Nitrogen management for rice may help production
ByBob Johnson Riceresearchersandgrowersareworking tofine-tune the timingandtypeofnitrogen applications to produce the best cropwith the least impact on the environment. Current studies examine how to adjust nutrientmanagement theyearafterground is fallowed due to drought. They’re also looking at the best alternative when aqua ammonia isn’t practical and at the most efficient timing for nitrogenapplications. “Splitting the nitrogen last year gave better yield than applying it up front,” said Bruce Linquist, University of California Cooperative Extension rice specialist. The goal of nutrient management studies is to find the right amount of fer- tilizer to apply at the right time to maxi- mize yield without causing pollution or encouraging diseases such as blast that thrive on excess nitrogen. “We are looking at alternatives to aqua fertilizer,” said Linquist, appearing recent- ly at the Rice Experiment Station in Biggs. “Weather or supply chain issuesmay force growerstorethinktheirnitrogenprograms.” Some current studies of nutrient man- agement options came after late-season rains twoyears ago. The rains forcedmany growers to try alternatives to their usual practice of applying most of their nitro- gen as aqua ammonia at planting or a few weeks later.
surements sensing the normalized differ- encerededge, orNDRE, canbeusedmuch like thehand-heldGreenSeekermeasure- mentsof normalizeddifferencevegetation index, orNDVI, toknowinadvancewheth- er a top-dressedapplicationat panicle ini- tiationwould significantly increase yield. Continuing trials are fine-tuning the response index or the ratio between the NDVI reading in the nitrogen-rich strip and the reading in the rest of the field to triggermidseasonnitrogen applications. Theprevious recommendationof aratio of 1.1 to1has been revisedafter 2020 trials to 1.07 to 1. “When a top-dressed nitrogen appli- cation was needed and applied, yields increased by roughly 200 to 1,000 pounds an acre,” Linquist said. “The larger the response index, the greater the potential yield increase.” These studies have resulted in greater confidence that the effect of in-season ni- trogen applications can be predictedwith some degree of confidence. “These are encouraging results that provide a more robust tool to accurately assess a rice fieldduring panicle initiation to determine whether the top-dress will produce a yield increase,” Linquist said. Another timely nutrient management study is focusedonhowfertilizerneedsare affectedby fallowinga field, asmanygrow- erschoose todounderdrought conditions. “There are 100,000-plus acres of rice fields fallowed this year,” Linquist said. “We’re comparing different nitrogen pro- grams. The straw will be further decom- posed and we know that straw affects ni- trogen uptake in different ways.” Studies this year at the experiment sta- tion are intended to help growers decide how tomanage fertilizer next year in fields thatwere fallowedbecauseof thedrought. Another study of planting dates for five commonmedium-grain varieties in 2020 showedthatdelayingplantingfromMay29 to June 12 slightly increased yields or had no effect. But further delay until June 26 resulted ina yieldpenalty of 1,500pounds an acre ormore. ( Bo b J o hn s o n i s a r e p o r t e r i n Sacramento. He may be contacted at bjohn11135@gmail.com.)
Bruce Linquist, a University of California Cooperative Extension rice specialist, discusses nutrient management studies at the Rice Experiment Station in the Butte County community of Biggs.
“Sincenoresearchhadbeendone inthis area, growers had no recommendations to go on,” Linquist said. “Given that un- certainty, an experiment was conducted comparing aqua ammonia, ammonium sulfate and urea applied as the nitrogen source. The study looked at different tim- ings of applications, splitting applications and theuseof specialtyureaproducts that delay or slow the release of nitrogen.” Researchers are repeating those trials again this year after finding last year that ammonium sulfate and urea were com-
parable to aqua ammoniumand that split applications produced the best yields. Another part of current nutrient studies at theexperiment station is repeating trials of threeureaproducts that slowthe release of thenitrogen.That couldmake iteasier to match available nitrogenwith cropneed. Theuseofaquaammoniainwater-seed- ed systems prevents loss of nitrogen to the atmosphere or nitrate leaching because flooded fields are anaerobic. But studies show there can be significant losses with aqua in drill-seeded rice. “When a field is drained that still has aqua nitrogen in the soil, oxygen is intro- ducedandmicrobes cancause theammo- nium to nitrate through a process called nitrification,” Linquist said. “Nitrate, in turn, can then denitrify when the field is reflooded and is lost as nitrogen fertilizer.” Studies at the experient station and a commercial field near Knights Landing showed theseaquanitrogen fertilizer loss- es in dry-seeded fields, which are drained and reflooded, can be in the range of 20 to 30 pounds an acre. There is further evidence of the value of nitrogen-rich strips. When an area of the field is fertilized heavily enough to ensure that nitrogen is not limiting, that can be usedtoestimatenitrogenneeds for therest of the field. For midseason fertilizer applications, researchers have found that drone mea-
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