Leaf Color Chart evaluates rice N levels

There is a new addition to the pickup caches in more than 100 Northern California pickups this summer.

It is a Leaf Color Chart (LCC) developed by University of California Cooperative Extension Butte County Farm Advisor Randall "Cass" Mutters to gauge the nitrogen content in rice.

A rice crop’s color is the key to determine nitrogen content, and Mutters has spent the past three years giving rice crop colors some very precise definitions. The LCC was created to make it easier and quicker than conventional leaf sampling and lab analysis to determine N content of a rice crop.

Timeliness is important in any rice crop, but becomes even more critical in high quality, specialty Akitakomachi rice for the Japanese market.

"The window of opportunity for correct nitrogen applications for this specialty rice is very narrow, and it is important from a marketing standpoint that growers hit it just right," said Mutters.

However, he focused his research on the most widely planted rice varieties. He planted almost 200 rice plots into the eight most popular rice varieties; applied various rates of nitrogen up to 180 pounds of N per acre and collected 30 leaves from each plot and then quantified those colors using a spectrometer.

Precise definition

He also worked with Minolta’s color lab in Los Angles and a company spectraphysicist to in New Jersey to precisely defining those colors to nitrogen levels in individual rice leaves and across whole rice fields.

"There are numbers to describe colors precisely, and that is what I worked with Minolta in developing," he said.

He then arrayed large chips of those eight colors from the lightest to the darkest on a flexible, gray, rectangular, high-impact plastic slate.

He put it in the field last year with a few cooperators and this season has more than 150 growers and pest control advisers trying it, giving him feedback and data to validate the simple, easy-to-use nitrogen evaluation color chart. It is an alternative to a chlorophyll meter that is can be used to mechanically evaluate leaf color.

There are several reason to quantify rice nitrogen levels. One is obviously to maximize yields and in the case of specialty rice, ensure quality. Another equally important one is to reduce potential nitrate leaching into the groundwater and streams.

The color chart system Mutters has developed also fits into the emerging precision ag era. "You could use this tool to apply only what the field needs or could vary the foliar N rates across a field based on the needs of different parts of the field," he said.

N rates in producing the specialty rice directly affect taste scores of the Akitakomachi rice.

Protein test scores

"Lower test scores are attributable to higher grain protein and moisture content at harvest," Mutters said. "Increasing protein results in decreasing test scores. Managing nitrogen to keep grain protein low is key," he said, adding that good yields and taste scores were recorded when leaf tissue N was around 2.5 percent at panicle initiation.

Mutters’ color chart is creating considerable interest, and UC Davis has applied for a patent on it. Mutters also has received inquiries from several other rice production nations about adapting the color chart for growers there.

The chart comes with a laminated instructions sheet. There are two ways to use the LCC; color matching individual leaves or using it to visually evaluate entire fields.

Mutters says it is best to color match whole fields from 9 a.m. to 3 p.m. "Sampling at a consistent time of day is important," he notes.

"You should position yourself with the sun at your back and look directly across the top of the leaf color chart and match the color of the field to the corresponding color panel," he said.

For single leaf evaluation, Mutters said a grower or PCA should sample Y-leaves because they are the most recently matured and are recognizable by their fully developed color.

"Collect several leaves across a field to obtain an average color value for each field," he said.

Color chips match

The eight color chips are the matched with nitrogen values for medium and long grain rice, medium grain and specialty rice varieties.

Mutters cautions that nitrogen content varies with plant growth stages. He gives the N values for those growth stages on the instruction sheet that accompanies the LCC.

"We anticipate when this season is over and we get feedback from those who used the chart that we can move forward into full utilization of the LCC," he said.

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