Phosphorous management

A 2009 study showed that delaying phosphorous applications until 35 days after planting provided yields comparable to applications just before planting. Researchers followed up on this work in 2010 with experiments in two phosphorous deficient fields and obtained similar results. Growers wishing to reduce the potential for algae growth with an alternative phosphorous timing are advised the following:

• If the field is not phosphorous deficient, then fertilizer can be applied any time, including the fall. (Most Sacramento Valley fields are not phosphorous deficient.)
• If the field is phosphorous deficient and a fall application is planned, a higher rate should be used. If the field is phosphorous deficient and an application is planned for spring planting, then phosphorous fertilizer should be incorporated.
• Fertilizer application can be as late as 35 days after seeding. Application is recommended when rice is above the water surface (three to four weeks). Also, irrigation water should be held for two weeks after application to avoid off-site phosphorous movement.

Greenhouse gas potential

Flooded rice fields contain conditions favorable for the production of greenhouse gases (GHG) such as methane and nitrous oxide. However, management practices play an important role in mitigating emissions in California rice fields.

Two on-farm experiments were conducted in 2010, with a conventionally farmed rice field growing M-206 and a drill-seeded field growing Koshihikari. Water management varied significantly during the season, with the latter field drained three times before permanent flood a month after planting. Both sites were drained one month before harvest. Several different nitrogen treatments were included to determine possible effects on emission levels.

Principal findings from this research include:

• Nitrification (conversion of ammonia to nitrate) appears to be the major process involved in nitrous oxide emissions, although denitrification (anaerobic conversion to nitrogen gas) may also contribute to overall emissions.
• Methane emissions were not directly affected by the addition of nitrogen fertilizer, but high fertilizer applications may lead to higher crop residue and eventually higher methane emissions.
• Frequent flood-drain cycles resulted in higher nitrous oxide emission events.
• Applying nitrogen deep into soil as aqua ammonia may reduce nitrous oxide and methane emissions (compared to surface nitrogen applications).
• Best management practices optimize agronomic efficiency, maximizing yields while minimizing GHG potential.

Nitrate leaching study

Water quality restrictions may eventually affect agricultural practices that allow nitrates to enter surface and ground water. To date very little information has been available to document nitrate leaching in flooded rice systems.

A study was conducted in 2010 to document the extent of nitrate leaching losses. Soil samples were taken to a depth of seven feet from seven fields representative of typical rice fields (relatively impermeable soils) and one field that is not (sandy soils).

If leaching were a potential problem in these fields, elevated nitrate concentrations would be found below the root zone.

Nitrate concentrations in excess of 10 ppm are considered a health hazard by the U.S. Environmental Protection Agency. In this study the highest nitrate level found was 4.2 ppm in surface soil.

In general, surface soils had more nitrate, ranging from 0.4 ppm to 4.2 ppm. Below the rooting zone, nitrate levels were all 3 ppm or less—in most cases less than 0.5 ppm. This suggests that nitrate leaching is not a big concern in California rice systems.

Web-based tools

The goal of this work is to develop a Web-based tool for rice weed management in alternative stand-establishment systems in the Sacramento Valley. Central to this approach is the ability to predict early-season weed emergence.

Researchers gathered historical air temperature data from 13 weather stations for the period April 15 through May 15 in the decade of 2000-2009. An average daily accumulation of thermal units was calculated to predict early watergrass emergence in the hottest and coolest parts of the region. Temperature sensors were placed in three locations in 2010 to compare thermal unit accumulation between May 8 and May 26.

Air temperature proved to be a better predictor of watergrass emergence than temperatures recorded at the soil surface. Work continues to refine the predictive model and further improve accuracy. A similar analysis for smallflower umbrella sedge is planned for 2011.

Stale seedbed systems

Unusually cool and wet weather in spring 2010 precluded research on nitrogen applications in stale seedbed systems. Although good recommendations have been developed for the use of stale seedbeds, work in this area has been suspended for now due to lack of grower interest.