MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

As discussed in a previous Field CAT Alert article, preemergence herbicides should be applied as soon after planting as possible because delayed application increases the risk of poor herbicide performance, especially for grass control. Delayed preemergence applications were covered in detail, but what if you are able to make your preemergence application and the weather is cold, or extremely wet or dry after planting?

An activating rainfall is required after preemergence herbicides are applied. This rainfall stimulates seed germination exposing the seedling to the herbicide in the soil. The herbicides in turn are activated by the rainfall, which helps to incorporate the herbicide into the soil and aids in the uptake into the weed roots or shoots. Soil moisture is an important component to ensure herbicide activity and seed germination, however, temperature is also a key factor.

If there was good moisture at planting and the time of your preemergence application, but no appreciable rainfall within a week or two, weed seeds can germinate and potentially grow through the herbicide sitting at the soil surface. Conversely, if cool wet weather occurs after planting, weed seeds may not germinate as quickly as they would under warm conditions. This can lead to a delay in germination that occurs after preemergence herbicides have been activated or after they have dissipated in the soil. Although a good rainfall is essential, a heavy rainfall, or extended wet weather, can lead to the herbicides being carried below the weed seed germination zone or hydrolysis of the herbicide. These situations can lead to a shortened period of preemergence weed control and the need for postemergence weed management.

Weeds that emerge from wet soil in dry weather may be controlled if rainfall occurs soon after weed emergence; however it is more common that a postemergence application will be needed to control the emerged weeds. The upside of this situation is that when rainfall does occur subsequent flushes of weeds will usually be controlled. However, when weeds emerge after the preemergence herbicides have been dissipated in the soil, the only option is postemergence weed control.

In situations where the soil is dry enough, rotary hoeing will control small weeds. Delaying your postemergence herbicide application as long as possible will allow you to control your emerged weeds and potentially avoid a second postemergence application. Keep in mind that research at MSU has shown that it is necessary to control weeds before they reach four inches of growth to prevent measurable yield reduction. Also be aware of the products applied preemergence when selecting postemergence applications, especially when products containing atrazine or Callisto are applied preemergence. Atrazine should only be applied postemergence in a premix or for emergency use due to critical timing and rate or carryover concerns. Callisto, Camix, Lexar, or Lumax cannot be applied postemergence where one of these products was applied preemergence due to rate restrictions and subsequent carryover concerns.

Herbicide options for postemergence weed control are often dependent on the hybrid planted in the field. Many postemergence herbicides can be used on any corn hybrid, while others such as Liberty and glyphosate require herbicide resistant hybrids. For details on postemergence herbicides programs in corn, see 2008 Weed Control Guide for Field Crops, Extension Bulletin E-434.

Another issue to consider with cool, wet weather and preemergence herbicides in the spring is crop injury. Preemergence herbicide selectivity is primarily through differential metabolism—meaning that crops are able to break down the herbicide more rapidly than weeds, thus the weeds are killed, but crops survive the effects of the herbicide. Cold soil temperatures can lead to crops under stress that are less efficient at metabolizing herbicides and are prone to injury. For the same reasons moisture is good for weed control, the potential for crop injury will increase with soil moisture content since more herbicide is available for absorption in wet soils.

Where delayed preemergence application with amide, amide: atrazine premixes, or other products are planned as discussed in the last Field CAT Alert article (May 8, 2008), patience should be exercised where conditions are cool and wet. While these products are labeled for this use, there will be an increased risk of injury due to stressed corn, and delayed preemergence applications should be postponed if possible until conditions have improved and the corn has begun to recover. The same precaution is true when applying postemergence herbicides after prolonged periods of cool weather. If there is ever a question, check the herbicide label or contact your extension personnel.