NDSU Extension - Griggs County


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September 25

NDSU Extension Griggs County column by Jeff Stachler

Waterhemp is Present in Soybean Fields in Griggs County

Determining why weeds are present in a crop after herbicide application(s) is very important.  If you did not do this within 3 weeks after herbicide application, then just before harvest is the last opportunity to determine the cause.  Weeds present at this time of the year can be caused by choosing the incorrect herbicide, applying the incorrect herbicide rate, applying herbicides ineffectively, not using the correct adjuvants or adjuvant rates, and resistance to herbicides.  The most concerning reason for lack of control is due to resistance to herbicides.  If weeds are left in the field that have survived herbicide applications, then weed control will become more difficult in future years.

Last Thursday, September 17th I drove through parts of Addie, Cooperstown, Romness, Tyrol, and Washburn Townships evaluating weed control in every soybean field I passed.  I evaluated weed control on weed-free (no plants in a field), occasional (A plant of the species as an occasional individual plant) with a rating of 1, large patches (A patch(es) of 8 or more plants of individual species scattered in a field) with a rating of 2, and wide spread (Numerous patches or individual plants of the species across the field) with a rating of 3.

Only 21% of fields were weed-free, which is too low based upon my experiences.  However if you consider broadleaf weeds as being the most problematic for resistance to herbicides, then weed control was not too bad in the county as 48% of fields had zero of fewer than 3 broadleaf plants in a field.  So complete weed control was close, however some plants were left behind that will cause future problems.

The most frequent weed found in Griggs County soybean fields was kochia which was present in 35% of fields.  The next most prevalent weeds in soybean fields were common lambsquarters and barnyardgrass, both at 17% of fields.  The fourth most prevalent weeds in Griggs County soybean fields was waterhemp, a pigweed species, and biennial wormwood, both at 14% of fields.  The sixth most prevalent weeds were Powell amaranth and volunteer corn, both at 10%.  All other weeds present were in fewer than 6% of soybean fields which included marestail (horseweed), common ragweed, hemp dogbane, redroot pigweed, marshelder, and foxtail barley.

The weeds with the greatest density or infestation level in fields included marestail at 1.7 on the scale of 1 to 3 mentioned above, kochia at 1.4, Powell amaranth at 1.2, and waterhemp and biennial wormwood at 1.1.  All other weeds were a rating of 1.

The number of weed species within a field was 35% of fields having one weed species, 31% of fields having two weed species, and 13% of fields having three weed species.  No fields had greater than three weed species within the same field.

Of the weed species currently present within soybean fields, kochia and waterhemp are the most concerning to me and are likely present due to resistance to herbicides.  If there are not many plants within a field please take the time to remove these plants from the field by hand before or at harvest so they do not become a bigger problem than they already are.  Kochia and waterhemp are difficult to control in soybean and the cost of herbicides will increase.  You do not want waterhemp any more than you want Palmer amaranth!!

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