Clearfield technology stewardship issue

When Clearfield rice and Newpath herbicide were first being developed, two other technologies offered the promise of red rice control in a growing rice crop — Roundup Ready rice and LibertyLink rice.

Both Roundup and Liberty (also labeled as Ignite) herbicides showed promise as red rice herbicides at that time. I remember a field day at Stuttgart where all three technologies were highlighted side-by-side.

It was generally assumed that with three different modes of action, you would not have to worry about resistance developing to any given one, especially if you threw in a soybean rotation every couple of years. There was talk back then of having stacked trait rice that might tolerate two or even all three herbicides.

However, the negative reaction of the European Union and others to GMO rice put an end to both the Roundup Ready and LibertyLink plans to go to market and both technologies have been on hold ever since.

I know that contamination of the latter has caused problems in the past couple of years, but I will restate my opinion that once we gain market acceptance, the technology will be sorely needed by rice farmers.

We are entering our seventh year of Clearfield rice. Due in part to the excellent performance of hybrid Clearfield varieties, the technology is in as much demand as ever. Unfortunately, there are many fields where Clearfield rice has been grown continuously or almost continuously for those seven years. Fields like those cause the most concern for the development of Clearfield-resistant red rice and of the development of other resistant weeds.

Due to the excellent work by Nilda Burgos, Dave Gealy, and University of Arkansas graduate student Vinod Shivran, we have a good understanding of the genetics of out-crossing between both hybrid and conventional Clearfield lines. We know that out-crossing with a hybrid Clearfield line will result in a wide array of biotypes versus typically just one with the conventional lines. Hybrid rice also has a slightly higher occurrence of out-crossing than conventional rice.

All this information reminds growers to pay extra attention to any escaped red rice in a hybrid Clearfield field, because the only chance to control it could be before it sets seed during the season.

In the case of conventional lines, most out-crossing results in a large late-maturing F2 type hybrid that will be very late going to seed the following year. Due to the late-maturing nature of the out-crossed plants, you will often get a chance to stop them from producing seed by combining the field and destroying the stubble.

If red rice makes it through the first two applications of Newpath, the last line of defense is the use of Beyond herbicide to clean up escaped red rice. Unfortunately, from a stewardship standpoint, some hybrid varieties will not tolerate late Beyond applications as well as the conventional lines.

Initially in our work, the hybrids such as XL 730 would tolerant late applications of Beyond up to 14 days after panicle initiation. However, newer lines such as XL 729 and 745 have not tolerated these later applications. This has forced us to limit applications of Beyond on the hybrids to prior to panicle initiation.

That sounds good until you try walking fields of hybrid rice to determine if you have red rice or not. Look hard and make the Beyond applications early if you think they are needed. As I said, this is the last line of chemical defense. The only alternative is hand-rouging.

Coverage may also be a factor. The 24c label for Arkansas will stay the same for conventional varieties at panicle initiation plus 14 days. Continual screening of both conventionally bred and hybrid Clearfield lines will be needed to maintain this database.

In addition, BASF is seeking a full registration for up to three applications of Beyond in-season. Some of our studies this summer will focus on Clearfield rice tolerance to multiple applications of 4 to 5 ounces per acre of Beyond at various timings.

One very positive thing that is going on in Clearfield production today is the use of other herbicides in combination with Newpath, especially other grass herbicides. While Newpath alone has shown excellent activity on most grass weeds in our program, from a resistance management standpoint, you cannot beat adding Command, Facet or a propanil-containing product into the program. This is very common in Arkansas.

It is good that all this tank-mixing or use of other products is going on for the management of resistance to barnyardgrass and other grass weeds in general.

The keys to prolonging the usefulness of the Clearfield technology include: crop rotation, the use of other herbicides in the program, and the use of Beyond or hand-pulling to eliminate any escapes.

As usual, the burden of providing stewardship to the Clearfield technology will fall to the grower in terms of cost. The lack of tolerance in the new hybrid lines will make it more challenging. I wish that I could say that another red rice technology is just around the corner, but at this time I cannot.

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