By David Nicolai, University of Minnesota Extension

Glyphosate-resistant biotypes of giant ragweed and common waterhemp have been confirmed within specific locations in Minnesota in 2007. Diversification with other herbicide modes of action via pre-emergence herbicides, post-emergence tank mix partners or rotation to LibertyLink corn are some suggested chemical diversification strategies that can slow the rate of glyphosate resistance and reduce economic risk. Application to small (2- to 4- inch weeds) is more effective than application to larger weeds and proper spray application coverage definitely does improve glyphosate performance.

Value of pre-emergence herbicides

Dr. Jeff Gunsolus, University of Minnesota Extension Weed Specialist presented the following glyphosate weed control guidelines in a recent University of Minnesota Crop E-News article at http://www.extension.umn.edu/cropenews/ [1].

The use of a pre-emergence herbicide into a Roundup Ready cropping system can improve the effectiveness of glyphosate treatment by controlling many of the early emerging weed species and creating an environment where more of the weeds are at the same height and fewer in number at the time of glyphosate application. This will improve glyphosate effectiveness.

Corn and soybean pre-emergence herbicides will be more effective on common waterhemp than on giant ragweed. For pre-emergence control in soybean, consider products such as: Authority First/Sonic, Prefix, Valor, and Gangster (list not inclusive); in corn, consider products such as: Lumax, Harness/Surpass, SureStart (list not inclusive).

Post-emergence options

Effective corn and soybean post-emergence herbicide tank mix options are as follows:
• In soybean, glyphosate plus FirstRate has been shown to be quite effective on giant ragweed (unless the biotype is ALS resistant) and glyphosate plus Flexstar or Cobra can improve common waterhemp control.
• In corn, glyphosate tank-mixed with plant growth regulator or pigment inhibitor herbicides or Liberty plus atrazine on LibertyLink corn often improves control of both weed species.

Research supports diversification

Research conducted in Minnesota from 2004 to 2006 indicates very clearly that the use of a one-half rate of a pre-emergence acetanilide herbicide (e.g. Harness/Surpass) in corn reduces the risk of poorly timed post-emergence applications. In plots not treated with an acetanilide herbicide, a delay in glyphosate application by one week after weed populations exceed 4 inches in height (around V4 corn) resulted in yield losses of 12 bushels per acre, delay of glyphosate application for another week resulted in an average yield loss of 27 bushels per acre. Early weed control in corn is very important and in many fields provides consistent economic returns while reducing the risk of poorly timed glyphosate applications.

In summary

Chemical diversification 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. Points to consider this year as you are planning your weed management program:

• Utilize other modes of action through use of a pre-emergence herbicide or a tank-mix partner.
• Consider alternating Roundup Ready crops with LibertyLink technology or a conventional hybrid or variety.
• As the growing season moves into high gear don’t forget to scout your fields approximately 10 to 14 days after your first glyphosate application to detect weed escapes.
• If weeds survived, try to determine why. Was the failure caused by misapplication, poor weather, poor timing, or later weed flushes? Get on these problems early while there is still some time in the current growing season to address them.
• Keep in mind that many post-emergence herbicides have crop size restrictions.
• If your glyphosate applications have failed to control the same weed species, in the same area of the field for several years, you may have a resistant weed problem. The sooner you act on a potential problem, the better.
• Other weed species that are difficult to consistently control in glyphosate- dominated cropping systems are: common lambsquarters, common ragweed and in no-till fields — horseweed.
• For more details on the biology and management of these weed species go to: The Glyphosate, Weeds and Crops Web site at: http://www.glyphosateweedscrops.org/ [3]
• For further information on the performance of various herbicide programs in University of Minnesota research trials, check out the Applied Weed Science Research Web site at: http://appliedweeds.cfans.umn.edu/research.html [4]

(David Nicolai is a crops educator with University of Minnesota Regional Extension in Hutchinson.)