2/18/19: Waterhemp - Your #1 Weed Concern

Many of you are planning your chemical programs for this upcoming season.  Yes, control of marestail, ragweed, velvetleaf, lambsquarter and cocklebur are still very important, but your primary focus should be on waterhemp.  Here are a few reasons why waterhemp is now considered the number 1 weed of concern in the MidWest.

  1. Competitive through sheer numbers, waterhemp at 20 plants per square foot can reduce bean yields up to 44%.
  2. They just keep coming, a higher percentage of waterhemp emerge later in the season compared to most other annual weeds.  Research has shown even if waterhemp don’t emerge  from the soil until beans are at the V5 stage they can still reduce yields up to 10%. 
  3. Fast grower, waterhemp gets the edge because it grows faster that most crops or weeds.  They can grow 1 to 1.5 inches per day during the growing season.
  4. Prolific producer, they can produce more than 250,000 seeds per plant, but can produce up to million seeds if there is little or no competition.
  5. Creates genetic diversity, waterhemp comes in male and female plants.  More genetic diversity means it’s easier for waterhemp to evolve and spread plants with resistance to various chemicals.
  6. Remarkable ability to adapt and extremely hardy, very hard to control if plants reach more than 6” tall.  Many waterhemp populations now exhibit multiple herbicide resistances within one plant.

Although there are many ways weeds escape control in crop fields, one the leading causes of waterhemp control failures is emergence of plants following postemergence herbicide treatments.  Remember, waterhemp can have as much as 50% of it’s emergence after mid-June.  That is why it is vital that you plan on split applications of herbicides with residual activity, with the first around planting time and then additional residual included in the post application.  When selecting herbicides for residual control, group 14 and 15 herbicides will provide the most consistent control with group 15 products the only chemistry with sufficient residual activity to be included with the post application.  Timing of the second application is critical for consistent results, with the second application being made about a week earlier than normal so that the initial application is still active.

Selecting herbicides that will provide full season residual will not only provide the greatest opportunity to maximize yield potential from cleaner fields but the reduction in the seed bank will greatly reduce the threat of new resistence.

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