In the 1990s, when I was beginning my academic career at the University of Nebraska-Lincoln Panhandle Research and Extension Center in Scottsbluff, Monsanto still held the patent for glyphosate, and it could only be purchased in the US as Roundup herbicide. It sold for about $90/gallon, which in today’s dollars would be about $180/gallon. Needless to say, growers were not interested in applying a quart or two per acre in the 1990s. In fact, many growers did not want to apply even the labeled rate of 16 oz/acre. They were always asking “How low can I go?” It is a question that is once again being asked as glyphosate supply dwindles and prices skyrocket.
The answer at the time to “How low can you go?” was: quite low if you are only interested in grass control, the weather cooperates, and you do everything right in your application. Roundup was seldom used for broadleaf weed control because the rates required for broadleaf weed control were too high and too expensive. Glyphosate was almost always tank mixed or premixed with an herbicide for broadleaf weed control – recall the old Monsanto products Landmaster BW (glyphosate + 2,4-D) and Fallowmaster (glyphosate + dicamba). With today’s high prices and lack of glyphosate availability, growers should consider using glyphosate rates that are effective for grasses (16 to 24 oz/acre of a 3 lb ae/gallon product) and adding a broadleaf herbicide to the tank to handle the broadleaf weeds.
In studies conducted in eastern Washington and northeast Oregon, Dr. Judit Barroso, Oregon State University, and I looked at using soil residual herbicides to control Russian thistle in no-till fallow to reduce the dependence on glyphosate for Russian thistle control. In 2015, we both identified populations of Russian thistle that were resistant to glyphosate at 4 to 8 times the rate that controlled susceptible Russian thistle, so we were interested in finding alternatives to glyphosate in no-till fallow. We found that Spartan Charge, Fierce, and metribuzin can all be used for Russian thistle control in fallow. To reduce the risk for crop injury to subsequently planted winter wheat, a late-fall application of Spartan Charge may be the preferred treatment in low-rainfall regions where winter wheat–fallow is commonly practiced. A late-winter application may be preferred in higher rainfall regions where a 3-year rotation (e.g., winter wheat–spring wheat–fallow) is common. Fierce should be considered if other broadleaf weeds, such as tumble mustard or prickly lettuce, are of concern. Fierce also provides control of downy brome and other grasses in fallow. The use of these soil-applied herbicides will reduce the need for the frequent application of glyphosate for the control of Russian thistle and other weeds in no-till fallow.
The use of residual herbicides in burndown herbicide treatments is being promoted throughout the country this season with limited glyphosate inventories. As somebody with little hair left on his head, it is interesting for me to see us reliving the 1980s and 1990s again, at least as it relates to glyphosate use. Just as herbicide resistance has forced us to look at older herbicide chemistries for weed control, today’s herbicide marketplace is forcing many to look back in time to when glyphosate was expensive and the question was not how many quarts do I need to apply, but rather how few ounces can I get by with?
In addition to tank mixing glyphosate with herbicides for broadleaf weed control and/or with residual herbicides, it is important to make sure you use effective additives such as ammonium sulfate (AMS). Water quality is also an important issue. The AMS Sprayer Mix Calculator on the Wheat and Small Grains website can help you determine how much AMS is needed to offset the cations in your water. The Extension publication titled, The Impact of Water Quality on Pesticide Performance from Purdue University does a great job of explaining how water quality impacts pesticide performance. Glyphosate is a translocated herbicide, and it performs best when plants are small and actively growing, so avoid applying glyphosate in weather that is not conducive to rapid plant growth. With the current high prices and low availability of glyphosate, we need to forget the sloppy ways of the past 20 years and remember the things we used to do when glyphosate cost the equivalent of $180/gallon.
For questions or comments, contact Drew Lyon via email at drew.lyon@wsu.edu or phone at 509-335-2961.