Posts Tagged ‘field corn weed control’

Corn Herbicide and Soil Insecticide Interactions

Friday, March 30th, 2012

Mark VanGessel, Extension Weed Specialist;

Last year a few farmers had severe corn injury because they had used an organophosphate insecticide and an ALS-inhibiting herbicide at planting. Remember, there a number of corn herbicides that have label precautions about use when an organophosphate insecticide is used at planting. The herbicides include both soil-applied and postemergence herbicides. This includes the insecticides Counter (terbufos), Lorsban (chlorpyrifos), and Fortress (chlorethoxyfos). The list of herbicides is quite extensive and includes multiple herbicide families so follow this link for a detailed list of herbicides

Unfortunately, most corn seed companies no longer designate hybrids as either IT or IR. So if you are not sure, take the cautious approach and assume the hybrid is a “standard” hybrid (no enhanced tolerance for imidazolinone herbicides); and follow the most restrictive guidelines.

Cleaning Equipment to Prevent Spreading Weed Problems Around

Friday, September 9th, 2011

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. 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 and others are due to poor herbicide performance as a result of the summer drought. 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. A new weed species 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 and leave the seeds where you found them.

Fall Control of Perennial Weeds

Friday, September 9th, 2011

Mark VanGessel, Extension Weed Specialist;

Fall is the best time to treat most perennial weeds because it is the time that plants are best able to move the herbicide to the roots where it will do the most good. When considering fall weed control the emphasis should be on what the patch of weeds will look like next spring or summer not the amount of dead stems this fall. Also, it is important to consider that a fall application will not eradicate a stand of perennial weeds; the fall application will reduce the stand size or the stand vigor. Fall applications of glyphosate is the most flexible treatment for most perennial weeds such as artichoke, bermudagrass, Canada thistle, common milkweed, common pokeweed, dock, hemp dogbane, horsenettle and johnsongrass. Rates of 1 to 1.25 lb acid per acre are consistently the most economical (or about 1.5X the normal use rate for annual weeds). Allow at least 7 days after treatment before tilling, mowing, or planting through the treated area. Dicamba (Banvel) at 2 to 4 pints is also labeled for artichoke, bindweeds, dock, hemp dogbane, horsenettle, milkweeds, pokeweed or Canada thistle. Allow 10 days after treatment before disturbing the treated plants. Planting small grains must be delayed after dicamba application 20 days per pint of dicamba applied. Fall herbicide applications should be made to actively growing plants. Allow plants to recover after harvest before treating them. Consider keeping the combine header as high as possible so the weeds are quicker to recover; or combining around the weed patches and then spraying those patches immediately after harvesting. Weed species differ in their sensitivity to frost; some are easily killed by frost (i.e. horsenettle) others can withstand relatively heavy frosts. Check the weeds prior to application to be sure they are actively growing.

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


Postemergence Pokeweed Control

Friday, May 20th, 2011

Mark VanGessel, Extension Weed Specialist;

A few questions have come in about controlling common pokeweed postemergence in field corn. We had a trial a few years ago with tall pokeweed (sprayed in late June) and had results similar to a study contacted at Southern Illinois University. Dicamba [Banvel, Clarity, Sterling]; Distinct; NorthStar, and Callisto were the best treatments for conventional corn hybrids. Glyphosate was also effective if Roundup Ready corn was planted. Our trial did not include Lightning, but the SIU trial reported good control with Lightning with Clearfield corn. For soybeans, glyphosate is the best option. In non-Roundup Ready soybeans, Synchrony was fair (but requires STS-soybeans) or FirstRate which was only fair in the SIU trial.


Soil-Applied Herbicides to Emerged Field Corn

Friday, May 6th, 2011

Mark VanGessel, Extension Weed Specialist;

There are times (like this year) that corn has been planted and is emerged without residual herbicides being applied. Or to reduce the risk of crop injury, the residual herbicides are applied as the corn begins to emerge. Most of the soil-applied herbicides can be applied to emerged field corn. Only those products containing atrazine will provide control of weeds that have already emerged; so if there are emerged weeds, you may need an additional product to help control them. The following is a table for applying residual herbicides to emerged corn with maximum height of corn at time of application.

Herbicide Maximum Corn Height
Atrazine 12 inches
Callisto 30 inches or 8 leaf
Dual II Magnum/Cinch 12 inches
30 inches with RR corn
Outlook 12 inches
Princep do not apply to emerged corn
Prowl/Prowl H2Oa 30” or 8 collar, whichever is more restrictive
Sharpen do not apply to emerged corn
Topnotch/Harness/Degree/Breakfree 11 inches


Basis no later than 2 collars
Bicep II Magnum/Cinch ATZ 12 inches
Bullet 5 inches
Expert 12 inches with RR corn
Fultime/Keystone/Breakfree ATZ 11 inches
Guardsman Max 12 inches
Harness Xtra/Degree Xtra 11 inches
Hornet WDG 20 inches
Lexar 12 inches
Lumax 12 inches
Sequence (RR corn only) 30 inches
SureStart/TripleFlex 11 inches
Verdict do not apply to emerged corn


Protecting Corn Yield With Postemergence Programs

Friday, April 29th, 2011

Mark VanGessel, Extension Weed Specialist;

Modified from an article by Bob Hartzler, Iowa State University.

Farmers have an array of products that allow weeds to be effectively controlled postemergence. These new technologies include herbicide resistant hybrids, such as Roundup Ready and Liberty Link corn, and several newer herbicides (Callisto, Status, etc.) While postemergence herbicides (2,4-D, Banvel) have been used successfully for more than 30 years to control weeds in corn, the new products offer greater flexibility in application timing, reduced risk of crop injury, and a broader spectrum of weed control. However, an understanding of weed/corn competition is needed to use these products most efficiently.

Most summer annual weeds (giant foxtail, pigweeds, common lambsquarters, etc.) begin to emerge near the time of corn planting, but significant numbers of weeds continue to emerge into late June and July. A temptation for many farmers relying on postemergence herbicides is to delay application until the crop canopy is large enough to shade out late-emerging weeds. Delaying application of postemergence herbicides may result in cleaner fields at the end of the growing season, but this approach may have serious economic consequences. A regional project investigated the effectiveness of using only glyphosate for weed control in Roundup Ready corn (Gower et al. 2003). Glyphosate was applied at several times during the growing season based on the size of the dominant weeds in the field. A total of 35 experiments were conducted in nine states, including Delaware. Most sites had high weed densities. In these studies, weed control continually improved as applications were delayed. For example, a single application when weeds were 12” tall resulted in 95% control, whereas spraying 2” weeds resulted in only 73% control. The reduced weed control was due to weeds that emerged after application, rather than an inability of glyphosate to kill the larger weeds. Looking only at weed control would suggest that delaying herbicide applications is an effective strategy to enhance weed control.

However, corn subjected to weed competition from emergence to postemergence application began to suffer yield losses when herbicide application was made to 4” weeds. Applying the herbicide when weeds were 4” tall resulted in a 3% yield loss, and each delay approximately doubled the yield loss. The reduction in corn yields due to competition prior to the postemergence application illustrates the risk of delaying treatment in hopes of minimizing problems with late emerging weeds.

Table 1. The effect of application timing on weed control and corn yields.
Adapted from Gower et al. 2003. Weed Technol. 17:821-828.

Application timing
(Weed Size)

Weed control Corn yield loss1
(Early-season competition only)

Corn yield loss2
(Early- and late season competition )



73 0 7


83 3



90 6



93 14


12″ 95 22


1 Weeds emerging after herbicide application controlled with hand weeding.
Weeds emerging after herbicide application allowed to compete with corn.

Reference: Gower, Loux, Cardina, Harrison, Sprankle, Probst, Bauman, Bugg, Curran, Currie, Harvey, Johnson, Kells, Owen, Regehr, Slack, Spaur, Sprague, VanGessel and Young. 2003. Effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn: Results of a 2-year multistate study. Weed Technol. 17:821-828.

An efficient approach is an early application of glyphosate to protect the corn yield and in addition, include a herbicide with glyphosate that will provide residual control. Herbicides to consider include: atrazine, Callisto, Hornet, Resolve, Sandea, or Steadfast. Herbicide selection needs to be based on weeds present in the field. Be sure to consider corn height restrictions as well.


Water is Needed to “Activate” Soil-Applied Herbicides

Friday, April 29th, 2011

Mark VanGessel, Extension Weed Specialist;

Herbicides applied to the soil surface require rainfall or irrigation to move them into the soil where the plants will absorb them; or to be mechanically incorporated (field cultivator). Some areas have not received much rainfall since the herbicides were applied. Some products, like atrazine or mesotrione, may be taken up by the roots and provide some control. But Dual, Harness, and Prowl all need to be absorbed by emerging shoots, so they will not control weeds once they emerge. If you have irrigation and your corn herbicides have been applied but you have not received rain, you should consider irrigating to activate those herbicides.


Inconsistent Control With Burndown Herbicides

Friday, April 22nd, 2011

Mark VanGessel, Extension Weed Specialist;

There are a lot of fields that have been sprayed with a burndown and the control was not as good as expected. It is not uncommon for early season burndown applications to be erratic, but this season seems worse than others. The most common complaints have been with grass control, henbit, and chickweed. A few observations from my experiences: glyphosate is good on most of these species, but is not great. Glyphosate often provides good to excellent control of grasses if the rate is adequate (at least 0.75 lbs acid equivalent) and the plants are growing. However, annual ryegrass (aka Italian ryegrass) is hard to kill with glyphosate and requires close to a 2X rate if spraying in early spring. The addition of a triazine will significantly reduce the control of annual ryegrass. I often see only fair control of henbit with glyphosate. The addition of a triazine herbicide like atrazine or simazine will help. That is the tough choice, adding a triazine may help with some species, but can reduce the control of other species. You have to determine what weeds you have and which are going to be the most difficult to control and decide.

As far as paraquat, adding a triazine for the burndown before corn almost always improves control. However, grass control of annual ryegrass or grass cover crops will probably not be acceptable due to significant regrowth.


Rates for Residual Herbicides in Corn

Thursday, April 14th, 2011

Mark VanGessel, Extension Weed Specialist;

What rate of residual herbicide to use? In soybeans, use the full labeled rate because you need 4 to 6 weeks of residual control if spraying 3 to 4 weeks prior to planting, or you need the burndown activity at these rates because the weeds have gotten large. In my opinion, there is no clear cut answer for corn. A number of issues need to be considered:

Is this a no-till or conventionally tilled field?

· If it is no till you may need the higher rates because the amount of plant residue in the field;

· How far ahead of no-till planting are you spraying (so how long do you need to have residual control); and

· If spraying ahead of no-till planting, will you be coming back with additional preemergence herbicide at planting

Are you planning on a POST application? In our work, a preemergence followed by postemergence is the most effective and most consistent approach. A total preemergence approach can work if you have light weed pressure, have a dry year that limits a second flush of weeds, do not have large seeded species (morningglory, Texas panicum, cocklebur), do not have perennial weeds (horsenettle, yellow nutsedge, bermudagrass), are able to get your preemergence herbicides incorporated with timely rainfall or irrigation, or you are just plain lucky. Otherwise, most fields will need a postemergence spray.

My point of reference for a solid, one pass herbicide program is either Lexar or Lumax with additional atrazine; or a full-rate of a premix with atrazine and grass herbicide (Bicep, Harness Xtra, etc) PLUS pendimethalin (Prowl) or rimsulfuron (i.e. Basis). Any of these programs can include simazine. And I have seen all of these programs requiring postemergence treatments for acceptable weed control. None are consistently effective for full-season control.

If you need to spray postemergence then it makes sense to use a less intensive program at planting. But keep in mind, the less intensive soil-applied herbicide approach you use, the more likely you will need to spray earlier, may need to use more than one herbicide postemergence (particularly to provide residual control), and will need to be more timely with your postemergence treatment. If you can spray your postemergence herbicide before the weeds are 3 to 4 inches tall, and you are willing (able) to spray herbicide combinations (i.e. not rely solely on glyphosate), then reducing soil-applied herbicides by up to 20 to 30%, or not including all of the tankmixes, is a sound decision.