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Fall Residuals and Glyphosate-resistant Ryegrass
Author: Tom Barber, Extension Weed Scientist

residual ryegrass controlFall Herbicide Applications: Over the last several years fall herbicide applications have been increasing in use and popularity. Growers seem to like the idea of spraying a residual herbicide in the fall that will prevent any winter annual weed emergence, hopefully prior to planting. Two problems exist with this system. One, by preventing any winter annual emergence, soil erosion will increase and with average rainfall during winter months, most fields will have to be re-hipped by spring. The second misconception regarding fall herbicide applications is that they will work well into the spring and help to control initial flushes of glyphosate-resistant Palmer pigweed. This is simply not true. Residual activity of herbicides applied in the fall begins to decrease rapidly in early March, when soil temperatures begin to increase. The last two springs have been extended with cool, wet weather, and residuals seemed to last longer. However, as soon as temperatures began to increase at a constant rate last spring, we saw flushes of winter annuals late which resulted in one of the worst problems with glyphosate-resistant horseweed that we have experienced in a long time. Previous recommendations for horseweed control involved a residual when spring burndown was applied. This is still the most effective way to reduce late emerging horseweed flushes in-crop. It seems many producers relied too heavily and were too confident in fall applications to get them through to planting. In most situations it would be better to plant a cover crop that is easy to kill, such as wheat or cereal rye.  This will help reduce populations of winter annual weeds due to competition and will prevent bed erosion. Then apply an early spring burndown strategy with the use of residual herbicides to keep fields clean until planting. Recommendations for fall applications are only necessary for control of glyphosate-resistant Italian ryegrass. This problem adds another level of difficulty and should be addressed in the next couple weeks.

What about glyphosate-resistant ryegrass? Glyphosate-resistant ryegrass is becoming an increasing problem, especially in the southern half of Arkansas. Timing residual applications following corn or early bean harvest is not the ideal timing to control ryegrass flushes through the winter. At best these applications in August or through September may only last long enough to get the first flush of ryegrass. In heavy ryegrass infested fields, fall applications should be delayed until the 3rd week of October through early November for best control. Effective rates include Boundary 2pt/A, Dual Magnum 1.3pt/A, and Zidua 2.5oz/A. Depending on the weather, fall applications of herbicide may not last through early March. Last year the cool, wet conditions in spring allowed for extended control with fall applied herbicides. It is inevitable that these residuals will lose effectiveness, usually sometime in late February, or early March. Therefore be prepared to make an application of Gramoxone or Select when glyphosate-resistant ryegrass flushes emerge. In our experience, Gramoxone (paraquat) provides a higher level, and faster control of ryegrass in the spring than Select (clethodim), however in either case, two applications may be needed with high populations.

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