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Pigweed increasingly problematic

Pigweed has rapidly become a serious weed for Arkansas farmers. I constantly hear all sorts of horror stories about fields growing up with pigweed after being treated with every imaginable combination of herbicides. There is no silver-bullet pigweed control program, but there are some things we can do to increase our success.

Properly timing applications is the most important thing we can do. Several herbicides provide good control for a short period of time, but pigweed germination is not influenced by day length and, therefore, the weed will germinate all summer behind every pivot rotation or when moisture is available.

Pigweeds grow fast and will quickly get too tall for good coverage with post-directed herbicides. For this reason, they must be controlled often or with a residual herbicide.

Contrary to most weed control situations, the more sand in the soil, the more difficult it is to control pigweed. I believe this is because our soil residual herbicides are leaching below the top 0.25 inch of soil, where most of the pigweeds are germinating.

Each pigweed plant is capable of producing over 100,000 seeds. Eighty to 90 percent of the seeds are viable and will germinate if conditions are right. Some remain viable buried in the soil for five or six years.

Pigweed seeds are extremely small and do not germinate very deep in the soil. Reduced tillage operations bring less seed to the top 0.25 inch of the soil profile and will reduce the number of seedlings. This may be insignificant in the big picture when there are several billion seeds in a field where pigweeds were allowed to produce seed any time within the past five or six years.

So where to from here? In our research verification fields, this is our approach:

Pigweeds and sandy soils may be the best fit for Roundup Ready cotton and we have elected to plant Roundup Ready cotton in all our heavily infested pigweed fields. But glyphosate is not the total answer.

In our conventional-till fields, we incorporate a yellow herbicide preplant. In our no-till or stale-seedbed fields, we omitted Prowl because it could not be incorporated.

Although Prowl is labeled as a pre-emergence herbicide and many farmers have been successful with surface-applied Prowl, others have experienced brittle or girdled stems and cotton injury. Due to this injury potential, we still have not added Prowl as a pre-emergence herbicide to our recommendations.

We have chosen Caparol and Zorial for pre-emergence herbicides. Both have activity on pigweed and offer some residual control. Zorial will stay in the top soil layer better than almost any other herbicide, especially on row-watered cotton. Every time water moves up the bed, the Zorial moves with it.

Glyphosate over the top at three- to four-leaf is deadly on pigweeds and should be adequate with Caparol + Zorial pre-emergence. As soon as we can get back in with a post-directed application, we use glyphosate plus another residual product like Caparol.

At layby we have elected to use a herbicide with some residual control in combination with MSMA or glyphosate. Although we would like to use Bladex, our supply is limited. We have, therefore, selected Goal — or Valor if the Section 18 is approved — to provide a few days of residual control after layby until our cotton can canopy.

Sides of beds and middles will be treated with glyphosate under a hood until layby and glyphosate or glyphosate plus a residual at layby.

Reflex is probably the most effective herbicide we have tested for residual control of pigweed. We experienced excessive injury with Reflex and felt the potential for injury was too great for us to recommend or continue testing it. We will, however, continue to devote considerable research time and effort toward finding more economical and efficacious control tools for pigweed. The silver bullet may just be in the next set of plots.

Kenneth L. Smith is an Extension weed scientist at the University of Arkansas.

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