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.

Jbond@drec.msstate.edu