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As glyphosate-resistant Palmer amaranth continues to spread throughout much of the Southeast, nearly all growers find themselves altering their weed management plan to limit this threat. In order to achieve adequate control though, growers need to fully understand the pest they’re at war with. In a recent interview with, University of Georgia Extension Weed Scientist Stanley Culpepper, he detailed the biological attributes that make Palmer amaranth so challenging to control, as well as some best management practices to mitigate the devastating impact of this weed species.
Question: What are the major problem weeds in the Southeast?
Culpepper: Currently, there is only one weed of significance in cotton and that is glyphosate-resistant Palmer amaranth. Morning glory takes a distant second place. Management of these weeds is not an easy process and consists of aggressive programs comprised of several critical steps.
Question: What are the steps recommended for controlling glyphosate-resistant weeds?
Culpepper: First, no one will be successful if they do not understand the biological attributes that make Palmer amaranth so special such as seed production, growth rate, spread and an exceptional ability to prosper in challenging environments. Once you understand Palmer amaranth, an integrated and diverse management program must be implemented. None of my growers will be growers for long if they rely on a single management approach. Successful programs will include a sound herbicide management system, often including at least six herbicide modes of action and hand weeding, plus either tillage and/or cover crop residue. The next step is the hope that Mother Nature will help out with a favorable growing environment for herbicide activation, especially on dryland.
Question: When should growers begin scouting for Palmer amaranth and when should they begin treating?
Culpepper: Timeliness is a critical component to a successful management program. When dealing with a weed that can grow up to two inches per day under ideal conditions in the summertime, you have to be prompt. If you’re even two days late in treating this weed, you may not harvest your crop. In our world, management begins in March and often does not end until November, which coincides with the emergence period of Palmer amaranth.
Question: What does a season-long management approach look like?
Culpepper: In our cotton system, season-long control begins with residual herbicide application prior to Palmer amaranth emergence, and then overlapping residual herbicide applications throughout the cotton season until cotton canopy closure. Some growers will make at least five herbicide applications from burndown through cotton layby. Although herbicide programs can be extremely effective, the system will likely require some hand weeding plus either tillage or cover crop residue to assist in the battle. Currently, in Georgia, more than 92% of our cotton growers are hand weeding 52% of our cotton crop, which stresses the seriousness of the situation. Additionally, most growers are spending in excess of $80 per acre to manage this one weed on nearly every acre of cotton. We must improve our economics in managing this pest and that is where our research is focused currently.
Question: Which cover crops are growers having the most success?
Culpepper: We work with everything, but rye has shown the most success in our area. When done correctly, rye can actually reduce Palmer amaranth germination by at least 75%. Many scientists often suggest the impacts from rye cover crops on weed control are from an alleopathic effect, but our efforts suggest this is not the case with mature rye and Palmer amaranth. Our efforts suggest the rye cover crop reduces sunlight, soil temperatures and soil moisture, which are critical for Palmer amaranth germination. As we continue to improve the system, it is showing numerous additional benefits, including improved thrips control, improved water management, less herbicide use and less wind/water erosion.
Question: What are the possible effects of allowing weed escapes?
Culpepper: If growing in dryland, a female Palmer amaranth plant will produce about 400,000 seeds when competing with the cotton crop during the season. It could produce more in irrigated acres. No one can get all the Palmer amaranth plants in their fields, but we certainly are going to do our best at removing them from the field before seeds are released into the soil seedbank.
Question: How has resistance changed the way growers deal with weeds?
Culpepper: If you don’t control glyphosate-resistant Palmer amaranth, you simply don’t farm cotton in Georgia. Thus, every single cotton grower in my state has changed his or her farming operations to manage this resistant weed. Each grower is finding effective solutions, but one thing is for sure, and that is they are using integrated programs with great diversity. For those few areas where resistance is not observed, it is important to note that a preventive program is about 1/3 the cost and far more successful than programs dealing with resistance after it has arrived.
Question: Where do you see the future of weed control if we continue on our current path?
Culpepper: In terms of management, our current path is more sustainable than it may have ever been. Nearly every cotton field has six or more classes of herbicide chemistry, is hand weeded and also often includes either tillage or cover crop residue. However, I am concerned about the loss of conservation tillage production due to Palmer amaranth and the massive economic burden this plant is placing on our family farms. Our hope is to reverse the loss in conservation tillage and to convince traditional tillage growers to move toward conservation systems as these systems, when done correctly, appear to be the best chance for economic sustainability. Regardless of which approach our growers implement, we must stay aggressive, and eventually we’ll find a more economically sustainable approach.
Prevention is key when dealing with a resistant weed species. If Palmer amaranth takes over the fields, it is very likely that the crop will not get harvested. It is critical to manage resistant weeds in a timely manner through a variety of tactics including herbicide application, hand weeding, tillage and cover crops. Culpepper stressed the importance of contacting your county extension agent to figure out how to most effectively tackle this monumental problem. You can also visit roundupreadyplus.com to find additional regionalized crop recommendations.
Individual results may vary, and performance may vary from location to location and from year to year. This result may not be an indicator of results you may obtain as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible. 10;10; ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. Roundup Ready PLUS® is a registered trademark of Monsanto Technology LLC. Deltapine® is a registered trademark of Monsanto Company. All other trademarks are the property of their respective owners. ©2013 Monsanto Company.