Problems with glyphosate-resistant Italian ryegrass escalated during 2012 in Mississippi. In my opinion, last year was the worst to date for this weed. Several factors probably contributed to the problem. The warm, dry winter in 2012 influenced spring burndown in fallow fields because Italian ryegrass never stopped growing.

Mississippi harvested an average of 2.6 times more wheat in 2011 and 2012 than in 2009 and 2010. Italian ryegrass in Mississippi is worse after a wheat crop because of the prevalence of Hoelon- (Group 1 herbicide) and ALS-inhibiting herbicide- (Finesse, Osprey, Powerflex; Group 2 herbicides) resistant populations.

Due to the lack of herbicide options for Italian ryegrass in wheat and the frequency of resistant populations, a tremendous amount of Italian ryegrass goes to seed in fields of Mississippi wheat. Unfortunately, many of these populations are also resistant to glyphosate.

The reasons can be debated, but the fact is glyphosate-resistant Italian ryegrass has spread at an alarming rate across Mississippi, especially considering Italian ryegrass seed are not spread by wind. At this time last year, 14 Mississippi counties were known to contain populations of Italian ryegrass that could not be controlled with glyphosate. Thirteen of those counties were located in the Mississippi Delta. By April 2012, an additional 17 counties had been added to the list, bringing the total to 31.

Management tips for glyphosate-resistant ryegrass

Glyphosate-resistant Italian ryegrass worsens

Glyphosate-resistant Italian ryegrass has spread from the Delta into the Hills with confirmed populations in most of the counties in the Jackson area and several counties in central and north-central Mississippi.

(See http://msucares.com/crops/weeds/resistant/index.html for a detailed listing of Mississippi counties with glyphosate-resistant Italian ryegrass populations.)

Italian ryegrass began emerging earlier this fall than in 2011 because of the rain and cooler temperatures during September. Our Italian ryegrass emergence monitoring sites indicated that emergence began during the week of Sept. 17 in many areas of the Mississippi Delta.

In years past, research at the Delta Research and Extension Center in Stoneville has demonstrated that September or even early-October is too early to apply a fall residual herbicide for glyphosate-resistant Italian ryegrass. Fields should not be treated with a fall residual until the latter half of October (November is even better, weather permitting) to maintain control through the fall emergence window, which usually ends during the middle of December.

With that said, some fields will contain glyphosate-resistant Italian ryegrass that has been emerged and growing for several weeks by the time a residual herbicide is applied this fall. This initial flush can be controlled with tillage in fields that have not yet been tilled for 2013. Where fall tillage has already been completed, paraquat (at 0.75 pound active ingredient per acre) should be included with any fall residual herbicide.

Weed scientist Tom Eubank and I stress the importance of controlling glyphosate-resistant Italian ryegrass prior to planting corn at all of our local and area Extension meetings. Corn acres will likely continue to increase, and unfortunately glyphosate-resistant Italian ryegrass competition can be devastating.

We conducted research in 2012 where corn was planted into plots with varying levels of glyphosate-resistant Italian ryegrass control. Corn yields were optimized where Dual Magnum plus paraquat was applied in November. Corn yields were also good in plots that were disked twice in November, treated with Select Max (12 fluid ounces per acre) in January and paraquat (1 pound active ingredient per acre) in late-February.

Controlling glyphosate-resistant Italian ryegrass is expensive. Most of the successful management programs cost approximately $40 to $44 per acre. Spending that amount of money on management of one weed species when no crop is even in the field is hard to justify. However, based on a corn price of $6.90 per bushel, the benefit-cost ratio for applying Dual Magnum plus paraquat in November and controlling GR Italian ryegrass escapes with paraquat in February was 13:1. So, in the first year of this research, for every $1 per acre spent on herbicide to control glyphosate-resistant Italian ryegrass, $13 per acre in revenue from increased corn yield was realized.

In fields with severe glyphosate-resistant Italian ryegrass infestations, 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.

For a comprehensive glyphosate-resistant Italian ryegrass control program, see Mississippi State University’s Information Sheet 1355 “Herbicide Programs for Managing Glyphosate-Resistant Italian Ryegrass,” which can be accessed at http://msucares.com/pubs/infosheets_research/i1355.pdf.

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