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Research project shows: Earlier flooding can cut herbicide costs

When Trey Koger set up the winter flooding research project for the Mississippi Rice Promotion Board in the fall of 2002, he hoped to be able to determine if timely winter flooding could eliminate a preplant herbicide application in rice. Two years into the project, Koger is confident that if rice fields are flooded early enough in the fall, winter weeds won't germinate and growers may not need to spray before planting.

“Our take-home message to growers from this research is if you establish a flood before winter weeds germinate, which was before Oct. 1 in the two previous years, then you eliminate any winter weeds from germinating,” said Koger, a weed biologist with the USDA Agricultural Research Service's Southern Weed Science Research Unit in Stoneville, Miss.

In the fall of 2002 Koger went to a working farm and with the cooperation of rice grower Steve Prather flooded four fields under various timings and durations.

The fields were about 40 acres each in size. Two were flooded in early October and the other two were flooded in the middle of December. The floods were dropped off one of each of the early-flooded and late-flooded fields in mid-January. Floods remained on the other two fields until mid-March. The system gave Koger four flooding regimes to evaluate.

“In each of the four fields the upper portion of each field was not flooded so that we could observe normal winter weed emergence under non flooding systems,” said Koger.

The winter weeds he evaluated began to germinate the middle of October and germination continued until the end of December.

“In the fields where we got the early flood, the producer was able to drop in and plant without applying a preplant burndown application,” said Koger. “However, in the field where we had a late flood and winter weeds germinated, the weeds survived the flood. They didn't grow, but they were there in the flood. As soon as we pulled the water off, the weeds just took off.”

Koger also determined when growers pulled the flood off wasn't as important a factor as when they flood, in terms of weed control, just as long as the flood was on at least through January.

“In some years, producers are able to get sufficient amounts of rain after rice harvest so they don't have to pump water into the field for a winter flood,” said Koger. “If they put the boards in early enough, and get the flood on by early to mid-October, they may be able to save money by not needing a preplant herbicide application.

“If a grower can catch rainwater, then winter flooding will be cheaper than a preplant burndown application to control winter weeds,” he said. “It also decomposes the rice straw so you don't have to burn it off, or do much tillage before planting the next crop.”

In the fall of 2003, for the second year of the research project, which is funded by the Mississippi Rice Promotion Board, Koger moved his plots on site at Stoneville. He has smaller plots, which will allow him to verify his first year findings about flood timing and also evaluate what, if any, effect winter flooding has on yields of the following year's rice.

“Our second objective is to see how those different flood durations affect the following year's crop,” said Koger. “Our thought is that when you have an extended flood you might deplete the oxygen level in the soil to the extent that the vigor of the following year's crop is reduced.”

By moving the experiment to small plots at the Stoneville research area, Koger can monitor and evaluate the yield of each flooding practice. He followed the same flooding schedule in 2003 as in 2002.

Koger is also comparing rice degradation under the four flooding options.

“In the winter of 2002-03, we had so much rain that our flooding treatments didn't have a comparable influence on degradation when compared to non-flooded fields, and we believe that's going to be the case in most years. Rice degradation is going to take place with or without the floods.

“The length of the flood versus its impact on straw degradation is not as important to us as determining when growers need to flood to reduce the need for a preplant herbicide next year,” he said.

Koger is also monitoring soil temperatures to find out at what temperatures winter weeds begin to germinate.

“It's tied to soil temperature, so it's going to vary across years,” said Koger. “In 2002, it was around 60 degrees. When the average 24-hour soil temperature got below 60 degrees, they started to germinate.

“Another activity we are looking at is the denitrification in the soil and how the different flooding treatments affect that process. We want to see what conditions interact with the denitrification process, which is how the soil breaks down the nitrogen to allow the plants access to it.”

Flooding of the small plots in the fall of 2003 marks the second year of a three-year project. Koger plans to have preliminary published data after completion of the 2004 season.

Eva Ann Dorris is an ag journalist from Pontotoc, Miss. She can be reached at 662-419-9176 or [email protected].

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