When battling glyphosate-resistant weeds, there is no rest for the weary. Since mid-August, I have received dozens of calls about controlling glyphosate-resistant Palmer amaranth after harvest. Everyone is becoming aware that battling it will require a long-term investment of time and resources.
Unfortunately, the rains across much of the Mississippi Delta in August combined with the cooler temperatures over the past few days have brought another opponent back into the game, glyphosate-resistant Italian ryegrass. Even though glyphosate-resistant Palmer amaranth is still the top priority, in areas with a history of problems with glyphosate-resistant Italian ryegrass, this weed cannot be ignored.
The weed science research program at Stoneville, Miss., has been actively monitoring glyphosate-resistant Italian ryegrass emergence across the Delta for three years. Cool, wet conditions during July 2009 caused severe Italian ryegrass infestations in early August. Although temperatures were conducive for Italian ryegrass germination in early September 2010, we observed little to no ryegrass emergence until November because it remained extremely dry across most of the Delta from August through October.
We began our glyphosate-resistant Italian ryegrass emergence monitoring on Aug. 15 this year and found emerged plants in Tunica County on Aug. 30 and in Bolivar, Tunica, and Yazoo counties on Sept 7. If the current pattern of moderate temperatures and periodic rainfall continues, glyphosate-resistant Italian ryegrass will likely continue to emerge across much of the Delta.
As with Palmer amaranth in row crops or barnyardgrass in rice, the best time to control glyphosate-resistant Italian ryegrass is before it emerges. At Stoneville, we have evaluated fall applications of residual herbicides for control of glyphosate-resistant Italian ryegrass since 2006. Among currently labeled herbicides, fall applications of Command, Dual Magnum, and trifluralin (Treflan, Triflurex, etc.) have performed most consistently for controlling ryegrass.
The problem is Italian ryegrass has a broad emergence window ranging from fall through early spring, and a single herbicide application will rarely control this weed throughout its emergence time. Based on research conducted from 2009 through 2011, Tom Eubank and I are confident that the most effective timing for application of a residual herbicide in the fall for glyphosate-resistant Italian ryegrass is around Nov. 1.
What about the Italian ryegrass that is emerging now? Plants that are emerging now can pose a serious threat to spring burndown programs if allowed to grow until spring. However, if a residual herbicide is applied in September, the control will run out before the end of the glyphosate-resistant Italian ryegrass fall flush.
Over the last two years, glyphosate-resistant Italian ryegrass has ceased emerging in mid-December and begun again during February. Early-emerging glyphosate-resistant Italian ryegrass can be easily controlled with normal fall tillage in fields where these operations have not yet been completed.
In fields that have already been prepared for next year, a higher rate of paraquat (Gramoxone Inteon or equivalent 3 pound product) tank-mixed with the fall residual herbicide in October or November will control glyphosate-resistant Italian ryegrass that emerged prior to the residual herbicide application.
Fall residuals/spring burndown
One of the main objectives in our Italian ryegrass control research in 2010-11 was integrating the fall residual herbicide programs with spring burndown applications. The take-home message was glyphosate-resistant Italian ryegrass management must include both fall and spring control tactics.
One benefit of fall management, whether it is a residual herbicide application or tillage, is keeping the ryegrass population low enough for good spray coverage during spring burndown. We have observed fair glyphosate-resistant Italian ryegrass control with clethodim (Select Max or equivalent 2 pound product) applied in January followed by paraquat in early March.
However, where ryegrass densities were high, spray coverage from postemergence treatments was poor, so some plants survived while smaller plants never got treated because the herbicide was intercepted by the dense ryegrass canopy. When the same clethodim followed by paraquat spring program was preceded by a double-disking treatment in November, glyphosate-resistant Italian ryegrass control was 94 percent.
During most years in the Delta, November is not a good time to disk a field, so finishing field preparation earlier in the fall, and then following that with a combination of a residual herbicide plus paraquat in late October to early November will ensure having manageable populations of glyphosate-resistant Italian ryegrass in the spring.
In our research last year, we observed 98 percent glyphosate-resistant Italian ryegrass control in late March when a November application of Dual Magnum (1.33 pints per acre) was followed by clethodim (Select Max at 12 ounces per acre) in January OR paraquat (Gramoxone Inteon at 4 pints per acre) in February. When Dual Magnum was utilized in the fall, only one cleanup application was needed in the spring. But, when the plots were only disked in the fall, then both clethodim and paraquat were required in the spring.
Glyphosate-resistant Italian ryegrass infests a large acreage in the Mississippi Delta and has spread to Arkansas and Louisiana. Its broad emergence window and rapid growth can make it difficult and expensive to manage. The good news is that glyphosate-resistant Italian ryegrass does not possess the same devastating potential as Palmer amaranth.
Although it can negatively affect crop emergence and seedling growth (especially corn), it is a winter weed and control can be addressed before planting without the complicating factor of having a crop present. Also, we have seen cases where aggressive control tactics employed over two to three years have reduced populations to much more manageable levels.
For a comprehensive glyphosate-resistant Italian ryegrass control program, see Mississippi State University’s “Herbicide Programs for Managing Glyphosate-Resistant Italian Ryegrass”, which can be accessed at http://msucares.com/crops/weeds/herb_shortlist_lolmu_08-20-10.pdf.