Sweetpotato Herbicides- FAQs

Stephen Meyers, Sweetpotato Extension Specialist
By Stephen Meyers, Sweetpotato Extension Specialist May 29, 2015 13:28

Sweetpotato Herbicides- FAQs

Due to a lack of post-emergence herbicides for use in sweetpotato, sweetpotato weed management is front-loaded with pre-emergence herbicides early in the growing season. For the last several years the following herbicide program has provided the most consistent control of the greatest diversity of weeds:

2 to 3 oz/acre Valor pre-transplanting followed by

1.5 to 2.66 qt/acre Command within 48-72 hours of transplanting followed by

a cultivation prior to vine running (3 to 4 weeks after transplanting) and immediately followed by 0.75 pt Dual Magnum followed by

Select, Poast, or Fusilade (or equivalent products) as needed for control of escaped grasses.

While the program works well, it is not a one-size-fits-all and may not work for every production field in every year.

With that in mind, each year I receive these questions:

  1. When is the best time to apply Dual Magnum and at what rate?

Dual Magnum (active ingredient = S-metolachlor) is registered for application any time after transplanting until 60 days before harvest. In Mississippi the maximum labeled rate is 1.3 pints/acre and must be applied in a single application. The best application timing will depend on what weeds are present in each production field. In production fields with sedges, Dual Magnum should be applied to a clean production field immediately after transplanting. The standard 0.75 pt/acre recommendation has provided control of sedges equivalent to higher rates. In fields where sedges are not present or at very low densities, Dual Magnum applications should be delayed until at least 10 days after transplanting. Why delay this application? Dual Magnum applications made immediately after transplanting and followed by moderate to heavy rainfall can result in modest plant stunting, reduced yields, and misshapen storage roots. Crop tolerance increases every day through at least 14 days after transplanting.

Remember: Dual Magnum does not offer any post-emergence weed control.

  1. How do I apply Valor?

Valor (ai = flumioxazin) applications in sweetpotato require attention to detail. Once Valor is applied, movement of treated soil will dilute its concentration in the soil and limit weed control. Prior to application, production field rows should be rolled or knocked down to the height of transplanting equipment. If row knockers on the transplanting equipment will be in contact with the soil, remove them prior to transplanting. Ensure that press wheels close the transplanting trench to avoid a concentration of herbicide from washing into to the trench and coming in direct contact with slips. The original supplemental labor for Valor stated that it should be applied 2 to 5 days before transplanting. The current label does not provide exact dates. Valor can be applied at bed formation to help maintain a weed-free row until transplanting. However, 2 to 5 days pre-transplanting remains the standard.

Properly rolled rows prior to Valor application will minimize soil disturbance at transplanting.

Properly rolled rows prior to Valor application will minimize soil disturbance at transplanting.

When using Valor pre-transplant in sweetpotato, row knockers may need to be removed to minimize soil disturbance.

When using Valor pre-transplant in sweetpotato, row knockers may need to be removed to minimize soil disturbance.

  1. Do I need to use Command?

In the standard herbicide program Command (ai = clomazone) does a few things. Primarily, it helps to cover soil that was previously treated with Valor but disturbed during transplanting, provides excellent control of common grass weed species that Valor may only suppress, and helps to maintain weed control through the vine running cultivation. If Valor applications are made as described above, and receive an activating rainfall, it is possible to have sufficient weed control through vine closure. However, it is likely that grasses will need to be sprayed with a grass-selective herbicide prior to vine closure. Another option would be to cultivate when grass escapes begin to emerge as long as the cultivation sufficiently covers emerging weeds within the planted row. Following cultivation, Dual Magnum may be applied or a subsequent cultivation could be utilized at vine running.

  1. Why should I wait to cultivate?

Research conducted both at LSU and Mississippi State University suggest that delaying cultivation until at least 3 to 4 weeks after transplanting improves season-long weed control. Cultivation removes the layer of treated soil from the surface and brings fresh weed seed from the soil seedbank to the surface. If you don’t see weeds, don’t cultivate. Of course, there are exceptions to every rule. For example, if heavy rainfall threatens the integrity of the row, cultivation will be necessary to ensure proper crop growth and development.

 

Disclaimer: All pest management options come with risk. The statements made in this post are based on years of research data from across the Southeastern U.S. The use of product names is not an endorsement of any product. Herbicide formulations and regulations are subject to change. Read and follow all label directions.

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Stephen Meyers, Sweetpotato Extension Specialist
By Stephen Meyers, Sweetpotato Extension Specialist May 29, 2015 13:28
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