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Preemergence Herbicides are a Proactive Approach for Weed Management

prepared by Tom PetersExtension Sugarbeet Agronomist, NDSU & U of MN 
and Rich ZollingerExtension Weed Specialist, NDSU

Agriculturalists frequently advocate for use of preemergent herbicides. You heard state Extension Specialists recommend this in presentations during winter meetings and you have read it in trade magazines while you relax at home. Now it is time to put what you have heard and read into action. However, you are not so sure anymore, for some reason. The following evidence is intended to encourage you to apply herbicides preemergence.

Argument one, there is no rain in the forecast. Answer, it is true, residual soil –applied herbicides must be activated by precipitation to effectively control weeds and factors such as temperature, sunlight, and soil type influence herbicide behavior in soils. However, herbicides can lay on the surface for an extended period and remain affective. Most soil-applied herbicides used by farmers 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.                        

Daryl Ritchison, Interim Director of the North Dakota Agricultural Weather Network (NDAWN) summarized climate data from Fargo from 1881 to 2014 and found there was at least 0.25 inches of precipitation on an average once every 10 days in May and once every 7 days in June. Research conducted at the University of Arkansas indicated herbicides remained effective after lying on the soil surface for 14 to 21 days before activating precipitation. They stated 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.

Argument two, I need to finish planting to maximize yield potential. Answer, weeds cost your crop water, nutrients, and yield. Preemergence herbicides increase yield potential by preventing or suppressing early weed competition and reducing weed species mixtures, which can increase the simplicity and effectiveness of postemergence herbicides. Finally, preemergence herbicides results in a narrow distribution of weed sizes and improves consistency of postemergence weed control.

Preemergence herbicides fit in a planned weed management strategy. They often have a unique site of action (SOA) that compliment postemergence herbicides and reduce the onset of weed resistance. Finally, preemergence herbicides protect from the unknown; weather conditions that may not permit the timely application of postemergence herbicides.
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