Improving Fertilizer Guidelines for California's Changing Rice Climate, 2015

 

Project Leader

Bruce Linquist, UCCE rice specialist, Dept. of Plant Sciences, UC Davis

he goal of this project is to develop fertilizer management guidelines that are economically viable and environmentally sound. Research objectives in 2015:

• Determine the potassium status of rice soils.

• Develop management practices for growing rice under alternating flooded/dry conditions.

• Quantify rice yield variability in the Sacramento Valley. Examine midseason nitrogen status and topdressing.

Potassium status

In 2012 and 2013, 55 rice fields were identified in the Sacramento Valley for potassium status research. Samples were taken from the soil, water, and rice flag leaves in all fields and analyzed for potassium. In addition, growers were asked about yields, straw management, and winter flooding practices.

Soil potassium values ranged from 35 to 350 ppm. There was no relationship between soil potassium values and the amount of potassium that had been added and removed. Soil potassium values were lowest in the southeast part of the valley, followed by the northeast and northwest. Highest values were in the southwest. All fields below the threshold of 60 ppm were on the east side.

When soil potassium values were below 60 ppm, half the flag leaves sampled had values below the critical level of 1.2%. Potassium fertilizer should be considered when soil potassium levels are below 120 ppm.

There was a significant difference in the concentration of potassium in irrigation water. The Sacramento River had the highest potassium values (1.18 ppm), while the Feather River averaged 0.79 ppm. Well water had the highest overall potassium concentration (2.3 ppm), but it was also highly variable. Recycled irrigation water averaged 1.4 ppm and also was variable.

Preliminary analysis of the data indicates that it is difficult to make refined recommendations for potassium, likely due to differences in soil mineralogy. In 2016, mineralogical traits will be collected and analyzed to help fine-tune potassium recommendations.

Findings from this research will be developed into a format that is useful to growers and made available for online use, as well as distributed through other outlets to ensure that growers can further improve potassium management.

Alternating wet/dry rice

Current water management practices keep California rice fields continuously flooded through the majority of the growing season. This strategy helps provide high yields, good weed control, and efficient nitrogen use. Nonetheless, there is interest in exploring alternative production practices such as alternating flooding with periods of dry soils. This practice goes by the acronym AWD.

A study at the Rice Experiment Station has been evaluating this practice for three years. In all AWD treatments, grain yield was the same as in the conventional water-seeded treatment. Similarly, the optimum nitrogen rate required to achieve maximum yields was similar among treatments. The AWD treatment reduced greenhouse gas emissions by more than 70% and grain arsenic by 50%. This management strategy may also have positive effects on methyl mercury cycling.

Research from 2015 suggests the window during which a field can be safely drained is fairly large—up to 11 days without a yield penalty. While these results are encouraging, it is not clear how easy this method would be to implement at the field scale.

Rice yield variability

California rice yields are among the highest in the world. However, over the past 15 to 20 years, yields have stagnated. The goal of this area of research is to identify ways to further increase yields through improved management.

In 2015, researchers focused efforts on conducting a yield gap analysis of rice systems in the United States. As a first step, all California statewide variety trial data from 1999 through 2013 were compiled into a central database. Yield information was linked to site-specific daily climate and soils data. A procedure was developed to calculate climate variables, such as average temperature during flowering, that is specific to each data point in the statewide variety trials.

Using this data and others from the Southern U.S., researchers tested a model (ORYZA) to estimate yield potential in various regions. ORYZA can adequately simulate yield potential for environments similar to the one it was developed in (Southern U.S.) after basic calibration of phenological parameters.

Modeling of environments that experience relatively cool temperatures (California) required more extensive calibration. Complex phenomena like cold induced sterility will require updating components of the model.

Issues with the ORYZA model prompted exploration of the response of California rice varieties to exposure to cold temperatures during the period from panicle initiation to 50% heading. One set of experiments sought to quantify the benefits of increased water depth during this period to protect spikelets from cold induced sterility. Results showed that raising water between panicle initiation and heading did not affect blanking but did result in an increase of about 10 grains per panicle. This is a single study and interpretations of results should be made with caution.

Another area the modeling exercise with ORYZA highlighted is the importance of stand density. An experiment with M-206 at the Rice Experiment Station examined the effect of seeding rate on yields. The optimal plant density to achieve maximum yields was 25 plants per square foot, which was achieved at a seeding rate of 50 seeds per square foot. For this variety, that equates to planting at a 140 pounds/acre rate.

Using county level yield data, the next step in this research is to determine current yields for each region and then determine the yield gap in each region.

Midseason nitrogen status

The objective of this study was to examine the potential of using remotely sensed data to determine the need for a midseason top-dressed nitrogen application.

A nitrogen rate trial was conducted near Arbuckle and at the Rice Experiment Station. Five nitrogen rates were used to provide different color and yields. An additional experiment on 29 farmer fields also was conducted. Plant samples at panicle initiation and heading were taken, as well as readings from a handheld crop sensor.

For the nitrogen rate trials, a fairly typical response was observed. At the Arbuckle site, maximum yields were achieved at 67 pounds/acre. At the RES, maximum yields were achieved at 160 pounds/acre. An unusual outcome of this study was the very high yields achieved with no fertilizer nitrogen applied.

In this study, the handheld sensor was not a good predictor of biomass at panicle initiation and of nitrogen concentration in the plant. However, it did correlate well with aboveground nitrogen content and was useful for estimating final yields and, thus, could possibly be used to determine whether a top-dressed nitrogen application is necessary. This is a preliminary analysis, however. Additional data are still being analyzed. Given these initial promising results, along with the increased availability of remotely sensed data, further research is warranted.