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Monitoring red rice outcrossing

Clearfield rice fields were among the cleanest in regions infested with red rice in 2006. Many will agree.

Looking more closely, though, there were Clearfield rice fields with a few remaining red rice plants either randomly dispersed within the field or growing in small patches along the borders. These would be the infamous escapes.

Those along the edges may have not been sprayed or had poor coverage because of interfering objects or applicators trying to avoid drift onto an adjacent susceptible crop.

These escaped red rice plants can outcross with Clearfield rice if they flower at the same time. Therefore, growers who plant Clearfield rice should monitor escaped red rice and manage possible outcrossed plants. Why? Because Clearfield technology is the best tool in an integrated program for controlling red rice in rice.

Assuming there are escapes, how much outcrossing can we detect? How big of a problem can it be? It depends. In some cases, zero — not a problem.

We are conducting an outcrossing monitoring program in collaboration with Arkansas county Extension agents and BASF. In this work, about 40 percent of fields were infested with strawhull red rice, 33 percent with blackhull red rice, and about 60 percent of fields with brownhull red rice did not have any outcrossing. The main reason is no overlap in flowering.

If the escaped red rice flowers earlier or later than Clearfield rice, there is nothing to worry about. To know for sure, scout the field closely around flowering time. Otherwise, you just have to assume that outcrossing occurred and proceed with managing the potential outcrosses.

Where outcrossing occurred, strawhull red rice (the most common type) had less outcrossing (about 100 per 100,000 plants) than blackhull red rice (about 450 per 100,000 plants).

Brownhull, a rarer type of red rice, does not outcross often (only four of seven fields). However, when it does, the outcrossing rate is much higher (about 750 per 100,000 plants).

We also want to caution growers that there is a wide range of outcrossing rates within a type of red rice. For instance, among the strawhull-infested fields, the outcrossing rate ranged from 15 per 100,000 plants to 190 per 100,000 plants. Among blackhull-infested fields, the range was from 70 per 100,000 plants to 1,440 per 100,000 plants.

As we have mentioned during field days, not all strawhull red rices nor all blackhull red rices are the same. Besides the wide disparity in their flowering dates, they also vary widely in plant height and other characteristics.

In the year following Clearfield rice, the escaped red rice population may be composed of Newpath-susceptible plants from the soil seed bank and possibly resistant red rice plants, which are products of outcrossing that occurred in the prior season.

With Clearfield 161, the first generation outcrosses will look similar in one field if the field was infested with only one type of red rice. For example, crosses with strawhull red rice will generally produce upright red rice plants that are taller than both the Clearfield and red rice parents, and the plants will generally be late-maturing. Most of them will not be flowering at the time of rice harvest.

The good thing is these first generation plants are easy to spot when the rice is mature. We say “generally” because some strawhull and blackhull red rices will produce early-maturing crosses.

Blackhull red rices, which are mostly awned, will also produce plants with pink awns when hybridized with rice. David Gealy’s work at the USDA rice research center in Stuttgart, Ark., also shows this. If the first generation outcrosses are able to produce seed, the second generation (F2) outcrosses will be detected the third time Clearfield rice is planted in the same field.

F2 plants will segregate into all sorts of plant types — short, intermediate, tall; extremely early, normal maturity, late. The short, early-maturing types will escape detection and replenish the seed bank. These Newpath-resistant types will persist.

Those with normal maturity will flower with the existing rice crop and produce new outcrosses. The late ones would behave similarly to the first generation crosses.

Is the development of resistant red rice populations from outcrossing manageable? Yes. But it has to be a priority for all of us. We recommend the following:

• Prevent survivors from seeding. Watch for red rice that attempts to put out another panicle after harvest, then spot-treat with 2 to 3 quarts of Roundup. In case some plants survive this application (big plants), mow down the survivors to prevent them from reseeding.

• Alter tillage practice. Avoid tillage in the fall so as not to bury resistant red rice seed into the subsoil.

• Modify flooding practice. Keep the field flooded in the winter to encourage predation by ducks and deterioration of seed on the soil surface.

• Use burndown treatment and slightly delay planting. Conduct shallow tillage in the spring and delay planting somewhat to allow for early red rice germination, then treat with glyphosate; or, do not till in the spring, wait for early red rice emergence, burndown with glyphosate, and then plant no-till.

• Prevent survivors from seeding. Monitor hot spots in the field during the growing season and rogue or spot-spray escaped red rice in these areas. Use GPS coordinates if possible.

Crop rotation is the best partner for Clearfield rice. However, there is room for improvement in our rotation recommendations.

For example, a field was planted with Clearfield rice in 2004, Roundup Ready soybeans in 2005, and conventional rice in 2006. Moderate red rice infestations, in patches, were observed in this field in 2006.

Another field was planted with Clearfield rice in 2004 followed by Roundup Ready soybeans in 2005 and 2006. There was still red rice germinating in 2006.

The seed bank can be reduced drastically in one season, but it cannot be completely depleted quickly.

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