Time to Scout for Weeds

Kurt M. Vollmer, Postdoctoral Researcher – Weed Science, University of Delaware; kvollmer@udel.edu

With most of the corn and soybeans planted, now is the time to start scouting for weeds. Doing so will prevent major headaches later in the growing season. While scouting, be sure to note the weed species present, height, life-cycle, and severity of the weed infestation. When looking at fields this year, pay attention to those areas that were drowned out last summer. The weeds in many of those spots produced seed and now have very high seed banks. So while weed pressure in the rest of the field may not be too heavy, weeds present in these spots may be at unacceptable levels.

In particular, Palmer amaranth can quickly become unmanageable if not spotted early. Many herbicide labels suggest spraying this weed when it is less than 4 inches tall, but the UD Weed Science program recommends applying postemergence herbicides before its 3 inches tall. Our research with soybean shows that the best time for this second application is no later 28 days after applying a residual herbicide. Furthermore, Palmer amaranth can quickly exceed 4 inches, and research at the University of Maryland has shown that delaying the postemergence application to 32 days or longer can result in reduced levels of control. Remember, the earlier Palmer amaranth is spotted the better. Furthermore, keep in mind there could be several days between scouting and actually getting the sprayer into the field, allowing Palmer amaranth to reach heights that prevent complete control.

Last Year’s Drowned Out Corn, This Year’s Weed Problems

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

A number of fields last summer had poor stands of corn due to the wet soils. As a result, the weed control in those areas was very poor and the number of weed seeds in the soil is probably very high.

Be sure to scout those areas separately from the rest of the field to evaluate if additional weed control is needed or if the area needs to be treated the sooner to achieve good spray coverage.

  • Start clean, no weeds should be present at time of planting
  • Apply residual herbicides within 1 to 2 weeks of planting
  • Use full herbicide rates
  • Scout to be sure the residual herbicides were activated and evaluate if postemergence herbicides are needed
  • Treated emerged weeds while they are small and most susceptible (less than 4 inches tall)
  • Scout again to be sure treatments were effective and determine if a follow-up treatment is needed

Options for Harvest-Aid Treatments for Field Corn

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

There have been several questions regarding options for harvest-aid treatment. A harvest-aid may be a consideration to dry down vegetation prior to harvesting and to reduce foreign matter in the harvested grain. There are a number of things to consider including: what weeds need to be treated; how will harvest-aid be applied; stage of the crop, and size of the weeds.

Be realistic on expectations of these products; while they are herbicides they were developed to control much smaller weeds than the size treated as harvest aids. Most of the products listed below will not kill plants; they are likely to burn off leaves, but not impact lower stems or vines. These products will not affect weed seed production. Reducing leaf material and foreign matter entering the combine should be the goal, not killing the weeds present.

A few more considerations:

  • With the hot temperatures, the risk of drift will increase with these products
  • Spray coverage is important for the contact herbicides (Defol and Gramoxone), so be sure to apply in 20 gpa or higher
  • If morningglory is present, can a combine run through the patches without pulling down the corn?
  • This time of year will favor translocation, so glyphosate is likely to cause more damage to desirable plants
  • Read the product label carefully for all instructions and restrictions

Products labeled:

Defol (sodium chlorate) is labeled for applications 14 days prior to harvest. Defol will dry down plants but it does not have herbicide activity. Dry down is slow; expect at least 14 days for dry down. Control of morningglory can be erratic.

Glyphosate is labeled but must be used with care do to potential injury to desirable vegetation. Apply glyphosate at 35% moisture or less and black layer has formed. Allow 7 days between application and harvest. Refer to the glyphosate label for rates.

Gramoxone Inteon is labeled for broadcast treatments. Application rates are 1.2 to 2 pts/A plus a non-ionic surfactant, and must be applied at least 7 days prior to harvest. Be sure to read the label for all precautions.

Aim is labeled for applications up to 3 days before harvest. Aim will only effect a few weed species and will not dry down grasses.

2,4-D amine is labeled but due to volatility and off-target movement, use of 2,4-D is not recommended. Applications with air temperatures above 85 degrees increases the likelihood of off-target movement. Application timing is after the hard dough or dent stage.

Considerations for Postemergence Corn Herbicides

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Many corn fields need postemergence treatment for weeds, but it maybe a week or two before the corn starts to shade the row middles. In this case, select an herbicide that will provide some residual weed control. Liberty, glyphosate, dicamba, or Aim/Cadet will not provide residual control. Consider atrazine if the corn is less than 12 inches, or a product with mesotrione (Callisto), Capreno, or Impact/Armezon for broadleaves. If you expect grasses to be a problem, then products such as Halex GT, Armezon PRO, or Capreno will help with residual control. Other products are labeled for corn less than 12 inches tall; refer to the Mid-Atlantic Field Crop Weed Management Guide for a complete list.

Use Caution when Selecting Adjuvants

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

The weather patterns lately have resulted in situations where the risk of crop injury from postemergence herbicides is higher. Specifically, prolonged periods of overcast skies, cooler weather, and plenty of rain. If postemergence herbicide applications are made as the days turn hot and sunny, the risk of injury is greater. This is due to the wax layer on the leaves not having developed and the leave surface being “tender”. If spraying during these sensitive periods, switch to “softer” additives if the label allows it; for instance MSO increases the risk of injury over COC; and non-ionic surfactants (NIS or 80-20’s) reduces the risk further. Consider using the lower allowed rate of the surfactant or nitrogen. Be sure to read the label and see what is allowed by the manufacturer.

Corn Height Restrictions for Postemergence Herbicides

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Corn herbicides need to be applied at the correct timing to avoid crop injury; and the weeds need to be small (3-4 inches depending on the herbicide) for effective control. Many labels state from corn emergence to a certain size corn. However, some herbicides require the crop to be at a certain size before the herbicide can be applied (i.e. Status require corn to be at least 4 inches tall or V-2 stage). Almost all herbicides have a maximum crop size and this can range from V-2 to V-8. Maximum size depends on the herbicide and can vary based on whether the herbicide is applied over the top of corn or with drop nozzles. Applications after this time can result in crop injury and possibly yield reduction. Some labels refer to crop size based on height of corn in inches, collar stage, or leaf stage. Refer to the herbicide label to ensure applications are made at the appropriate crop stage. When corn height and collar number are given for the same herbicide, base decision on whichever feature is first attained. If tankmixing, use the guidelines based on the most restrictive herbicide.

Also, this year corn within the same field maybe at different stages. Evaluate the field and select the herbicide that allows application to the stages of corn that represent your field.

Weed Control After Recent Rains

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Rainfall over the past week have complicated weed control for fields planted before the rain. The rains have moved most of the herbicide out of the weed emergence zone and will there is probably not much left to provide residual control. With that said, we do not know if the herbicide is all gone, and thus replanting with something other than the original crop, may lead to crop injury.

So, be sure to visit your fields soon, and often, to look for new emergence and time your postemergence sprays before weeds get too large. Fields may need to be treated sooner than you normally would spray since herbicides providing residual control are probably gone.

Areas with drowned out crops may need to be replanted. Weed control in areas being replanted will be challenging. Be sure to start clean and kill any weeds that might be present. Consider if you will be able to get into these replanted areas later. If you are only replanting areas that were drowned out, it may mean that you will need to drive through taller corn to get to these spots. If that is the case, will you be able to get in there when the replanted sections need to be sprayed with postemergence herbicides. The earlier planted corn may be too tall to allow a sprayer to get in when the replanted corn is 10 to 14 inches tall. Therefore, you may need to think about relying more on residual herbicides with these replanted areas.

Drowned out areas that are not replanted should be sprayed to prevent weeds from getting established and ultimately producing seeds. Weeds growing in these areas can produce a tremendous amount of seeds that could cause problems for the next few years.

Killing Corn in Order to Replant

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Some corn fields have to be replanted and killing the existing corn can be challenging. In general, killing corn before it has 2 to 3 collars is difficult because the growing point is below the ground. In a multi-site trial conducted a few years ago, paraquat plus a triazine herbicide (metribuzin or atrazine) was more effective than paraquat by itself. When paraquat plus a triazine was applied to 2 to 3-inch tall corn, control ranged from 67% to 98%. However, when the corn was allowed to reach at least 5 inches tall, control was at least 95% at all six sites. Select Max applied 2 to 3-inch tall corn, averaged 81% control, but at the taller stage, control was not as good as the paraquat treatments. Select Max also requires at least a 6-day period between application and replanting corn.

So, you will need to balance how quickly you want to replant your corn with effectiveness of the herbicide treatments. Applying paraquat plus atrazine (or metribuzin) to 5-inch tall corn was the best option for effective kill.

Postemergence Options for Palmer Amaranth Control in Corn

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

The recent rains in many areas will have moved many of the soil-applied herbicides out of the top one to two inches of soil. As a result, there may be a number of fields requiring postemergence treatments.

Postemergence control of Palmer amaranth requires an effective herbicide to be applied to small (3-inches or shorter) Palmer amaranth plants and often requires a herbicide that provides residual control. Emerged Palmer amaranth control in corn can be achieved with a Group 27 herbicide plus atrazine. These herbicides include mesotrione (active ingredient in Callisto and Halex GT), topramezone (Impact or Armezon), and tembotrione (Laudis, Capreno, DiFlexx Duo). All of the Group 27 herbicides should include atrazine, so these applications need to be applied before the corn is 12 inches tall. All of the Group 27 herbicides can provide 2 to 4 weeks of residual control, depending on rates and soil texture.

Liberty is an option with Liberty Link corn. Dicamba will control small Palmer amaranth in areas where it is appropriate to use it, this includes Status and Diflexx. Liberty and dicamba do not provide residual control and should be tankmixed with atrazine or a product like Dual or Harness, or Zidua provide residual control. Be sure to check the label for maximum corn size with any herbicide applied postemergence.

Water is Needed to “Activate” Soil-Applied Herbicides

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

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 herbicides were applied. Weed control will start to decline if water is not received within 5 to 7 days after applications. Some products, like those that contain atrazine, mesotrione (Callisto), or isoxaflutole (Balance) may be taken up by the roots and provide some control of seedlings after they have emerged. However, Dual, Harness, and Zidua are absorbed by emerging shoots, so once weeds have emerged these products will not provide control. If you have irrigation and your corn herbicides have been applied but you have not received water, you should consider irrigating to activate those herbicides. Another caveat is that early-season competition from grass can reduce yield.