ISSUE 5 June 10, 2010
STATUS LISTING IN THE ND WEED GUIDE
Please revise the Status information on page 19 in the 2010 ND Weed Control Guide:
Application window = V2 to V10
Rate range: = 2.5 to 10 oz/A is accurate by label, but for practical purposes the rate range is 2.5 to 5 oz/A.
WEED CONTROL IN BEANS
Soybean stage and weed stage are nearing the application window for postemergence herbicide application. Make application to small weeds - small weeds are much easier to kill than large weeds.
Do not be surprised to see patches of weeds escape glyphosate (only) applications. Repeated glyphosate applications have allowed weeds that were once killed with an X rate of 1 pt or 1.5 pt/A to survive those rates and require higher rates to achieve similar control. It may be only a concept but 100% weed control (through tank-mixtures, other modes of action, and repeat applications) will not allow weeds to go to seed and thus no seed production. 100% weed control will not allow weeds that have developed some level of resistance to glyphosate to increase. Research by Dr. Jeff Stachler at NDSU has shown the progeny of weeds with low level resistance to have an increased of level of resistance than the parent material.
Several have asked us over the years which weed would we first expect to be glyphosate resistance.... interesting question. Many candidates to choose from like wild oat, green foxtail, and buckwheat. Dr. Stachler is documenting common ragweed resistance to glyphosate in eastern ND but the weed that we feared and has more impact on a state-wide basis is kochia (see below). There are reports from South Dakota and now in North Dakota of kochia escaping normal applications of glyphosate. The weeds group at NDSU thought kochia may be the first to show resistance based on previous documentation of resistance to several other modes of actions. It appears kochia resistant biotypes may be on the increase.
I get many questions on how to kill dandelion in soybean. People think there is some hidden secret but in summary there isnít. Think about it - dandelion is a perennial. What herbicides kill other perennial weeds like Canada thistle, sowthistle, and field bindweed? - there basically four: glyphosate, 2,4-D, dicamba, and Tordon. None of these can be used in soybean with the exception of glyphosate in RR soybean. For a good review of dandelion control see the "Weed of the Year" in the 2009 ND Weed Control Guide or at:
For weed control in conventional soybean, greater weed management must off-set and surpass the convenience and forgiveness of glyphosate use in RR soybean. More timely applications, proper herbicide rates, use of superior adjuvants, adequate coverage with contact herbicides (Basagran, Flexstar, many others) are required to achieve adequate weed control. The NDSU soybean micro-rate (explained on page 25 of the weed guide) can give wide spectrum grass and broadleaf control of SMALL weeds.
GLYPHOSATE-RESISTANT KOCHIA CONFIRMED IN KANSAS
Kansas State University Research Scientists and Monsanto have been evaluating kochia populations now confirmed as glyphosate-resistant. (February 26, 2010)
Kansas State University scientists have completed long-term evaluations of a limited number of kochia populations that are now confirmed to be glyphosate-resistant. These populations have undergone both greenhouse and field testing by both Kansas State and Monsanto personnel.
Dr. Phil Stahlman of Kansas State University has listed two to five individual sites in western Kansas with glyphosate-resistant populations on the International Survey of Herbicide Resistant Weeds web site (http://www.weedscience.org) following lengthy evaluations of greenhouse and field studies. These sites were also under investigation by other Kansas State scientists and Monsanto researchers, focusing on the variability of the resistance and difficulties in proving heritability - a trait required for confirmation of resistance.
Kochia control by glyphosate can be adversely affected by both growth stage and environmental conditions, with erratic performance fairly common. Initially lack of control was thought to be due to factors or circumstance other than resistance. The areas that practiced low use rates were the first to exhibit lack of control of kochia not due to environmental factors.
NDSU Extension Weed Specialist