If glyphosate is no longer effective at controlling perennial johnsongrass, but there is a low cost herbicide which is just as effective on johnsongrass with the same proven safety record, but is only going to be effective for a few years, would you use it? How would you use it?
In Argentina as of 2005, johnsongrass has developed resistance to glyphosate. In several southern U.S. states as of 2007 glyphosate is no longer effective on some johnsongrass.
In California, as of 2012, glyphosate is still effective at controlling perennial johnsongrass. “Glyphosate” is that “low cost herbicide” for as long as its effectiveness continues. Other glyphosate resistant weeds have been reported in one area and in a short time the same resistance has shown up around the world. No one knows how soon glyphosate resistant johnsongrass will occur in California, but from past experience with other glyphosate resistant weeds, it is likely to occur in the near future.
Glyphosate is such a unique and powerful herbicide, which will not likely be replaced anytime soon, so we should use it in the most effective way possible, now, while it is still effective. This is particularly true of perennial weeds.
Glyphosate and other herbicides can be used to almost eradicate johnsongrass from farms in California. According to experienced weed scientists, using other herbicides in place of, or in addition to, glyphosate may slow the development of glyphosate resistance. Because it is a relatively easy weed to spot in fields and around the farm, it should be controlled before it is allowed to set seed.
Fortunately it seeds are short lived for a weed seed, which means controlling all established perennial plants and then controlling new seedlings for a few years before they set seed, can almost eradicated johnsongrass. And for the seeds already in the soil, there are many other herbicides which have pre-emergence activity to control johnsongrass seeds.
Almost eradicated, because there are usually a few missed plants, or new johnsongrass seed comes in planting seed, or seed is re-introduced through wind and water from surrounding areas. These few seedling plants must be continuously controlled a couple of times each year to maintain effective control.
Also, if glyphosate has been sprayed and plants survive, it is tempting since glyphosate has always worked in the past to spray glyphosate again. These survivors could be the first resistant plants, so be careful to not miss glyphosate resistance when it occurs.
Remember johnsongrass is not a problem now primarily because of the effectiveness of glyphosate. Before glyphosate there was a lot more johnsongrass in many areas of California and it was very difficult to control.
When glyphosate is no longer effective on johnsongrass in California there are other herbicides which can be used, but they are not as effective and are more expensive. Get rid of 99% of the problem now while there is a very effective and inexpensive herbicide available. Be prepared for the time when glyphosate is no longer effective on johnsongrass in California.