Farm Progress

Should dry weather and planting delays be a concern when using preemergent herbicides?

March 8, 2018

1 Min Read
PRE PROS: Preemergence herbicides can improve weed control and increase yield potential.

Having second thoughts about using preemergence herbicides this spring?

Tom Peters, North Dakota State University and University of Minnesota Extension sugarbeet weed specialist, and Richard Zollinger, former NDSU Extension weed specialist, have answers to two things that might be holding you back.

What if it is dry?
It is true, residual soil-applied herbicides must be activated by precipitation to effectively control weeds. Factors such as temperature, sunlight and soil type also influence herbicide behavior in soils. However, herbicides can lay on the surface for an extended period and remain affective. Most soil-applied herbicides today have a medium or low vapor pressure, meaning they generally will not volatilize (evaporate) during warm and dry conditions. Second, these herbicides are bound to soil particles and organic matter (adsorption) and will not move provided the soil does not blow.

Research conducted at the University of Arkansas shows herbicides remained effective after lying on the soil surface for 14 to 21 days before activating precipitation. Therefore, the challenge in dry conditions is not degradation of herbicides prior to precipitation, but adequate moisture for weeds to germinate and inadequate precipitation for herbicide activation.

Is there enough time? It’s important to get the crop planted on time.
True, but preemergence herbicides can increase yield potential by preventing or suppressing early weed competition. They also reduce weed mixtures and produce a narrower distribution of weed sizes, making postemergence herbicide control more consistent. Preemergence herbicides often have a unique site of action that complement postemergence herbicides, which help reduce the onset of weed resistance. Finally, preemergence herbicides are a hedge against weather conditions that may delay postemergence herbicide applications.

Source: North Dakota Crop and Pest Report

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