Over the past several years, I have had numerous complaints of glyphosate not controlling goosegrass. With each of these past complaints, our field and greenhouse research showed that resistance was not the issue. Unfortunately, that has now changed.
I collected goosegrass seed from plants that had survived 1.5 pounds of glyphosate. The field where these goosegrass seeds were collected had been in soybeans eight of the last 10 years with the growers using repeated glyphosate applications. The farmer noted poor control during both 2009 and 2010.
In Knoxville, during March 2011, we initiated greenhouse research comparing a known glyphosate-sensitive biotype to the suspected glyphosate-resistant biotype. Glyphosate was applied at 0.25, 0.5, 1, 2, 4 and 8X the normal use rate (X rate = 0.75 lb ae or 22 ozs Roundup PowerMax) when plants were 3 inches tall.
At 21 days after application, the X rate of glyphosate controlled goosegrass from the suspect location only 45 percent. Control of the known sensitive population was 98 percent with the X rate and was 97 percent with the 0.5X rate.
In order to get the same level of goosegrass control of the suspected glyphosate-resistant population we had to use 6 pounds of glyphosate (8X rate).
A separate study by my colleague Tom Mueller on the now confirmed glyphosate-resistant goosegrass population would suggest that the mechanism of resistance is target site.
Eleven years ago, goosegrass in Malaysia was reported to be resistant to glyphosate. The goosegrass had developed resistance in orchards in that country where glyphosate had been used extensively for weed control under trees. The level of resistance they reported was eight- to 12-fold and was first reported to be a response to target site mutation.
The goosegrass we examined from Dyer County, where the suspect population was collected,would seem to react very similar to glyphosate as the resistant goosegrass in Malaysia.
Where should we go from here? I really think this year we should do our best to try to stomp this out before it gets out of hand. In a recent survey conducted at the Dyersburg grain conference, 80 percent of the growers indicated that they were going to use a pre-applied herbicide. Folks, that needs to be 100 percent!
Moreover, we need to have either Dual Magnum or Warrant applied to every soybean and cotton field. Those two herbicides should do a good job on both glyphosate-resistant goosegrass and glyphosate-resistant Palmer amaranth, so you are killing two weeds with one stone. These herbicides arealso in the Roundup Rewards program and you should take advantage of it.
As the summer progresses, please try to keep an eye out for goosegrass that glyphosate is not controlling. This is particularly true for those of you in Dyer, Crockett and Obion counties in Tennessee. This is also a good idea for all in the Mid-South.
If we look at the recent history of glyphosate-resistant weeds, rarely does the resistant biotype appear in just one area. A good example would be Palmer amaranth glyphosate-resistant biotype which developed at more than one location at about the same time.
As a result, if Roundup does not appear to be controlling the goosegrass be sure to follow up with a clethodim (Select Max) type product.
What does this mean? It means that now for the first time we have to manage a glyphosate-resistant summer annual grass weed. Fortunately, some of the management strategies we are employing to control glyphosate-resistant Palmer should help us control glyphosate-resistant goosegrass. Namely, utilizing a pre-applied herbicide and following up with a timely post-applied herbicide.
One final thought is that some folks are using Ignite to control weeds that have escaped from glyphosate. Of the four major grass weeds in Tennessee — goosegrass, crabgrass, johnsongrass and broadleaf signalgrass — Ignite is the weakest on goosegrass. Goosegrass control will be inconsistent with Ignite once the grass gets more than aninch tall.
The keys to managing these glyphosate-resistant weeds have not changed with this new addition. One still needs a pre-applied herbicide to become activated (for goosegrass Dual Magnum in soybeans, Prowl in cotton) followed by either another residual herbicide (Dual Magnum or Warrant) early post or a timely post-applied (Select Max, Fusilade, etc.) herbicide.