Glyphosate-resistant biotypes of giant and common ragweed and common waterhemp have been confirmed in Minnesota and are listed on the International Survey of Resistant Weeds web site at: http://www.weedscience.org/in.asp Both species appear to be resistant to approximately four-times the labeled use rate of glyphosate (4X).
In the short time frame presented to us during the growing season, separating glyphosate nonperformance due to resistant weed biotypes from other factors is an inexact and qualitative process but a quick response could help reduce the spread of glyphosate resistant weeds and set-up long-term management plans.
A long-term plan would likely include chemical diversification that can provide consistent economic performance to the grower who uses Roundup Ready Technology and can help to reduce the probability of glyphosate-resistant weeds diminishing the economic value of this technology.
Whatever the cause, the short-term the goal is clear - weed control. Determining the presence of surviving plants early in the growing season allows time for successful control. Therefore, begin the process by scouting your fields 10 to 14 days after your first glyphosate application.
Glyphosate-resistant weed biotypes are difficult to detect because of their low level of resistance and the numerous other factors commonly associated with nonperformance such as:
• Misapplication – e.g. using too low of a rate for a given weed size.
• Improper timing – e.g. application on weed sizes exceeding label recommendations.
• Weed flushes after application of glyphosate, a non-residual herbicide.
• Cold weather conditions several days before or after application.
• Rainfall shortly after application.
• Dust or water with a high concentration of dissolved salts preventing absorption
Points to consider in assessing the likelihood of glyphosate-resistance biotypes would include the following:
• Recognize glyphosate-resistant biotypes early. Scout your fields approximately 10 to 14 days after your first glyphosate application to detect weed escapes. Look for the presence of a single surviving species, generally growing in patches.
• If more than one species is not controlled, especially if grass weeds are present; consider the factors other than resistance that might have affected nonperformance. Note- multiple resistant weed species are possible as is the presence of a more glyphosate tolerant species such as common lambsquarters.
• Is the uncontrolled weed species listed on the glyphosate label as being controlled at the weed size and herbicide rate that you applied? Reduced rates and application to larger weeds can contribute to nonperformance issues without the weed biotypes being resistant.
• Some weed species have always been difficult to consistently control in glyphosate dominated cropping systems. Examples include: Asiatic dayflower, wild buckwheat and yellow nutsedge. Common lambsquarters control is more consistent when treated weeds are less than 2 inches tall and during periods of warm and sunny conditions.
• Did the glyphosate application fail to control the same weed species, in the same area of the field in previous years? Resistance development is difficult to determine at low populations, however, repetition of glyphosate failure is an indicator of resistance.
• Do field histories indicate extensive use of the same herbicide for several years?
Over reliance on a single herbicide is a primary contributor to selection for herbicide resistant weeds.
• Do surrounding fields extensively treated with glyphosate indicate a similar level of nonperformance? Over time, herbicide resistant weeds can spread via harvest equipment, wind or wildlife to surrounding fields.
• For the weed species in question do you see a range of symptoms from dead plants to severely injured to stunted with shortened internodes but with a healthy meristem? Genetic diversity within a resistant weed population often produces a range of herbicide-induced injury responses.
One of the short-term key benefits for differentiating the likelihood of glyphosate resistant weeds from other issues of nonperformance is to prevent the remedial treatment from being a repeat application of glyphosate only at a higher rate. The application of more glyphosate on glyphosate-resistant biotypes accomplishes nothing but could result in satisfactory performance if nonperformance was due to some of the other factors listed previously.
For effective weed control, alternative herbicides should be applied at their labeled use rates (don’t cut rates) on small weeds and to prevent crop injury, the maximum growth stage restrictions should be adhered to (see examples below). Inter-row cultivation is also an effective option.
Corn Herbicide - Maximum Growth Stage
Callisto - 30 inches or V8
Hornet - 20 inches or V6
Impact - Up to 45 days before harvest
Liberty - LL corn only; 24 inches or V7
Status - 36 inches or V10
Soybean Herbicide - Maximum Growth Stage
Cobra - R6
Flexstar - Prior to bloom
FirstRate - Prior to 50% flowering
For more details on the biology and management of the glyphosate resistant weed species mentioned in this article go to: The Glyphosate, Weeds and Crops website at: http://www.glyphosateweedscrops.org/
Thanks to Jeff Stachler, U of M and NDSU Extension Sugarbeet Weed Scientist for assistance with this article.