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A Mid-Season Look in the Field

Dave Pazdernik, Minnesota Soybean Director of Research

Dave Pazdernik, Minnesota Soybean Director of Research

Many Minnesota soybean growers have finished spraying this week so now is a great time to check fields for weed escapes and new weed flushes, especially pigweed and more specifically waterhemp and possibly palmer amaranth. 

Palmer amaranth, also known as Palmer pigweed, has been found in Iowa and it is critical to keep watch for the presence of this weed in Minnesota fields.  If Palmer pigweed is suspected in a field, please contact your favorite agronomist or extension person and have the weeds positively identified.  If confirmed, destroy all Palmer pigweeds as these weeds should not be allowed to produce seeds, which is critical to keeping this weed out of Minnesota fields. 

Palmer pigweed is extremely competitive and will outcompete most plant species, even common waterhemp. Palmer pigweed specifics:

  • Can grow up to 2.5 inches per day, easily reaching 7 feet and appearing like a small tree.
  • Has the capability of producing one million seeds per plant.
  • A significant amount of the plant growing in the U.S. is glyphosate tolerant.
  • Sources of the weed include seed from animal feed stocks being trucked from the southern U.S. and vehicles and birds coming from other regions of the country.

There are a number of herbicides that control Palmer pigweed, but the main key to control is ensuring that you spray these weeds when they are less than 4-inches tall, and definitely not any taller than 6 inches.  Palmer pigweed is extremely difficult to kill with most herbicides if they are 6 feet or taller. Unfortunately, spraying Palmer pigweed over 6-inches tall is also a great way to develop herbicide resistant weeds. It is also important to determine the original source of Palmer pigweed seed.

For more information on Palmer pigweed and other difficult to control weeds, USB published “Take Action Herbicide Resistance Management,” an excellent booklet on herbicide resistant weeds and how to control them. The publication can be found at www.takeactiononweeds.com and is a guide to herbicide classification, identification of many common weeds and numerous weed management fact sheets. 

Waterhemp still more common in Minnesota

Today, you are much more likely to encounter common waterhemp than Palmer pigweed in your fields.  Most pigweed that has emerged in Minnesota in May and June has been redroot pigweed, but as the temperatures warm and growing degree days accumulate, waterhemp, which begins to emerge in June, will continue to emerge throughout much of the growing season. 

Therefore, it is critical to watch for new flushes of waterhemp throughout the growing season. Waterhemp germinates in the top quarter to half inch of soil, and sunlight increases the amount of waterhemp germination. Minimizing sunlight contact with the soil and not doing any soil tillage (cultivation) are two keys to preventing new waterhemp flushes from emerging as the season progresses.

Additionally, no-till fields may have less waterhemp as the soil is less disturbed when compared to conventional till fields, and there is more trash on the soil surface to prevent sunlight from reaching the soil surface. Another common practice that significantly reduces new flushes of waterhemp is planting narrow row or drilled soybeans as these practices tend to lead to a faster canopy, again preventing sunlight from reaching the soil surface.

Waterhemp has also developed different degrees of tolerance to glyphosate and many other herbicides. If you notice large waterhemp plants in your fields two to three weeks after spraying, you have weeds that are termed “escapes.”  Escapes can come from a number of causes including but not limited to, weeds being missed by the sprayer, weeds being small and hiding under the canopy so they miss the spray, or weeds tolerant to the herbicide being used.  If there are some large waterhemp escapes in your fields, it would be advantageous to destroy them as they could be a source of seed for herbicide tolerant weeds in the future. 

Two weeks ago, the University of Minnesota Extension Service printed an article entitled “Waterhemp has emerged in Minnesota” (http://blog-crop-news.extension.umn.edu/2015/06/waterhemp-has-emerged-in-minnesota.html).  The first step in controlling waterhemp or pigweeds in general is proper identification of the pigweed species that you have in your fields. Identification of pigweed species can be very difficult, especially at the seedling stages. There is an excellent seedling pigweed identification guide listed in the extension article (http://www.ksre.ksu.edu/bookstore/pubs/s80.pdf) developed at Kansas State University. Both the Minnesota extension article and Kansas weed seedling identification guide are excellent sources of information on weed management.

Lastly, there are three weed science field days coming up on July 9 in Waseca, Minn.; July 28 in West Fargo, N.D.; and August 12 in Olivia, Minn. Learn more below:

Weed science field day July 9

Weed resistance field days July 28 and Aug 12

Dave Pazdernik is the Director of Research at Minnesota Soybean. He can be reached at (507) 388-1635 or dave@mnsoybean.com