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.