Nitrogen management in rice is a notoriously tricky proposition, but a University of Missouri scientist believes there's a simple and cost-effective way to measure whether a crop needs additional applications of fertilizer.
“It's an unending problem in rice,” said Gene Stevens, MU Extension agronomist at the Delta Research Center in Portageville, Mo. In corn, he said, the yield response curve to nitrogen goes up gradually before it hits a plateau. “With rice, you get a really steep increase in yield response, but if you put on too much, you see a dramatic decrease.
“It's a lot trickier than it is in corn,” he said. “You want to find that peak point.”
Stevens has discovered that a good old-fashioned yardstick works about as well as anything else to determine that “peak point,” and at far less cost and complexity than some other methods. “It's a low-tech management technique,” he said. “Everybody's got a yardstick.”
At the MU Delta Center Field Day this summer, Stevens presented his research results using the low-tech yardstick nitrogen management system for rice. Other scientists shared their research results into cotton, rice, irrigation, weed control, soybean breeding and other areas.
Rice crops sometimes demand two or more applications of nitrogen, Stevens said. The largest application — usually 90 pounds per acre — occurs before the field is flooded. In mid-season, when the plants begin to reproduce, 30 more pounds might be necessary. A third application of 30 pounds often is applied a week later.
“Sometimes,” he said, “that pre-flood application is all you need. If you put on too much, the plants get too tall and lodge. And they're more susceptible to disease.”
He tested the rice plants at mid-season with a Minolta SPAD chlorophyll meter, with “marginal results.” The device costs about $1,400.
The California leaf color chart, which displays different shades of green for comparison with leaves, is cheap and available but unreliable because it depends largely on an individual's color perception. “The difference in one person's eyesight from another is amazing,” Stevens noted.
“Even laboratory tissue tests haven't done very well” in showing rice nitrogen needs, he said.
The plant area board method, which measures the height and width of plants, is inexpensive and has worked well in research settings. But farmers are unlikely to use it, Stevens said. “There are some problems with it.” It employs the metric system, with which many Americans are uncomfortable, and it requires “a complex formula” to determine the nitrogen needs.
Stevens' yardstick method works on the same principle as the plant area board. “With too little nitrogen, you get a smaller canopy,” he said. “You lay that yardstick in the water between the rows, and then you count the numbers you can see. If there's no canopy, you'll see them all. If there's a lot of vegetative growth, you won't see any.
“It seems to correlate well with the nitrogen rates,” he said. “It's a new thing; we haven't completely tested it. I think we'll be able to nail down the critical levels.”
Plant height measurement “works pretty well, but I've seen tall plants that still need nitrogen,” he said. “But you're OK if you have good height and plants that cover the water, so to speak.”
Many rice farmers turn to crop consultants for advice on nitrogen management, he said. “For them, time is money.” The yardstick method took about three and a half minutes per plot — “a lot faster than anything else we tried.”
Most important, he said, is that the method — like a yardstick — is simple to grasp. “It's something easy, something people will use.”
Forrest Rose is an Extension and ag information specialist with the University of Missouri.