By Bob Johnson, Ag Alert
Rice researchers and growers are working to fine-tune the timing and type of nitrogen applications to produce the best crop with the least impact on the environment.
Current studies examine how to adjust nutrient management the year after ground 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 nitrogen applications.
“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 fertilizer to apply at the right time to maximize 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 recently at the Rice Experiment Station in Biggs. “Weather or supply chain issues may force growers to rethink their nitrogen programs.”
Some current studies of nutrient management options came after late-season rains two years ago. The rains forced many growers to try alternatives to their usual practice of applying most of their nitrogen as aqua ammonia at planting or a few weeks later.
“Since no research had been done in this area, growers had no recommendations to go on,” Linquist said. “Given that uncertainty, an experiment was conducted comparing aqua ammonia, ammonium sulfate and urea applied as the nitrogen source. The study looked at different timings of applications, splitting applications and the use of specialty urea products 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 comparable to aqua ammonium and that split applications produced the best yields.
Another part of current nutrient studies at the experiment station is repeating trials of three urea products that slow the release of the nitrogen. That could make it easier to match available nitrogen with crop need.
The use of aqua ammonia in water-seeded 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 introduced and microbes can cause the ammonium 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 these aqua nitrogen fertilizer losses 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 used to estimate nitrogen needs for the rest of the field.
For midseason fertilizer applications, researchers have found that drone measurements sensing the normalized difference red edge, or NDRE, can be used much like the hand-held Green Seeker measurements of normalized difference vegetation index, or NDVI, to know in advance whether a top-dressed application at panicle initiation would 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 trigger midseason nitrogen applications.
The previous recommendation of a ratio of 1.1 to 1 has been revised after 2020 trials to 1.07 to 1.
“When a top-dressed nitrogen application 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 nitrogen applications can be predicted with some degree of confidence.
“These are encouraging results that provide a more robust tool to accurately assess a rice field during panicle initiation to determine whether the top-dress will produce a yield increase,” Linquist said.
Another timely nutrient management study is focused on how fertilizer needs are affected by fallowing a field, as many growers choose to do under drought conditions.
“There are 100,000-plus acres of rice fields fallowed this year,” Linquist said. “We’re comparing different nitrogen programs. The straw will be further decomposed and we know that straw affects nitrogen uptake in different ways.”
Studies this year at the experiment station are intended to help growers decide how to manage fertilizer next year in fields that were fallowed because of the drought.
Another study of planting dates for five common medium-grain varieties in 2020 showed that delaying planting from May 29 to June 12 slightly increased yields or had no effect. But further delay until June 26 resulted in a yield penalty of 1,500 pounds an acre or more.
