Studies conducted at the Delta Research and Extension Center in Stoneville, Miss., indicate residual herbicides applied in the fall provide the most efficient control of glyphosate-resistant Italian ryegrass.

Glyphosate resistance first appeared in Italian ryegrass in Chile in 2001 and in the United States in Oregon in 2004.

The documented appearance of glyphosate-resistant Italian ryegrass in Mississippi in 2005 “is the first ever in field crops,” according to Jason Bond, a weed scientist at the center. “The two cases in Oregon were both documented in orchards.”

Bond says repeated applications of glyphosate on Italian ryegrass over the years “have put almost perfect selection pressure for the weed to develop resistance.

In addition, “we have a high percentage of crops grown using some form of reduced tillage practices, so that’s a double whammy on the selection pressure.”

Italian ryegrass in Mississippi also has documented resistance to ALS herbicides. However, ALS resistance has so far been documented only on roadsides.

Currently, glyphosate-resistant Italian ryegrass is confined to between 10,000 acres and 30,000 acres in Washington, Bolivar and Sunflower counties in Mississippi, but based on observations this spring, much more acreage is infested, according to Bond.

Early screenings of suspected resistant plants showed a three-fold tolerance to glyphosate, according to Bond. “We now think this resistance is probably much higher than that. In the field, we are not seeing control at as much as a gallon per acre in some cases.”

Initial greenhouse studies first focused on controlling resistant Italian ryegrass with post-emergence herbicides, Bond said. “The studies indicated that Gramoxone Inteon provided a percent mortality that was manageable.” Select Max at 8 ounces and Gramoxone Inteon at 40 ounces provided 95 percent or higher control in the early timing (two to three tillers). Gramoxone Inteon at 40 ounces and 56 ounces provided 100 percent control at late timing (10 to 12 tillers).

After the initial work was concluded, Mississippi weed scientists honed in on salvage applications in the spring on larger ryegrass. They conducted two studies, a Select Max rate study with and without glyphosate and a sequential study with different herbicides applied before and after ryegrass flowering.

Select Max was applied at three rates, ranging from 6 ounces to 12 ounces of product per acre, with and without glyphosate. According to Bond, “There was a benefit on control for the two lower rates of Select Max when glyphosate was added. At the highest rate of Select, there was not a benefit when glyphosate was added. However, none of the treatments provided more than 70 percent control. The study showed that a single pass application in the spring really isn’t a viable option.”

In the sequential application study, weed scientists looked at glyphosate alone, glyphosate plus two rates of Select Max (6 ounces and 9 ounces) and Gramoxone Inteon alone.

“The best treatments in the study were either glyphosate plus the two rates of Select Max prior to flowering or Gramoxone Inteon alone, followed by the highest rate (4 pints) of Gramoxone Inteon. All those treatments were very expensive.”

Scientists also noted that while Italian ryegrass was controlled, “we had a lot of residue left on the soil surface at the time of planting. This poses a serious problem for our producers because most of our equipment is not set up for a high level of residue.”

Later work turned more toward controlling ryegrass sooner “so we might reduce the amount of residue in the spring,” Bond said. “We conducted a post-emergence timing test with multiple rates of glyphosate and glyphosate plus other post-emergence herbicides including Gramoxone Inteon, Ignite, dicamba and 2,4-D. Several of those are also very important treatments in the Delta for resistant horseweed.”

Bond said the fall treatments were more consistent than early- or late-spring treatments. “In the spring, we needed Gramoxone Inteon at the higher rates to really knock it down.”

Another two-year trial looked at Treflan incorporated, Prowl H2O surface-applied and incorporated, multiple rates of Dual, Command and KIH 485, which is an experimental herbicide, and Valor at a site where there was heavy pressure of resistant Italian ryegrass.

The applications were made in early November and evaluated six months later. “Our best options were the two highest rates of Dual, all rates of Command and the higher rate of KIH 485. Those products held all the way to planting and we didn’t have much residue on the soil surface. The others broke in the spring and required a follow up application.”

Another test compared Gramoxone Inteon and residual herbicide tank mixes at three timings. Residuals included Direx, Linex, Goal and Valor. The best treatment was Gramoxone Inteon plus Direx in the fall. “Out to 140 days after the fall application, we still observed greater than 90 percent control.”

One problem is that many growers may not find out they have resistant Italian ryegrass until the spring. “But with Gramoxone Inteon and Direx, you can knock it down as late as February. It’s not perfect, but it leaves a manageable amount of residue.”

Bond concluded that single pass applications in the spring “are not what we want. We can knock it down with sequential applications, but you have to have some tank mixes of some expensive products. The best of those is sequential applications of Gramoxone Inteon at 4 pints followed by 4 pints. That’s a full gallon to knock down the ryegrass.

“Gramoxone Inteon in the fall works well on small Italian ryegrass, but we can have multiple flushes, so that treatment may not be very effective. When Dual, Command and KIH 485 are added, you get control for quite some time.”

Bond says tillage can be a benefit where it fits the producer’s system. “It’s not so much an issue in cotton as it is in soybeans where we do have a higher percentage of our acres in some form of reduced tillage.”

Another issue is that grower may not have fall herbicide applications budgeted, Bond notes. There is also a consideration for soil loss if the ground is bare from herbicide application. “We are going to address some of those issues in future studies.”

Bond says weed scientists will also be conducting grower surveys to determine the full distribution of resistant ryegrass. “Along with that, we want to determine how glyphosate tank mixes with dicamba and 2,4-D targeted at horseweed can impact ryegrass control.”

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