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Pre-plant Herbicide Concerns in Cool, Wet Soil

One of the biggest concerns in agriculture today, revolves around weed resistance to herbicides and how we manage it. Years of repeated use from single site mode of action herbicides has created a barrier that is becoming increasingly harder to overcome. One of the best practices to combat against herbicide resistance is to apply pre-plant, or pre-emergence, herbicides. Spraying herbicides before planting and before crop emergence can help delay weed emergence. Premixed chemicals that contain several sites of action can help limit future resistance. As we think about our chemical program as planting approaches, one should keep in mind the affect cool and wet weather may have on those herbicides.

Herbicide injury can result from the antagonistic effect herbicides have on crops in cool wet conditions. Plants uptake these herbicides at a rapid rate, but cannot metabolize the chemicals as rapidly as plants growing in drier conditions are able to. The added stress from the cold and wet conditions slows the plants ability to break down herbicides. Injury can also occur as a result of direct seed to herbicide contact from wet soil conditions. Some of the most common herbicide injuries in wet soil conditions are found in the Chloroacetamide chemical family. These soil applied grass herbicides include acetochlor (Degree, Harness), s-metolachlor (Dual II Magnum), alachlor (Intrro), and dimethenamid-P (Outlook). Pre-emerge applications of Growth Regulators (2, 4-D) can also cause injury with similar symptoms. Symptomology of injuries includes the stunting of shoots, which can result in seedlings not emerging from the soil, and twisted leaves or “buggy whipping” at emergence.

Chloroacetamide herbicide injury to Corn

Photos courtesy of Purdue University

If the weather stays cool and wet, the last thing we want to do is abandon our pre-emerge herbicide plan. It may be necessary to switch to a different group of herbicides. For example, one could use Valor or Rowel on Soybeans, instead of Outlook. It is also important to follow the herbicide label and apply the recommended rate. Applying a half rate of herbicide could help to reduce crop injury. Finally, waiting to apply pre-emerge herbicides immediately after planting, when conditions are warm, can be beneficial. Please feel free to reach out and consult with us for help determining which hybrids are most sensitive to these herbicides on poorly drained soils.

The weather forecast influences every action we take in agriculture. Whether it be tillage, planting, or herbicide applications, the capability to change management decisions is necessary. Just as we may switch to a shorter season hybrid, as planting progresses later, we may have to adjust our herbicide program as well. Herbicide injury can often be confused with other symptoms of stress. If you need help diagnosing herbicide crop injury, please feel free to contact us here at HarvestMax and we would be happy to help in any way that we can.

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