Midseason Weed Control Issues in Corn and Soybean

 

As we proceed through the growing season, there are a few issues surrounding weed control and herbicide use. Below are some items to consider:

 

 

We are receiving calls about weeds breaking through soil-applied herbicides. With all of the recent rainfall and especially if reduced herbicide rates were used, a postemergence herbicide may be necessary to clean up some of the escaped weeds. However, keep in mind there are crop height restrictions on many of the post herbicides. For a listing of additional herbicides and maximum corn heights and information on maximum weeds sizes for these products please refer to the Penn State Agronomy Guide and check the most recent herbicide label for specific use guidelines. (Or refer to the herbicide label for additional use information at the Crop Data Management Systems website or Greenbook Crop Protection website.)

 

 

Since the past weeks have been rainy and cloudy and the upcoming forecast is for hot/humid weather, there is the potential for crop injury when post herbicides are applied. The general rule of thumb is to allow a couple sunny days or so to pass after coming out of a rainy, overcast period before applying herbicides. Since the plants are stressed, this allows them time to build up a thicker leaf surface and to get their metabolic processes functioning at a faster pace to detoxify the herbicide. Also, with all the moisture and sunlight the plants will be growing very quickly and are succulent, so consider using nonionic surfactant (NIS) instead of crop oil concentrate (COC) or methylated seed oil (MSO) as the spray additive. Keep in mind also that certain pesticide formulations (e.g., EC) can increase crop injury. And finally, with the lack of spray days due to poor weather conditions, it is tempting to put “everything” in the tank at once (i.e., other pesticides, various adjuvants, foliar fertilizers, etc.) to reduce the number of trips across the field but this can sometime cause serious crop injury since the plants are sensitive and can’t fully metabolize and handle all of the products at once.

 

 

Certain herbicides especially, glyphosate and the PGR herbicides (dicamba, 2,4-D, etc.) can cause problems outside the field boundaries. To help reduce drift, use drift reducing nozzles such as venturi or air-inductions (AI) style tips. Most manufactures now make these tips as extended range models in order to reduce the spray pressure down to 15 psi. Also consider the difference between particle drift and vapor drift. Particle drift occurs when small droplets actually move and deposit onto leaf surfaces (this can be prevented by choice of nozzle, pressure, spray volume, application time, etc.). Vapor drift is related to the function of the herbicide formulation (e.g., ester vs. amine) and does not matter what kind of tip or pressure is used. It is impacted by temperature and relative humidity. For a more detailed discussion on spray drift and ways to reduce it, see "Reduced Spray Drift from Glyphosate and Growth Regulator Herbicide Drift Caution" from Purdue University.

 

However, keep in mind, if using herbicides that require good spray coverage (e.g., Liberty, Gramoxone, Cadet, Cobra, Reflex, etc.), AI tips may not be the best option unless certain adjustments are made to allow for better coverage including, higher spray volume, pressure, and boom height.

 

 

Proper sprayer cleanout is important especially when moving from one crop to another. There are still many reports of PGR herbicide injury on soybeans when applicators don’t properly clean out the sprayer after a corn (or Xtend soybean) application. Dicamba residues left in the sprayer still pose one of the biggest threats to non-Xtend soybean (and certain vegetable crops) when not thoroughly cleaned out. Dicamba-containing products include: Banvel, Clarity, DiFlexx, Xtendimax, Engenia, FeXapan, Sterling, Status, Yukon and others. To get the most effective sprayer cleanout, simply rinsing with water will not work especially with plastic tanks and rubber hoses. Usually it requires a few steps with inclusion of ammonia and/or tank cleaner.