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Fall residuals for glyphosate-resistant Italian ryegrass

Fall residuals for glyphosate-resistant Italian ryegrass

Postemergence options for glyphosate-resistant Italian ryegrass in the spring are extremely limited and require at least two applications to even approach complete control. Residual herbicides applied in the fall offer the best opportunity for control.

Across the Mississippi Delta, it appears that most farmers are over the hump on harvest if not fast approaching the finish line.

The weather over the last month has allowed for extensive field preparations for 2011 behind corn, soybean, rice, and even cotton in some cases.

The lack of rainfall during August and early September also prevented excessive weed germination following harvest. However, most areas in the Mississippi Delta received at least some rainfall over the past couple of weeks and the summer temperatures are starting to moderate. This means growers in areas with a history of glyphosate-resistant Italian ryegrass should begin to make preparations for another onslaught of this weed.

Last year, glyphosate-resistant Italian ryegrass began emerging during July or August, and this early emergence complicated management programs. However, the late-summer weather conditions in 2010 have been the complete opposite of 2009. We began scouting for emerged glyphosate-resistant Italian ryegrass in mid-August this year, monitoring eight sites from Tunica, Miss., to Yazoo City, Miss.

As of Sept. 21, we had found no emerged glyphosate-resistant Italian ryegrass at any of the sites.

There was a period during the first week of September when the temperatures were conducive for Italian ryegrass germination. In general, if day-time temperatures are consistently below 87 degrees for a week, Italian ryegrass will begin emerging. But, temperature in early September was irrelevant because most areas were so dry. Only when Italian ryegrass gets the right combination of moisture and temperature will seedlings begin emerging.

Historically, the average high temperature at Stoneville falls below 87 degrees on Sept. 14, so Italian ryegrass should begin emerging soon.

In the Mississippi Delta, we have been evaluating management programs for the weed for several years. The primary take-home messages from this work are that postemergence options in the spring are extremely limited and require at least two applications to even approach complete control, and residual herbicides applied in the fall offer the best opportunity for control.

Because so few options exist for controlling glyphosate-resistant Italian ryegrass in the spring, we have begun to focus most of our management strategies on fall applications of residual herbicides. Among the labeled herbicides for which we have multiple years of data, fall applications of Dual Magnum, Treflan, and Command have provided the best residual control of glyphosate-resistant Italian ryegrass.

November applications effective

Most years, herbicide applications in November were very effective and controlled glyphosate-resistant Italian ryegrass until spring. However, in a lot of cases, field preparation for the following year is completed prior to November (depending on the crop) in Mississippi. So last year, we set out to determine if the residual herbicides could be applied to coincide with fall tillage without sacrificing efficacy against glyphosate-resistant Italian ryegrass.

In one study, Dual Magnum (1.33 pints per acre), Treflan (3 pints per acre) and Command (2 pints per acre) were applied at monthly intervals from mid-September through mid-November. Because of the rainfall totals in our area last fall, the September and October applications were nearly complete failures. By April 1, none of the herbicides controlled glyphosate-resistant Italian ryegrass more than 65 percent when applied in September or October.

Delaying the application until mid-November was better, but no treatments provided complete control. Rainfall through the fall and winter definitely impacted our fall residual herbicide treatments last year, and we also endured a huge flush of the weed in March. But the bottom line was that residual herbicides can be applied too early if rainfall totals are high throughout the fall and winter months. Even when we delayed application until November 2009, follow-up applications in the spring were required to control escapes.

For the money, Treflan at 3 pints per acre in the fall is a good choice for controlling glyphosate-resistant Italian ryegrass. However, Treflan does not fit everyone’s production systems. Treflan must be incorporated into the soil within 24 hours of application, and the Treflan label is fairly detailed on suggestions for mechanical incorporation.

In our small-plot research, we incorporate Treflan with two passes in opposite directions with a tandem disk. However, if the disk inverts clods but does not break them up well, then glyphosate-resistant Italian ryegrass will emerge from the underside of the clods because no herbicide is present. Bedded fields also complicate the use of Treflan because if Treflan is applied in front of the hippers, then you risk burying the herbicide below the zone where the ryegrass will germinate.

So, even though Treflan may be attractive economically, it does require more planning and the additional tillage required for incorporation may outweigh the reasonable herbicide cost. Because they are soil-applied herbicides, Dual Magnum and Command still require incorporation, but this can be achieved with rainfall rather than mechanical incorporation. Dual Magnum and Command may be applied to the surface in no-till areas or following the final tillage pass in conventional tillage.

Because options for controlling glyphosate-resistant Italian ryegrass in the spring are limited and since we had escapes following all of our fall residual herbicide treatments in 2009-10, I reluctantly admit there are no one-pass programs for managing the weed. And, in years when the weather is similar to 2009-10, two-pass programs including herbicide applications in the fall and spring may not be adequate where weed density is high.

Similar to glyphosate-resistant Palmer amaranth management, glyphosate-resistant Italian ryegrass control will require a multi-faceted approach. Herbicide options are limited and Italian ryegrass has a history of rapidly developing resistance to herbicides. With that, tillage will be an integral component of management in the future.

At this point, we do not know what the impact of burying seed with tillage has on germination and emergence the following year. We also do not know from what depth the seedlings will emerge. At the very least, if the majority of fall-emerged glyphosate-resistant Italian ryegrass plants are destroyed with aggressive tillage, then postemergence herbicide coverage should be better on surviving plants and those that emerge in the spring.

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