From: Daniel Stephenson, Ph.D (Weed Scientist), Sebe Brown (Extension Entomologist) and John Kruse, Ph.D. (Cotton and Feedgrain Specialist)

Historically, many corn producers in Louisiana desire to plant corn in February. LSU AgCenter weed scientists and entomologists suggest burndown applications occur 4 to 6 weeks prior to planting to prevent competition from weeds and to remove vegetation that may be infested with insect pests – collectively known as “breaking the green bridge”.

Fields intended for corn should have already received a burndown application; however, weather conditions during January and early February may have prevented herbicide applications. As an example, 14.5 inches of rain were recorded at the Dean Lee Research and Extension Center in Alexandria since January 1.

The wet fields prevented ground application of burndown herbicides. Also, there were only a few days since January 1 that an airplane was able to make these applications due to wind conditions. As a consequence, Louisiana producers may be faced with weedy fields that are intended for corn.

A failure to “start clean” can greatly influence corn yields. Data have shown that corn determines its leaf orientation very soon after emergence. Leaf orientation perpendicular to the planted row is desired for maximum light interception, which influences growth and yield potential. If a spiking corn plant perceives any competition from an adjacent winter weed, the leaf orientation will be altered, thus potentially reducing that corn plant’s ability to intercept enough light for maximum yield. Therefore, planting into a weed-free field is very important.

Focus on Weed Control

Traditionally, a burndown application of glyphosate plus 2,4-D has been the standard protocol. This treatment usually provides good to excellent control of many winter/spring annual weeds common in Louisiana fields.

When applied 4 to 6 weeks prior to planting, a producer has time to evaluate the efficacy of glyphosate plus 2,4-D and decide if an additional herbicide treatment is needed prior to planting. If a producer is prevented from applying the burndown application in a timely manner, then weed competition and insect pressure may be an issue for emerging corn. Henbit in particular may be a refuge for cutworms and spider mites.

If a field scheduled for corn has received a burndown application, then these fields need to be evaluated to determine if corn will be planted into a “clean” field.

If the weather has prevented a burndown application and a producer intends to plant corn within the next few weeks, several factors must be considered.

The first issue is the 2,4-D plant-back restriction, which is 7 days for corn. If you are within this window, then you should not apply 2,4-D, to prevent herbicide injury to the corn. Second, maximum efficacy of glyphosate will not be observed until 21 to 28 days after application, so glyphosate applied 7 to 10 days before planting may not provide acceptable weed control and may allow insect populations to survive.

If a producer is within 7 to 14 days of planting corn, he/she should consider the following burndown treatments:

• Gramoxone SL at 1.5 qt/A plus atrazine at 1 lb ai/A plus 0.25% v/v nonionic surfactant.
• Gramoxone SL at 1 qt/A plus Leadoff at 1.5 oz/A plus 0.25% v/v nonionic surfactant.

Gramoxone SL will provide control of existing weeds, but coverage is essential. Therefore, a minimum of 12 gallons of water per acre and flat-fan nozzles should be utilized to maximize coverage. Also, Gramoxone SL efficacy can be increased when the air temperature is high and cloud cover is minimal.

Atrazine or Leadoff will assist Gramoxone SL with control by providing residual activity on winter/spring weeds during the first few weeks after corn emergence – if beds are not disturbed at planting. However, if an organophosphate insecticide will be applied in-furrow when planting corn, then Leadoff cannot be applied or injury will occur.

Focus on Insect Control

At-plant bands or post-emergence pyrethroid applications can be used to control cutworms; however, the infestation needs to be detected early to minimize stand loss. Moist soils will help incorporate the application to improve efficacy on any cutworms that may be located below the surface.

Foliar insecticide applications can be applied in bands behind the planter in reduced tillage fields. At planting soil insecticides such as Lorsban 15G can be t-banded with corn to help control cutworms pre-emergence. Lorsban should not be planted in furrow due to possible phytotoxicity. It is important to note that the use of ALS inhibiting herbicides with organophosphates such as Counter and Lorsban have the ability to cause significant crop injury.

If producers used Leadoff in their burn down strategy then Counter should not be used at all, to prevent any negative effects between the two chemicals. Lorsban has greater crop safety than Counter when used in conjunction with ALS inhibiting herbicides.

Force 3G can also be used at plant to help protect against cutworms. Force 3G is a pyrethroid insecticide and the ALS interaction is not a factor. Counter is not effective for control of cutworms but useful for rootworms.

Planting corn into a weed-free field is a must to maximize yield. Regardless of when you apply a burndown treatment, a producer must strive to “start clean”.