Dry forecast: How will that impact weed control?

Weeds under environmental stress are hard to control.

Weeds emerging in a corn field.
Photo by Erin Burns, MSU

The dry weather we are experiencing throughout Michigan will have impacts on preemergence herbicides that were already applied at planting and future postemergence weed control.

Residual control of weeds

Back in 2021, we wrote a Michigan State University Extension article that applies to our current situation, “Dry conditions will impact early season weed control,” outlining questions on if your preemergence herbicide was wasted, impacts of dry weather on weed emergence, the need for scouting, and considerations for postemergence applications given weed escapes observed while scouting. Referring back to this article will be helpful when making a plan for the rest of this season. One question we have received a lot this year is one herbicide better than the other under dry conditions? The answer is not that straightforward and depends on a number of factors:

  • In order for herbicides to be effective on emerging weeds, the herbicide needs to be in the soil solution and in the weed seed germination zone. Rainfall incorporates the herbicide into this zone. Residual herbicides are not as effective in dry soils since less herbicide is in the soil solution where weeds are germinating.
  • To predict the activity of residual herbicides, an understanding is needed of chemical properties of the herbicide and how they interact with the environment.
  • Availability of a residual herbicide is an interaction between the solubility of the herbicide, the strength of binding onto soil colloids and organic matter, the environment, and the rate of herbicide applied.
  • Solubility influences how much rain is required for herbicide incorporation, how easily a herbicide will be absorbed by germinating weeds and crops, and if the herbicide will be subject to moving down the soil profile, potentially causing crop injury or loss to leaching.
  • Herbicides with low solubility often require larger amounts of rainfall to achieve incorporation.
  • Herbicides with high solubility are more easily incorporated with limited rainfall and generally prefer to remain in the soil moisture phase, so they are more available to germinating weeds. However, if the herbicide is highly soluble it will have a tendency to move with higher amounts of rainfall, so it is more likely to leach.
  • The strength of binding is measured by Soil Organic Carbon-Water Absorption Coefficient (Koc) value, which is the ratio of herbicide bound to the soil to that in the soil water. The higher the Koc value, the more strongly the pesticide is sorbed, and therefore, the less mobile it is.
  • In general, as solubility of the herbicide decreases, sorption increases.
  • Sandy or low organic matter soils will have less binding and allow for greater herbicide availability for crop and weed uptake. 

Table 13, “Toxicity, solubility, adsorptivity, and persistence of herbicides” in the 2023 MSU Weed Control Guide goes over these properties of commonly used field crop herbicides and will be helpful when making decisions on future residual herbicide use this season.

Emerged weeds

Weeds growing currently under the dry and recently hot weather may become more tolerant to herbicides than weeds growing under more optimal conditions due to two main factors.

First, weeds can develop a thick wax layer (cuticle) on the leaf surface that acts as a barrier to herbicide absorption. In Michigan, one of the first weeds that becomes difficult to control under dry conditions is common lambsquarters. During dry years, we get reports and an uptick in submissions of potentially glyphosate- (Roundup) resistant common lambsquarters to MSU Plant and Pest Diagnostics, but dry environmental conditions are to blame on poor control, as populations that have been screened in the clinic are susceptible under optimal greenhouse growing conditions. Including the correct adjuvant and use rate will help increase herbicide absorption. See the postemergence sections in the MSU Weed Control Guide or herbicide labels for adjuvant recommendations. Often times follow up herbicide applications will control these weeds.

Second, weeds that are stressed will not translocate or metabolize systemic herbicides very well under dry conditions. Systemic herbicides (examples glyphosate, 2,4-D, dicamba) control weeds by being transported throughout the weed’s vascular system. When weeds are stressed, translocation and subsequent metabolism are slowed and thus weed control is reduced. Making applications earlier in the morning after weeds have had time to recover from the hot, dry conditions of the previous day will improve control than applications later in the afternoon or evening.

Ultimately, the extended dry period we are currently experiencing will alter both weed emergence and performance of preemergence residual and postemergence herbicides. Deciding whether plans for postemergence weed control will need to be altered is going to rely on regularly scouting fields to identify what weed species are present and the optimal time to make postemergence applications.

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