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Clearfield/red rice out-cross confirmed in Arkansas field

The first Arkansas field with confirmed out-crossing of the trait in CL 161 rice (that conveys tolerance to the imadazolinone family of chemistry) to a population of red rice has been confirmed in a field in Jackson County.

The field is included in a survey of Clearfield rice fields conducted by the University of Arkansas weed science group. The purpose of the project is to detect out-crossing and determine the frequency at which we can expect out-crossing of Clearfield rice with red rice to occur under various conditions.

The study has been conducted with the cooperation of BASF and county agents of the Cooperative Extension Service. Ford Baldwin and Tomilea Baldwin of Practical Weed Consultants first examined the field.

They observed red rice plants that looked very much like those that Tomilea had observed in Clearfield rice plots while she was finishing a part of her Ph.D. program with the university. Her project then revolved around Clearfield rice and she happened to observe out-crossing with red rice in her plots.

After surveying the field in Jackson county, Ford and Tomilea suggested we might want to look at the field and include it in our program. The most obvious thing about the field was the presence of large, immature, compact-growing, erect red rice plants dotted across the field.

We were told that the field received two Newpath herbicide applications and a salvage application of Beyond herbicide in 2004. Based on circumstantial evidence, most involved believed we had a case of out-crossing.

BASF sent samples to its lab for analysis. That analysis has now confirmed that the red rice in question was in fact an F1 hybrid between some red rice plant and CL 161. We say F1 because this field was first planted with Clearfield rice last year and there were a few survivors from last year's Newpath application.

The confirmed hybrids that we saw this season would have been offsprings of cross-pollinated red rice plants from last year.

The F1 hybrid plants stand out because they are much taller than Clearfield rice and exhibit a dense and erect vegetative growth habit. They are also characteristically late in maturity. They are only now forming seed-heads (see photo) and some of them may not even get to flower.

These distinctive visual traits should help Clearfield rice growers and consultants watch for the occurrence of herbicide-resistant red rice. The appearance of F1 plants in the field so far have been consistent with the F1 hybrids obtained in various gene flow experiments with Clearfield rice and red rice conducted by the weed science section at the University of Arkansas and in related experiments conducted by David Gealy at the USDA-ARS, Stuttgart. The plants are now referred to as bull red rice by some people.

If the F1 hybrids are allowed to go to seed, the next generation (F2s) will also have tolerance to Newpath herbicide, but will display a wide variety of phenotypic or visual characteristics. For instance, the F2 generation will have very early to very late flowering plants. Several plants will shatter its grains severely like red rice, but some will not. Plant height will range from about 1.5 feet to 5 feet. Growth habit will vary.

The frequency of occurrence of these characteristics among F2 plants is currently being documented at the university as part of a graduate student program. The kaleidoscope of F2 characteristics is not yet fully understood and can vary according to the red rice parent.

The bottom line is that it will be more difficult to spot all out-crosses past the first generation. This is one reason that early detection of out-crossing will be critical to managing fields whenever out-crossing occurs. Whenever out-crossing is suspected, it is important that we confirm it, with a combination of herbicide, enzyme sensitivity and genetic tests, because some red rice biotypes (whether blackhull or strawhull) also flower late and exhibit an erect growth habit.

Fortunately, due to the late maturity of these F1 hybrids, the seeds were generally immature at the time the field was harvested. Many of the surviving red rice plants will not make seed. However, some plants could still produce viable seed if left until frost.

For red rice, it takes at least two weeks from heading to produce a few viable seeds. We recommend that the farmer burn the field to singe existing seed heads or perform some other operations to destroy surviving plants.

In addition, this field provides the first test of the BASF stewardship program. On this point both BASF and university recommendations agree. Under no circumstances should this field be put back into Clearfield rice production.

Crop rotation or leaving the field fallow for at least two years is the only way to insure that other herbicides can be used to control red rice that emerges in the field next spring. The most obvious choice for crop rotation is to Roundup Ready soybeans. This has been part of the stewardship program from the beginning.

In general, our university recommendation has been to rotate to soybeans regardless of the level of red rice control obtained or the presence or absence of out-crossing. In some cases, crop rotation issues are complicated if the land is leased, lease agreements exist about what crops will be grown, or the ground may not be practically suited to grow anything but rice.

However, some action must be taken or it is likely that seed will spread from field to field with tillage, planting, and harvest equipment. While cross-pollination has been getting the most press lately, it is probably the movement of seed from field to field with harvest equipment that poses the most risk of spreading resistance.

In addition, this case solidifies the stewardship and patent protection position of BASF of not allowing producers to save seed. After seeing this field, it is easy to imagine the problems if red rice seed is harvested and planted in a field the following year. This will especially be true next year if there are any F2 generations out there.

The basic program for not getting resistant red rice on your farm is plant certified Clearfield rice seed from approved rice processors, do not save seed, strive to attain 100 percent red rice control with your herbicide program, and rotate to a crop like Roundup Ready soybeans the year following Clearfield Rice. If you do these things, then the development of Clearfield red rice is pretty unlikely.

The longevity of the Clearfield system depends on our ability to minimize out-crossing and on how we manage it when it occurs. By utilizing correct stewardship practices, we should be able to prevent this identified population from spreading any further.

Bob Scott is an Extension weed specialist with the University of Arkansas. Nilda Burgos is a weed scientist with the University of Arkansas.

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