Dry weather after planting causes many concerns, including the impact of weeds on annual crops. Many herbicides lose effectiveness during dry periods; growers who use herbicides on corn and soybean crops are likely to be affected. Fortunately, an awareness of herbicide effectiveness and the aggressive use of mechanical weed control measures can make a difference.SOIL-APPLIED HERBICIDES
As illustrated above, when planting and spraying are close to the field preparation time (example a), there is more time to get the needed rainfall to make a surface-applied herbicide perform adequately. As time between field preparation and spraying increases (example b), there are fewer days after an application to get a timely rain. Thus, rotary hoeing becomes necessary sooner.ROTARY HOEING
Rotary hoeing kills weeds that have germinated but have not yet emerged. These weeds are in the "white root" stage of development. After weeds emerge, rotary hoeing is less effective. Rotary hoeing also helps place the herbicide in the upper soil surface so that when rains do fall, the herbicide is in a better position to be quickly taken up by weed seedlings and hopefully kill them. If it has not rained within seven days of the first rotary hoeing, make a second ass with the rotary hoe to kill the next generation of weeds.POST-EMERGENCE HERBICIDES
Post-emergence herbicides also may fail in dry weather. These treatments work best when weeds are actively growing. When weeds are stressed by lack of adequate soil moisture, chemical control declines. If you decide to apply post-emergence herbicides under very dry conditions, be aware that crop injury may occur and weed control will be poor.CULTIVATION
In all situations, be prepared to cultivate once or twice following rotary hoeing. Some growers mistakenly believe that soil loses more moisture when cultivated. But remember that weeds transpire water into the atmosphere every day they are in the field; the longer weeds live, the more soil moisture is lost and unavailable for the crop, and the harder they are to eradicate. So it is always a wise decision to cultivate weeds early on.
Your county agricultural agent
UW-Extension publication "Reduced Herbicide Rates: Aspects to Consider," (A3563).
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