Research Highlights

Investigating Italian Ryegrass Management Options

By Laura Temple

Italian ryegrass is second only to Palmer amaranth as a problem weed in Mississippi soybean fields, according to Jason Bond, Extension and research weed scientist with Mississippi State University. Its combination of natural herbicide tolerance and the herbicide resistance it has developed limits effective control options.

“Italian ryegrass is a winter annual,” Bond explains. “When it isn’t controlled, it is growing and maturing at planting, when soybeans are most vulnerable to yield loss due to weed competition.”

Mississippi was the first state to confirm the presence of glyphosate-resistant Italian ryegrass in row crops in 2005. In addition, some Mississippi populations of Italian ryegrass are resistant to various ALS- and ACCase-inhibiting herbicides. The weed continues to develop resistance to other herbicides in orchards and specialty crop fields.

Because resistance limits control options, Italian ryegrass has become the focus of ongoing weed control research led by Bond. Soy checkoff funding from the Mississippi Soybean Promotion Board makes this research possible.

Residual Fall Herbicides

Italian ryegrass emerges when temperatures consistently stay below 90°F and soil moisture is available. Most of the population finishes germinating by mid-December. Bond’s team is exploring the efficacy of residual herbicides applied after harvest to control Italian ryegrass before or as it emerges.

“Because of the soil texture in much of Mississippi, farmers do as much seed bed preparation as possible in the fall after harvest,” he says. “This research looks at timing herbicide application with fall tillage and other fieldwork to coincide with the first flush of Italian ryegrass emergence.”

Depending on fall weather, these herbicide applications would occur in October or November. In addition to testing herbicides labeled for this type of application, this research also seeks to broaden control options.

“Our studies include herbicides that could be labeled for a fall application preceding soybean, but aren’t yet,” Bond explains. “The residual herbicides last longer in the fall and winter than during the summer.”

Many of the herbicide treatments applied in the fall of 2021 provided at least 95 percent control of Italian ryegrass through early February, he reports. 

Sequential Residuals

“We’ve noticed more Italian ryegrass emerging in mid-February and March than we had seen in the past, so we also looked at sequential residual applications,” he adds.

In those studies, the second residual applications were made in January and February, traditional burndown herbicide timing for Mississippi soybean fields. His team found this approach very effective.

“The trick is managing annual limits for application of some of these herbicides that are also commonly used to control summer annuals,” Bond says. “One example is s-metolachlor, found in Dual Magnum and similar products.”

As this research continues, the team plans to replace s-metolachlor with other herbicides in the fall so it can be used for in-season weed control before reaching the labeled application limit. The studies mix and match herbicide components based on total weed pressure throughout the season. His team hopes to determine when specific herbicides offer the most value and yield protection. 

Bond notes that fall herbicide applications don’t fit every soybean production system. They should target specific problem weeds. Some fall herbicides also control broadleaf weeds, which can leave soils bare for longer in the winter, increasing erosion risks. 

Alternative Burndowns

Another aspect of this research is exploring burndown options to control Italian ryegrass. Currently, two effective, affordable herbicides can be used for burndown: paraquat and clethodim, the active ingredient in Select and similar products. 

However, Italian ryegrass is resistant to clethodim, an ACCase inhibitor, in some areas of Mississippi. In other parts of the country, Italian ryegrass has developed resistance to paraquat in orchards and vineyards. 

“As resistance gets worse, we need to be prepared to continue controlling problem weeds,” Bond says. “In this region, farmers rely heavily on paraquat for both burndown and soybean desiccation before harvest. This research will point us to options and management practices that can help us preserve its use.”

He has been encouraged by the early results of all these studies. However, the work will continue next season to provide more data in various weather conditions to guide development of future Italian ryegrass control recommendations.

Published: Sep 12, 2022