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Environmental Conditions can Influence Herbicide Performance (06/27/19)

Growers planting their crops experienced a variety of conditions in April and May 2019.

Growers planting their crops experienced a variety of conditions in April and May 2019. For example, a review of weekly average air temperatures collected at the Fargo NDAWN station in April and May indicated temperatures averaged 34 F for the week beginning April 8 and averaged only 47 F for the week beginning May 6. Average temperatures finally climbed over 55 F the week beginning May 27.

Inconsistent environmental conditions have continued during weed control. For example, warm for a few days and then a return to jacket or sweater weather. One would guess many growers began POST weed control programs in dry bean and soybean the last two weeks and will continue making POST sprays at least into early July. It is timely to consider how environmental conditions interact with herbicides and affect both crop tolerance and weed control.

Best weed control is usually when herbicides are applied to actively growing weeds. The literature suggests ideal temperatures for applying most POST herbicides range between 65 and 85 F. Herbicides applied under cooler temperatures, for example, below 60 F, or when weeds are not actively growing can take longer to effectively control weeds and be less effective in general.

Some herbicides may injure crops if applied above 85 F. Avoid applying volatile herbicides such as 2,4-D ester, MCPA ester and dicamba during hot weather, especially near susceptible broadleaf crops, shelterbelts, farmsteads, or gardens.

Temperatures following herbicide application can influence crop tolerance. For example, desired crops degrade herbicides by metabolism. However, metabolism slows during cool or cold conditions, which extends the amount of time required to degrade herbicides in plants. Likewise, rapid degradation under warm conditions allows crop plants to avoid herbicide injury. Herbicides may be sprayed following cold night-time temperatures if day-time temperatures warm to at least 60 degrees.

Finally, what are "adverse environmental conditions" or conditions to avoid. Mostly common, adverse conditions are prolonged periods without significant precipitation or low air temperatures. On the other hand, high relative humidity, adequate soil moisture, and moderate to warm air temperatures all favor enhanced herbicide absorption and resultant control. Remember that, if conditions occur for enhanced absorption into weeds, conditions are also favorable for enhanced absorption into the crop, which could result in crop injury.

 

Tom Peters

Extension Sugarbeet Agronomist

NDSU & U of MN

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