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More weeds, fewer options for rice weed control

Typically, glyphosate-resistant Italian ryegrass has not been a serious problem in rice in Mississippi because it prefers a better-drained, coarse-textured soil. But it has started to show up in clay soils. Controlling the Italian ryegrass – especially when all efforts are concentrated in the spring – can be problematic. Glyphosate-resistant Palmer amaranth has also become a problem in rice because it is often left uncontrolled in the soybean rotation.

While barnyardgrass is still the No. 1 weed for rice producers in Mississippi, two glyphosate-resistant weeds are starting to climb the ladder – Palmer amaranth and Italian ryegrass.

According to Mississippi State University Extension weed scientist Jason Bond, 18 Mississippi counties have reported glyphosate-resistant Italian ryegrass, along with eight counties in Arkansas and one parish in Louisiana. Resistance is suspected in several more northeast Louisiana parishes and two counties in west Tennessee. Two counties in North Carolina have also reported resistance.

Speaking at the Louisiana Agricultural Technology and Management Conference in Marksville, La., Bond says the weed likely first established along roadways, then spread to road ditches and to turn rows before tillage began to move it across fields.

Typically, Italian ryegrass has not been a serious problem in rice in Mississippi because it prefers a better-drained, coarse-textured soil. But it has started to show up in clay soils, Bond says.

Controlling the weed – especially when all efforts are concentrated in the spring – can be problematic, according to Bond. Trials in which Gramoxone was applied twice to standing ryegrass two weeks before planting rice achieved 100 percent control. “But we delayed heading from 7 to 12 days just from competition with ryegrass stubble. Yield was reduced 15 percent.”

The use of a fall residual applications can reduce the effect of the stubble, noted Bond. “The only fall residual herbicide we can safely use in front of rice is Command. The fall application of a residual allows you to clean up the escapes fairly easily in the spring. So with ryegrass, we’re on a two-pass herbicide program.”

A November disking can also control the first flush of ryegrass, noted Bond. “Then we can be off to a better start (with a burndown) in the spring because we are only controlling the later emerging plants.”

If you use spring tillage to control ryegrass, Bond suggests “dragging a scratcher over it to knock some of the dirt off the root balls.”

In any event, glyphosate-resistant Italian ryegrass must be completely controlled before planting,” Bond said.

Bond suggests that producers avoid multiple applications of clethodim on ryegrass, no matter what the crop. “We have resistance in Mississippi to that chemistry. We have really abused those clethodim formulations over the last few years.”

Glyphosate-resistant Palmer amaranth is a problem in rice because it is often left uncontrolled in the soybean rotation.  “It has steamrolled us,” Bond said. “It’s everywhere. We have 31counties in Mississippi with confirmed glyphosate-resistant Palmer amaranth. That’s basically every crop-producing region of the state.”

The best control option in rice has been a full rate of propanil plus Grandstand, Bond says. “We have ALS resistant Palmer amaranth in 27 counties in Mississippi. Regiment is a very good pigweed herbicide, but it is not a very good herbicide on ALS-resistant Palmer amaranth.

“The big key for control of Palmer amaranth is timing of application,” Bond said. “It’s absolutely critical. In our state, we sometimes wait and take out the broadleaves with a pre-flood herbicide. You can’t do that with Palmer amaranth. It can grow two inches a day if the weather is right. It can get away from you in a hurry.”

Bond said that Broadhead on small Palmer amaranth has been successful. “When Palmer amaranth gets too big the only thing that will kill it is a shovel, and you better dig the root up too.”

Barnyardgrass remains the No. 1 weed in rice in Mississippi, said Bond. But the number of effective herbicides for control is dwindling. “About 45 percent of barnyardgrass samples we evaluated from 2007 to 2011 were resistant to propanil, and about 25 percent were resistant to Facet. Fifteen percent were resistant to both propanil and Facet. Last year, we confirmed resistance to Newpath and Beyond in a barnyardgrass population from Sunflower County. This population is also resistant to Regiment and Grasp.”

“If you remove herbicides that show some level of resistance in barnyardgrass – propanil, Facet, Command, ALS inhibitors – it leaves you with Prowl, Bolero, and ACCase herbicides (Clincher and Ricestar HT) to build your weed control program. That’s very difficult. These are very good herbicides, but work better as complementary herbicides, rather than base herbicides.”

Whether the weed is barnyardgrass, Italian ryegrass or Palmer amaranth, Bond is promoting early applications to deal with them. “In 2011 research, we sacrificed 1.5 bushels of rice for every day we waited to spray barnyardgrass. Whether it’s barnyardgrass or Palmer amaranth, the earlier you get on it, the better off you’re going to be. We also recommend using multiple modes of action and multiple applications with post-emergence and residual control in the same tank.”

Rice flatsedge has also shown resistance to ALS inhibitors in Mississippi rice fields, Bond noted, although the weed was less of a problem in 2011. The weed is a fast developer and can go from seed to seed in four to six weeks, and faster if the weed is stressed. “The best treatment we evaluated was a full rate of propanil and a full rate of Basagran. A second treatment may be required post flood.”

The conference was sponsored by the Louisiana Agricultural Consultants Association.








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