• Associate professor, Plant Sciences Department, University of Tennessee.
• Research/Extension weed specialist.
• Major areas include weed management in cotton, soybeans, corn and wheat.
• Worked 10 years as a field agronomist with Pioneer Hybrid Int.
• Member of the Southern Weed Society and the Weed Science Society of America.
I have often heard folks talk about the good old days of this or that. Now I am doing it. The good old days of weed control in cotton just occurred in 2006 and 2007. This was the time span when Flex cotton was widely planted, and glyphosate controlled all the weeds except horseweed in the Mid-South. For the most part, cotton producers had learned to control horseweed with a good burndown program or tillage.
As a result, it was not often an issue in crops. Many producers had parked their often aggravating and always slow moving hoods and did all their cotton weed control with a 90-foot or 120-foot boom. Most likely, it will never be this easy again!
In the Mid-South, we have no less than six glyphosate-resistant (GR) weeds. These include horseweed (marestail), Italian ryegrass, giant ragweed, common ragweed, Johnsongrass and the weed that is of most concern, Palmer amaranth, which has proven to be the game changer with respect to weed control in cotton.
It is a highly competitive pest to cotton with recent research from Georgia showing as few as two Palmer plants/20 feet of row, lowering cotton yields by 23 percent.
As most cotton producers now know, Palmer amaranth hits a field in much greater numbers than two plants in 20 row-feet. Moreover, it will grow one to two inches per day, which was not an issue when glyphosate controlled it at any size. It is a huge issue when the only other post option, Ignite, will only control it up to four inches tall.
However, Ignite can only be used on glufosinate-tolerant cotton, which cuts down the number of acres available to this option. As a result, weed control has regressed, even with glufosinate-tolerant cotton, from a post-applied program to one that relies on pre-emergence, post-direct and layby herbicide applications.
Glyphosate effectiveness on many weeds, but particularly on Palmer pigweed, spoiled us. We are all looking for the next glyphosate to come on the scene and make weed control easy again. Unfortunately, there is nothing on the horizon that is as effective or easy to use as glyphosate. There are no new herbicides that can quickly come to the rescue.
The chemical companies cut back on research and development during the Roundup-dominant era. Research is now being ramped back up. Unfortunately, developing new herbicides does not happen overnight and, at a minimum, it takes 10 years from discovery of a new herbicide to when producers open the jug.
The other potential solution is the new genetic herbicide traits we have all heard about. These include the dicamba-tolerant technology and 2,4-D-tolerant technology. These technologies are much closer to being here than a new herbicide. In my opinion, the early press releases have oversold this technology as they will not be the new “glyphosate” in controlling GR Palmer.
Neither dicamba nor 2,4-D will control large Palmer pigweed like we could with glyphosate “back in the day.” However, in early work they do appear that they will control Palmer that is less than eight inches tall. In other words, these will be welcome tools to help us manage this weed, but we will still need to use pre-emergence herbicides as well as directed applications in most cases.
Cotton producers are now going through some expensive and frustrating times trying to manage GR Palmer amaranth. I am going out tomorrow to help a couple of producers decide whether to disk down some GR Palmer amaranth-infested crops or try to salvage them. It is a frustrating time for weed scientists in cotton country as well. We would like to have a better answer than “get the disk out.”
On the positive side, we can manage this weed if we change our weed control strategies. It will not be as cheap or easy as when glyphosate controlled Palmer, but with the use of a pre-emergence herbicide coupled with directed applications and in severely infested fields a timely application of Ignite, it can be done.
Larry Steckel is a weed scientist and Certified Crop Advisor based at the West Tennessee Research and Education Center in Jackson, Tenn.