistently kill large horseweed plants, nor plants that were “burned off” and are recovering (FirstRate is better option than Classic after the soybeans have emerged). These herbicides may provide some suppression, but results have been quite erratic the past few years. Horseweed plants are generally not very tolerant of shade and most soybeans will begin to canopy over the horseweed and out-compete them. Additional glyphosate applications will provide some suppression of horseweed and sometimes the soybeans have a chance to outcompete them.

Marestail Causing Headaches this Spring

Thursday, May 24th, 2012

Mark VanGessel, Extension Weed Specialist;

This spring has been very challenging for marestail control. A number of fields were sprayed early, when horseweed plants well under 6 inches tall, and got excellent control of emerged plants. But this spring we have seen fields with lots of plants that emerged after the initial burndown treatment. In many cases horseweed plants that emerged in April were controlled with a second burndown application that included Liberty or Gramoxone. More frustrating are those plants that were not killed with the initial burndown treatment. Most of these plants were treated when they were over 6 inches tall, or not treated with a full rate of the burndown mixtures. At this time, options are very limited for these fields and often decisions need to be made on a field by field basis.

Control of Palmer Amaranth

Thursday, May 24th, 2012

Mark VanGessel, Extension Weed Specialist;

Palmer amaranth was seen in a number of locations in DE and MD eastern shore last summer. We found fields near Dover and throughout Sussex County with Palmer amaranth infestations. We talked about this plant at most of our winter meetings. It looks much like smooth or redroot pigweed early in its growth. The link below will show you some bulletins on how best to identify Palmer amaranth, but the watermark is very diagnostic. However, many plants never develop these “V” markings on the leaves.

This species needs to be taken very seriously; it can overwhelm a field in a few years. It is a species that has developed resistance to glyphosate very quickly and once that happens, it will make control very difficult. (If you think glyphosate-resistant marestail has been a headache; Palmer amaranth is much worse.)

Palmer amaranth grows very rapidly, which means you have only a few days to make postemergence herbicide decisions and get the field treated. Without effective control, Palmer amaranth will grow 5 to 6 feet tall. If you know you have Palmer amaranth, or you suspect you might have it, do not rely on glyphosate alone for postemergence control. Options for corn include HPPD-inhibiting herbicides (Group 27, Callisto, Impact, or Laudis); ALS-inhibiting herbicides (Group 2, Resolve, Steadfast, Permit Plus, Capreno, plus many others); and plant growth regulators (Group 4, such as Status).

Postemergence options in soybeans include PPO-inhibiting herbicide (Group 14, Reflex) and ALS-inhibiting herbicides (Group 2, FirstRate, Pursuit, Classic, etc).

We had a number of reports of poor performance with glyphosate last year for Palmer amaranth control, and so tankmixes will be essential for resistance management. Most of these postemergence options need to be applied to Palmer amaranth before Palmer amaranth plants are 4 inches tall.

Weed Control in Winter Wheat

Friday, September 2nd, 2011

Mark VanGessel, Extension Weed Specialist;

In last week’s issue of Weekly Crop Update I explained why we need to consider fall herbicide treatments for small grains. When splitting nitrogen applications in the spring, neither one of the timings are a good for herbicide application when trying to achieve spraying small weeds that are actively growing and achieve good coverage.

1. Weeds are more susceptible in the fall.

2. Fall applications match better with weed development.

3. Weed emergence is primarily in fall.

4. Fall herbicide applications are not influenced by temperature as much as spring applications.

5. Coverage is better with fall applications.

6. Spreads out the workload.

For no-till fields, a non-selective herbicide needs to be used prior to planting. However, we do not have effective herbicides labeled for preemergence applications, so it is important that the field be scouted to ensure the crop is at the proper stage for herbicide application.

A few products can be used shortly after the crop has emerged. Axiom and Prowl H2O can be used at crop emergence (Axiom at the spike stage and Prowl H2O at 1 leaf stage); however they need to be tankmixed with other herbicides or followed by postemergence herbicides to provide a broad spectrum control.

Products that provide postemergence control include: Harmony, Harmony Extra, Starane Ultra, Osprey, PowerFlex, Axial XL. Others labeled with a limited fit include metribuzin, Finesse, Maverick, 2,4-D or dicamba.

Control of specific problem weeds:

Annual bluegrass: fall applications of Osprey are the most consistent. Fall application of PowerFlex is also good. Maverick is a last resort type treatment in the spring (Maverick requires use of STS soybeans).

Annual ryegrass: fall applications of Osprey, PowerFlex, or Axial XL work extremely well. Spring applications of PowerFlex and Axial XL are options, but neither can be applied in nitrogen without reducing the amount of nitrogen applied.

Roughstalk bluegrass: Osprey or PowerFlex perform well on this species.

Speedwells: We have had limited trials with the speedwell species, but fall treatments seem to be most consistent. Harmony Extra has little to no effect on this species, PowerFlex in the spring was rated as fair to good; and slightly better than Osprey (fair). Research at Virginia Tech has shown good results with Finesse postemergence, but this treatment requires the use of STS soybeans. Initial results with metribuzin show some utility for speedwells.

Jagged chickweed: This is another species we have limited trials for, but fall applications seem much more effective than spring treatments. Osprey, Harmony Extra, and PowerFlex seem to work well when applied in the fall.

ALS-resistant chickweed: This species is on the move with more reports each year. Harmony Extra, Osprey, and PowerFlex are all ALS herbicides (Group 2) and have no activity on this biotype. Rather, Starane Ultra or metribuzin in the fall have been the best treatments.

ALS-resistant horseweed: Another species with no trials. Starane Ultra lists horseweed as a species it will suppress. We do know that 2,4-D will control horseweed in burn-down situations, but we have not looked at low rates of 2,4-D in wheat for crop safety and effectiveness.

One common weed that is not controlled with fall applications is wild garlic.  But this weed needs to be treated with Harmony Extra (or similar products) in the early spring, about the time we apply the second nitrogen application.  We need to think of wild garlic (a late emerging perennial) separately from the annual weeds mentioned above.

A rotation to vegetables is an issue with many of these herbicides, including Osprey, PowerFlex, Finesse, Maverick, and metribuzin. Starane Ultra is a 4 month rotation to most crops. As you can see there is no one program that will provide control of all of our problem species. In most situations, a fall treatment will outperform a spring application, and you need to select the herbicide(s) based on the problem weeds you have in your field.

Metribuzin Use in Winter Wheat

Friday, September 2nd, 2011

Mark VanGessel, Extension Weed Specialist;

Metribuzin is a product used for years in soybeans and other crops for broadleaf weed control (formerly called Sencor or Lexone). It has been labeled for use in winter wheat, but the label does not recommend its use in our region. Metribuzin is one of the active ingredients in Axiom, and so it has been used on a limited basis in our region. Since metribuzin is a generic product there are different products available, but most go by the name metribuzin or some close version of this spelling.

After identifying ALS-resistant chickweed and looking for potential control options I began testing metribuzin, along with a number of other weed specialists in the region. We have had good results with control and very little injury with metribuzin.

The label reads, “metribuzin alone or with tank-mixture treatments are recommended for use in the following states” and none of the states in the Mid-Atlantic region are included. On the other hand, the label does not prohibit the use of metribuzin. Metribuzin label does allow for tankmixing herbicides, to broaden the spectrum of control. We have not tested all the possible combinations with newer herbicides (Axial XL, Osprey, or PowerFlex).

Rate is dependent on soil type and growth stage. Application timing is from 2-leaf stage of the wheat until 4 tillers. We have tested metribuzin primarily for ALS resistant common chickweed, and rates of 2 to 4 oz applied with a nonionic surfactant have worked quite well.

Some precautions on the label include: Do not apply to stressed crop (including dormant, drought, frost damage, disease); do not apply with liquid fertilizer; do not use on soils with less than 0.75% organic matter; do not apply more than 0.5 inches of irrigation for the first irrigation after application and do not exceed 1 inch for any subsequent irrigation; wheat varieties differ in sensitivity (some are more sensitive than others).

Metribuzin is also labeled for barley, but we do not have experience with it.

Palmer Amaranth is in the Area

Friday, June 3rd, 2011

Mark VanGessel, Extension Weed Specialist;

Last year I saw a few fields in the area (Delaware and Maryland) with infestations of Palmer amaranth. Palmer amaranth is a pigweed, which looks similar to the smooth pigweed that is so common (and often called redroot). However, Palmer amaranth is a very aggressive species that grows very rapidly. It is native to the southwest region of the US, and does better than most plants under dry conditions. Palmer amaranth has been described as pigweeds on steroids because of its ability to grow very rapidly, get very tall, and be very competitive with crops. Palmer amaranth is found throughout the southern US and is moving northward. Palmer amaranth is not as sensitive to Group 2 herbicides as smooth or redroot pigweed (this includes Pursuit, Sandea, Accent, Matrix, etc.). It is sensitive to PPO herbicides (Reflex, Valor, etc); atrazine, and HPPD (Callisto, Impact, and Laudis. Furthermore, glyphosate-resistant Palmer amaranth is present in Georgia, North and South Carolina and other southern states. I am not aware of any herbicide-resistant Palmer amaranth in our area.

It is critical that you control plants early; and that you do not allow the plants to produce flowers. Plants will produce a very high number of seeds that will quickly infest fields. In the southern cotton growing regions where they have herbicide-resistant Palmer amaranth, they have had to resort to hand weeding.

To help identify Palmer amaranth, see the chart, websites, and photos below.

Characteristics Redroot Pigweed Smooth Pigweed Palmer Amaranth
Stem hairs Hairy Hairy No hairs
Stems Often ridges running length of stem Often ridges running length of stem Mostly smooth
Leaf petioles Petioles no longer than length of the leaf Petioles no longer than length of the leaf Long drooping petioles
Seed head Short, stout, prickly Long, slender, slightly prickly Very long, thick, very prickly

A couple of good publications include:


Smooth pigweed


Palmer amaranth


Spartan Charge for Lima Beans

Friday, May 20th, 2011

Mark VanGessel, Extension Weed Specialist;

There is a 24c Label for use of Spartan Charge for lima beans in Delaware (not available in other states in the region). It is for control of ALS-resistant pigweed (Group 2 herbicides). It is a lower rate of the active ingredient (sulfentrazone) than is used in soybeans. The rate will provide early-season control of pigweed, but do not expect to see significant control of most species on the label due to this lower rate. The level of crop safety is marginal with Spartan Charge and so overlaps will cause injury. Also, sandy soils or sandy knolls in fields are likely to show injury. Injury is also likely if used early-season. Under conditions of cool soils and sandy soils, less than the labeled rate is suggested. We do not have experience with Spartan Charge on lima beans under a wide range of conditions, so be cautious and consider using it only in fields with known history of ALS-resistant pigweed.


Postemergence Control of Glyphosate Resistant Horseweed

Friday, June 11th, 2010

Mark VanGessel, Extension Weed Specialist;

Options for controlling horseweed resistant to glyphosate after the soybeans have emerged are very limited. Liberty Link soybeans are an exception, because Ignite 280 is fairly effective on horseweed (be sure to keep your rates up).

For non-Liberty Link soybeans the options are very limited. FirstRate or Classic are only effective on small, newly emerged seedlings. However, neither FirstRate nor Classic, will consistently kill large horseweed plants nor plants that were “burned off” and are recovering. These herbicides may provide some suppression, but results have been quite erratic the past few years. Horseweed plants are generally not very tolerant of shade and most soybeans will begin to canopy over the horseweed and out-compete them. In most cases, I have recommended to not spray emerged horseweed plants with another herbicide. Rather, make postemergence applications of glyphosate based on need to control other weed species. Additional glyphosate applications will provide some suppression of horseweed and give the soybeans a chance to outcompete them.