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How Does Cold Weather Affect Herbicides? (05/14/20)

Air temperature has been cold, but field conditions are acceptable to continue planting 2020 in some regions.

Air temperature has been cold, but field conditions are acceptable to continue planting 2020 in some regions.     I am writing for those interested in performance of burndown products in cold weather conditions. Herbicide performance in cold weather is dependent on both herbicide and weed species. Let us consider three herbicides, glyphosate, paraquat and auxin herbicides.

Glyphosate performance is best when it is warm or daytime temperatures at least in the mid 50 F. Consider increasing the glyphosate rate by a third to reduce the risk of not controlling winter or spring annuals when daytime temperatures are less than 50 F and night time temperatures range from 30 to 40 F. I suggest delaying glyphosate application until weeds resume active growth in areas receiving frost.

Paraquat is an excellent crop burndown product and should be considered in no-tillage or strip tillage situations and is not as temperature sensitive as glyphosate (or auxin herbicides). Please use non-ionic (NIS) surfactant with paraquat and follow mixing instructions. According to Brett Miller, Syngenta Technical Representative, NIS is the preferred adjuvant with paraquat, providing optimum distribution across the leaf surface. Miller emphasizes mixing order; add NIS to water before fertilizer solution, co-herbicide or paraquat is added to the tank. Tank mixing triazines (atrazine for corn or metribuzin in soybean) increases the speed and efficacy of paraquat in burning down weeds. 

Please use caution to determine / verify paraquat formulation and rate since there are 2 and 3 pound/gallon formulations in the marketplace. Use flat fan nozzles and a carrier volume of 15 gallons per acre.

Weed control is slowed when cold temperatures follow auxin products (2,4-D and dicamba). Be sure to scout weeds to evaluate cold temperature or frost damage to weeds. In general, winter annuals will not sustain much damage from cold temperatures, so take notice of emerged spring annuals. Apply auxin herbicides two- or three-days followings lows above freezing and when there is approximately 50% undamaged leaf tissue on the spring annuals. Weeds will need some time to develop new tissue if greater than 50% of leaf tissue is damaged by cold temperatures.

Bottom line is winter, or spring annual weeds must be actively growing for burndown products to perform.

Cold weather generally is not a concern for preemergence herbicides and can benefit certain products by reducing volatility or reducing microbial degradation.

 

 Tom Peters

Extension Sugarbeet Agronomist

NDSU & U of MN

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