Friday, April 12, 2024

At-Plant Preemergence Herbicides

What are the factors that affect activation? 

⋅ By Mike Marshall ⋅
Extension Weed Specialist, Edisto Research and Education Center, Clemson University.
Clemson University Extension recommendations for overlapping soil-residual herbicides start at planting. This is critical because soil-residual herbicides reduce the number of weeds emerging in future flushes when the subsequent postemergence herbicides are applied. Weeds like Palmer amaranth, sicklepod and morningglory continue to be problems in peanut fields in South Carolina. Here are some things to remember when planning your soil-residual program after peanut planting.


Study Herbicide Options

In peanuts, we have several options for preemergence herbicides, including Valor (SX or EZ formulations), Warrant, Outlook, Sonalan, Prowl, Dual Magnum, Anthem Flex and Strongarm. Warrant, Outlook, Dual Magnum and Strongarm are also labeled for application after peanuts emerge. Valor is our most effective soil-applied herbicide for control of herbicide-resistant Palmer amaranth.

Valor should be applied immediately after planting but within two days of planting. If two days have passed, then do not apply Valor SX if soil above the seed has cracked and herbicide spray will contact the seed. Also, seedling damage can occur if peanut emerges through Valor-treated soil and if an intense rainfall event splashes treated soil onto the new foliage of the emerging peanut plant. Be sure to clean out the spray tank/boom assembly containing Valor shortly after finishing spraying for the day.

Increase Control With Tank-Mixes

Tank-mixing preemergence herbicides increases the spectrum of weeds controlled. Combinations of Prowl plus Valor, Strongarm plus Valor, or Sonalan plus Valor are typical broad-spectrum combinations for weed control in peanut. Strongarm is recommended if morningglory is a significant problem. Also, Prowl is effective on large-seeded grasses, such as Texas panicum. Prowl or Sonalan can be tank-mixed with Valor and Strongarm. Prowl and Sonalan are a different mode-of-action from Dual Magnum, Warrant and other Group 15s. Consider this for resistance management. Sonalan must be incorporated with water after application.

Meet The Water Requirement

Surface-applied herbicides, such as Prowl or Valor, typically need a minimum amount of water after application to properly “activate” in the upper soil profile.

Activation means the herbicide is dissolved in the soil water solution and readily absorbed by weed seeds as they start to germinate. The difficult question growers often ask is how much is required for a particular herbicide. 

Several factors affect herbicide activation, including soil texture (sand, silt and clay), organic matter content, residue levels and temperature. In general, 0.25 to 0.50 inch is sufficient for most soil-applied herbicides within a few days of planting/application. In general, we like to see at least 1 inch in total precipitation within two weeks of a preemergence application for optimum residual activity. 

The amount of water required for a preemergence herbicide activation varies, with some requiring more than others. For example, Dual Magnum requires less water than Prowl for activation. If dry conditions prevail after application, plan on using a residual, such as Dual Magnum, Outlook, Zidua, Anthem Flex, Strongarm or Pursuit, during the at-crack paraquat early postemergence (around 14 days after planting) application. Again, it is important to maintain overlapping residuals during the season to reduce pressure on postemergence herbicides.

Always Read The Label

Check herbicide product labels for the most up-to-date information. While some labels are vague about rainfall minimum requirements for preemergence herbicides, I believe 0.25 inches is sufficient to activate most preemergence herbicides. Another consideration is that some preemergence herbicides will sit on top of dry soil and wait for precipitation, and when rainfall comes, the herbicide will activate, albeit less than the amount you applied from the sprayer. 

If a dry period follows a preemergence herbicide application, plan on spraying the first postemergence herbicide application sooner and tank-mix a residual herbicide, such as Dual Magnum, Warrant, Outlook, Zidua, etc., with it. This will extend the soil residual activity on weeds that have not emerged yet in the field. PG

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