Postemergence Herbicides in Wheat: What a Year (06/03/21)
What a week of temperature extremes. Many were still scouting for frost and freeze damage by the time forecast highs in the mid to upper 90’s were announced. Herbicide application questions over the last week have ranged from “is it too cold to spray herbicides” to “is it too hot to spray herbicides”. Add in many uneven crop stands and it has been a stressful week (and year) for making postemergence herbicide decisions.
The temperature question is always a difficult one. Herbicides work best when weeds are actively growing. In general, ideal temperatures for herbicide performance are the 70’s to mid 80’s for a high, with overnight lows not too cold. We can expect less weed control when temperatures are on the cool side and plant growth slows down. Spraying after a morning frost will provide poor results. On the other hand, weed growth slows down in extreme heat (especially if accompanied by dry conditions). So now we can expect reduced weed control as temperatures soar above the 90 degree mark late this week.
Increased crop injury from herbicides is also a concern in extreme cold or hot conditions. Most questions coming in this week have been about spraying wheat in the heat since many wheat fields are reaching the critical time to spray POST herbicides. Spraying wheat in the heat of the day when temperatures are above 90 will certainly increase the risk of crop injury. If possible, it would be best to park the sprayer during the peak of the coming heat wave when temperatures are forecasted to be in the mid to upper 90’s. If a field has to be sprayed, then applications made in the morning or evening when temperatures are cooler can help mitigate some of the injury. Keep in mind that we do not want to spray during a temperature inversion in order to avoid off-target movement of the chemical. I’m no meteorologist, but the forecasts I have seen at this writing (evening of June 1st) indicate there will be some temperature relief early next week. Weed control and crop injury will be more acceptable if applications are delayed until that time.
Now for the other bugger this year: growth staging a wheat field for herbicide application. Many areas that finally received some rainfall two weeks ago are now looking at wheat anywhere from seedling to jointing…all within the same field. Some wheat herbicides have very narrow application windows, while others are flexible enough to allow application anywhere from emergence to boot. Knowing the growth stage window of your herbicides will be important to avoid crop injury and/or unacceptable herbicides residues if pre harvest intervals are not properly followed. Page 16 in the 2021 North Dakota Weed Control Guide has a handy table showing the application timing for many small grain herbicides. Keep in mind that the herbicides with the widest window of application will not necessarily be the best herbicide for your given weed spectrum. Tight supply chains may also limit the ability to switch out a herbicide option. Many fields will need to be evaluated on a case by case basis in the coming days to determine the best time to pull the trigger on herbicide applications.
Extension Weed Specialist