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Minimize Fertilizer Nutrient Loss

Use nitrogen BMPs for sustainable rice production

By Dr. Dustin Harrell
LSU AgCenter
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Sustainable agriculture has become one of the most thought-provoking and frequently used terms over the past decade. It basically refers to our ability to minimize negative impacts on the environment while maintaining current production levels. One of the most important sustainable agricultural strategies to mitigate environmental impact is to minimize fertilizer nutrient loss.

Gaseous loss of nitrogen (N), particularly nitrous oxide, is deleterious to air quality. In fact, nitrous oxide has 310 times the global warming potential as compared with carbon dioxide. In addition, N losses in runoff events and leaching negatively impact surface waters.

Therefore, if we follow Best Management Practices for N fertilizer applications, we not only will be reaping the benefits of maximizing the return on every fertilizer dollar spent but also employing sustainable agriculture practices.

Rice often gets a negative label for its environmental impact, or carbon footprint, mostly because of potential emissions of methane from rice soils being submerged four months of the year. However, when N fertilizer is used in drill-seeded, delayed flood rice production according to Best Management Practice recommendations, it is one of the most efficient users of N of all agricultural row crops. On the other hand, if N fertilizer is applied improperly, it also can be one of the worst.

Varieties, Soil Types And Environments
As we begin gearing up for the coming rice season, I would like to remind you of a few Best Management Practices for N fertilizer use in rice production.

To begin, it is important that only ammonium or ammonium-forming fertilizers (urea) be used. Avoid all fertilizers that contain nitrate-N. All N in the nitrate form will be quickly lost via denitrification once a flood is established.

Optimum N rates vary according to the variety of rice grown, a soil’s ability to supply N over the growing season and environmental conditions. Research has identified a range of N rates that work across many different soils for each individual rice variety. These recommendations can be found in the Rice Varieties and Management Tips publication, which is available online (www.lsuagcenter.com) and in print at your local Extension office. Past experience should help you identify what N rate is best for your particular soil within that recommended range.

 
Fertilization Recommendations For Second-Crop Rice
 

Ratoon, or second-crop rice, should be fertilized with 75-90 pounds of nitrogen per acre when first-crop harvest is before August 15. When first-crop is harvested after August 15, fertilize with 30-45 pounds of nitrogen per acre.

When conditions appear favorable for good second-crop production (minimal field rutting, little or no red rice, healthy stubble), apply the higher rate of nitrogen. Apply nitrogen and establish a shallow flood within five days after harvest.

Research has consistently shown that nitrogen fertilizer should be applied and the field flooded as soon as possible after first-crop harvest to maximize second-crop yields.

When the main crop is harvested after August 15, the potential for profitable second-crop production is reduced because of the probable delay in maturity, especially at higher nitrogen rates, and the increased likelihood of unfavorable weather.

Source: LSU AgCenter 2011 Rice Varieties and Management Tips

This year, we have a new N recommendation range for CL151. The recommended N range for CL151 is 90-130 pounds of N per acre. This is lower than almost all other currently grown rice varieties. In our research trials, lodging began to occur most often at rates of 135 pounds of N per acre and above when using recommended seeding rates. Yields seldom were improved at higher N rates.

Starter N fertilization is not necessary in delayed flood rice production. Research has shown that starter N applications do not result in a yield increase. However, an increase in early season vegetative growth was often observed, which resulted in rice reaching the tillering stage of development faster. Therefore, a flood could be established earlier. This is particularly beneficial in weed management. Starter fertilizer N is the least efficient of all application timings and should not be counted toward your targeted N rate total. If starter N is used, no more than 20 pounds per acre should be applied.

Benefits Of Urease Inhibitor
Two-thirds of fertilizer N should be applied just before permanent flood establishment on a dry soil in delayed flood rice production. Flooding the soil stabilizes the N in the ammonium form that can be utilized by rice throughout the growing season as long as the flood is not lost.

The longer the urea or ammonium-based fertilizer stays on the soil before a flood is established, the more N will be lost from ammonia volatilization. If the soil is moist when the fertilizer is applied, higher rates of N volatilization should be expected. If the N is applied into a flooded field, the N volatilization losses will be even greater. The use of a urease inhibitor, such as Agrotain, will temporarily delay volatilization losses while the fertilizer sits on the soil surface before the flood is established. This is particularly beneficial if it takes longer than three to five days to flood a particular field, which is the case for most of our commercial rice fields.

Another loss mechanism of N that can occur in rice production is referred to as nitrification-denitrification losses. As ammonium-N from our fertilizer application sits on the soil surface prior to flooding, the ammonium N can be converted to nitrate-N via the nitrification process. Upon flooding and after the soil becomes anaerobic, a process called denitrification will quickly convert all nitrate-N to gaseous forms, which will be lost to the atmosphere.

Application Timing
The final N fertilizer application in delayed flood production should be applied at mid-season. Green ring is our visual indicator for identifying proper application timing. This time, urea or another ammonium N fertilizer can be flown onto a flooded field.

Rice has developed an extensive root system by mid-season that can actually out-compete volatilization losses at this stage of development. So, flying N fertilizer into the water at mid-season is not a problem. In fact, research has shown that N applied at mid-season is taken up almost completely in as little as three days.

Applications of fertilizer N in water-seeded rice systems must be treated differently. Generally, two-thirds of the N should be preplant incorporated (PPI) with the second application at mid-season. In a water-seeded pin-point flood rice system, the first N application can either be applied PPI or applied during the brief interval between draining for the rice to pegging and re-flooding, with the final application being made at mid-season.

By using these Best Management Practices for N fertilizer, we not only will be obtaining the highest fertilizer use efficiency possible in our rice production system, but we also will be using sustainable agriculture practices.

Dr. Dustin Harrell is an Extension agronomist and assistant
professor with LSU AgCenter. Contact dharrell@agcenter.lsu.edu.


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