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Weed Management

Poor Weed Control: Another Consequence Of Drought

The herbicides used in peanuts are not new. The most commonly used products have been available for more than 10 years, and growers know what to expect from each of them. However, the 2007 season was disastrously dry for producers across the Southeast, and herbicide failure was relatively common. In a year with so many problems, poor weed control was another item on the list.

Why does drought make weeds difficult to kill?
Postemergence herbicides work by penetrating into a leaf, moving to the site of action (such as photosynthetic reaction centers), and stopping some process that is critical for plant life. Prolonged drought will hinder herbicide activity in essentially two ways. First, each plant leaf is covered with a layer to wax called a cuticle. When the plant becomes drought stressed, the cuticle begins to thicken in order to protect the leaf and ensure that water doesn't diffuse out. The thickened cuticle serves as a great barrier to herbicide entry. Essentially, drought reduces the amount of herbicide that enters the plant.

Secondly, a drought-stressed plant is not actively growing and movement of water and nutrients within the plant is limited. Therefore, any herbicide that enters the leaf will not be readily transported to the site of action. So, prolonged drought will reduce herbicide uptake while not allowing the herbicide that was absorbed to accumulate to lethal levels.

What steps should be taken to improve weed control during a drought?
First, don't wait too long to spray. Drought causes weeds to emergence slowly and many producers will wait for more weeds to germinate before they spray. In the mean time, many weeds will have gotten too large to be effectively controlled. Secondly, don't reduce the herbicide rates. Expect weeds to be difficult to control, and plan to use higher than normal rates, even up to the maximum suggested on the product label. Third, consider using a crop oil adjuvant. A surfactant, or sticker, increases herbicide uptake by changing the properties of the spray droplet. Conversely, crop oils works by partially dissolving leaf waxes. Reducing cuticle thickness will allow more herbicide to enter the leaf. This is why increased herbicide injury is often observed when using crop oils.

Drought conditions will cause numerous production difficulties, poor weed control only being one of them. There is little that anyone can do under these conditions to obtain the full potential of a herbicide. However, spraying small weeds, with the appropriate application rate and correct adjuvant system will make the best out of a bad situation.

 

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