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Incorporating rice straw boosts nitrogen levels

Issue Date: March 15, 2006
Bob Johnson

In the future, early season water management could be the key to nitrogen management in rice, according to Bruce Linquist, University of California, Davis, postdoctoral researcher in agronomy and range science.

Linquist made his remarks at a UC Cooperative Extension rice meeting held in Yuba City in late January. Researchers at the meeting presented information on rice varieties, weed and disease control and fertilizer management.

Since rice growers began incorporating their straw in order to satisfy air quality regulators, UC researchers have shown that there is a fertility fringe benefit to giving up burning. Linquist said he believes the next advance in fertility management will be based on greater understanding of the role played by water management.

"If you incorporate your straw, you can reduce your nitrogen applications by 25 pounds per acre," Linquist said.

In recent UC trials, supplementing the grower standard with an additional 25 pounds of nitrogen led to a crop yield response when the grower was burning the straw. The question remained, according to Linquist, whether the response was great enough to warrant the expense of the additional fertilizer application.

He noted that there was no crop response when the grower standard was supplemented with 25 pounds of additional fertilizer when the straw was incorporated.

Half of the growers who switched to incorporating their straw have not changed their nitrogen practices at all, according to a recent survey of rice farmers from all of the major producing counties. A quarter of the growers responding to the survey have reduced nitrogen applications since they began incorporating their straw, but by an average of only 10 percent. And nearly 10 percent of the growers have actually increased their nitrogen applications since they began incorporating their straw.

Recent UC trials indicate that the use of starter nitrogen applications can boost rice biomass in the early weeks, and can also increase average yields.

"On average yields increased by over 700 pounds per acre with the application of starter nitrogen," Linquist said.

But the results of the starter nitrogen applications were not consistent, as virtually the entire increased average yield came in just three of the five fields in the trials.

Nitrogen efficiency in the trials was calculated in terms of how many additional pounds of grain were produced by each additional pound of applied nitrogen.

The differences in efficiency were staggering—ranging from less than zero to more than 57 pounds of grain from each additional pound of applied nitrogen.

Linquist said he believes that the key to these differences in yield response is differences in water management.

UC researchers at the Rice Experiment Station in Biggs are conducting a long-term study of the effect that different methods of seeding and water management can have on shifting weed populations. The study may also increase understanding of how water management can affect fertility.

Although all of the plots have received 150 pounds of nitrogen fertilizer, among the five different treatments in the experiment at Biggs the plots that are direct seeded with conventional tillage have consistently produced the highest yields.

The plots that are water seeded into stale beds have consistently had the lowest yields in the weed control experiment at Biggs.

Linquist believes the key to the lower yields is the loss of native nitrogen when you flush the fields.

The main purpose of the Biggs' study is not fertility efficiency but exploring whether different water, seeding and cultivation systems can be used to manage weeds that have become herbicide resistant.

"If you have a problematic field that's costing you a lot of money in herbicides, think about using one of these systems of stand establishment to bring down the weed seed bank," said Cass Mutters, UCCE farm advisor in Butte County. "In California rice we do not rotate, and we do not cultivate mid season."

The result of this cultural system is that late water grass weeds have resistance to most herbicides. To a lesser extent early water grass and barnyard grass weeds also have herbicide resistance.

"We want to shift the weed recruitment, we want to move it from resistant to susceptible," Mutters said.

The long-term trial at Biggs provides powerful evidence that spring tillage with water seeding shifts the weed population toward sedges. Drill seeding conventionally tilled plots encourages barnyard grass and sprangletop.

Another treatment at Biggs uses a minimum of soil disturbance, which brings seeds up toward the surface. The stale seedbeds are sprouted, treated with Roundup and then drill seeded. Both aquatic and grass weeds were reduced dramatically in plots under this treatment.

Yet another approach in the experiment is to cultivate in the spring, treat with Roundup and then water seed. This approach also generally reduced weed populations.

In the no-tillage plots that were treated with glyphosate and water seeded no additional herbicides were needed. This approach, however, can push planting back as far as late May.

In the Biggs experiment switching water seeded plots to no tillage has resulted in a 92 percent reduction in weed seeds. There has been an 80 percent reduction in weed seeds where drill seeded plots were switched to no tillage.

"You're investing your time in bringing your weeds back down to where your traditional weed control systems are effective," Mutters said.

(Bob Johnson is a reporter in Davis. He may be contacted at

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.

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