If crops look stressed, it’s hot and sunny, there hasn’t been a decent rain for a while and the radio announcers are talking about drought, then you need to do things differently when you apply herbicides. Research shows it takes higher herbicide rates and more potent adjuvant treatments to control drought-stressed weeds. Weeds toughen up when they face moisture stress, high temperatures, low humidity and bright sunny conditions. Growth slows down, so does weed metabolism and translocation. Even more importantly, the weeds develop a thick waxy cuticle barrier on their leaves to reduce water loss and that thicker cuticle makes it more difficult for herbicides to penetrate.
Under drought conditions, it usually requires using the higher labeled rates of the herbicides and adjuvants to control weeds. We see a big effect from adding more of an appropriate surfactant to glyphosate applications even when the herbicide manufacturer claims its formulation is ‘fully loaded.’ We rarely see foliar herbicides work well without adjuvants, even when the weeds have thin cuticles. The risk of not using an adjuvant with a foliar herbicide is not worth it, always use adjuvants if the label allows.
Some labels address tough drought conditions with language such as “best results are obtained when applications are made to actively growing weeds.” If realistic, waiting for the next rain event and the plant to restart growing can help, although earlier is usually better in weed control. When water is scarce, the crop and weeds are competing for the last drops of water in the soil and the damaging effect of weeds on yield will be the greatest.
Spraying when humidity is high usually helps a little and may be very significant with a herbicide such as glufosinate. The key is keeping the spray deposits moist. A humectant helps keep the spray deposit moist so the herbicide can penetrate into the weeds over a longer time. We suggest using a polymer drift mitigation adjuvant and a surfactant with humectant properties. Lastly, follow the weed stage and label restrictions. Our experience is that larger weeds need higher herbicide rates and the best adjuvants because they are the ones most toughened up by drought conditions.
Nobody likes a dry season and having to spend more money when yields and profits are likely to be lower than usual. However, weed control under dry conditions is probably the investment that gives the highest return to the grower. It’s essential to get the adjuvant treatment right under those conditions, or the grower will be wasting his herbicide money. Tough environmental conditions separate the men from the boys in the adjuvant world.
The bottom line is that we need to pay close attention to when and what we apply under hot and dry conditions. Read the herbicide label to see what adjuvants the company recommends under drought conditions. Most will say to use more, a different type, or waiting until a rainfall and the weeds start growing again. Choosing the right tank-mix adjuvant system on top of the built-in one is always a good investment when you treat weeds under drought conditions.
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Weeds toughen up when they face drought conditions.