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. Both species appear to be resistant to approximately four-times the labeled use rate of glyphosate (4X).
In the short timeframe 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 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 10-14 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:
Points to consider in assessing the likelihood of glyphosate-resistant biotypes include the following:
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 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 in. or V8
Hornet: 20 in. or V6
Impact: Up to 45 days before harvest
Liberty: LL corn only; 24 in. or V7
Status: 36 in. or V10
Soybean Herbicide – Maximum Growth Stage
Flexstar: Prior to bloom
FirstRate: Prior to 50% flowering
For more details on the biology and management of the glyphosate-resistant weed species mentioned, go to Glyphosate, Weeds and Crops.