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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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