Dustin L. Harrell, Kongchum, Manoch
Dustin Harrell, Manoch Kongchum and Nutifafa Adotey
Rice production is a traditional row crop enterprise with a rich history in Louisiana. In 2016, rice was produced in 32 parishes on approximately 420,000 acres by just over 1,000 farming families. The economic impact of rice production in the state was valued at $427 million in 2016, which illustrates the importance of the industry to local farming communities and the state as a whole. If rice production is to remain sustainable in Louisiana, management practices must be focused on maximizing fertilizer efficiency and minimizing environmental losses.
Rice production practices in Louisiana have evolved over the years to allow more efficient use of fertilizer. With the introduction of Clearfield rice, which is a herbicide-tolerant rice developed at the LSU AgCenter and released in 2002, most rice is drill-seeded rather than water-seeded, which had been necessary to suppress the red rice weed. With drill-seeded rice, the establishment of a permanent flood is delayed until the rice reaches the 4- or 5-leaf stage of development. Prior to the Clearfield technology, fertilizer had to be applied into water, which reduces nutrient efficiency. This one shift in production practices has had a tremendous positive impact on the efficient use of fertilizer.
Nitrogen is the most important fertilizer nutrient in rice production because it is used in the greatest amount and it represents the greatest fertilizer expenditure on a rice farming budget. Rice is the most efficient user of nitrogen fertilizer as compared to all other cropping systems when research-based best management practices are followed. When these practices are not followed, rice can be one of the least efficient of all cropping systems.
In drill-seeded rice production — now the most common production practice in Louisiana — nitrogen fertilizer is applied at two application timings. The first application timing is just before the permanent flood is established. The second application is at midseason. The pre-flood application is the most important because this is when the greatest amount of nitrogen fertilizer is applied. It is also where there is the greatest risk of nitrogen loss. Research in Louisiana has shown that pre-flood nitrogen fertilizer should be applied: 1) on dry ground and 2) flooded as soon as possible. Urea, which is the most common nitrogen fertilizer source in rice production, can volatilize (turn into ammonia gas) and be lost to the atmosphere when left unprotected on the soil surface.
Research at the H. Rouse Caffey Rice Research Station has shown that in some years approximately 30 percent of nitrogen fertilizer can be lost if left unprotected on the soil surface (Figure 1). Flooding a rice field will cause the soil to go anaerobic (no oxygen), which will stabilize ammonium-nitrogen derived from the breakdown of urea. This allows rice to take up the nitrogen when it needs it. Urea begins to volatilize slowly at first and then increases over time when it is not protected by the irrigation water. After urea is left exposed on the soil surface after three to five days (depending on environmental and soil conditions), the gaseous loss potential of nitrogen would be sufficient enough to justify treating the urea fertilizer with urease inhibitor, which contains the chemical NBPT. The urease inhibitor will temporarily delay the breakdown of urea and greatly suppress volatilization losses (Figure 1). It is also imperative that preflood nitrogen is applied on dry ground and not on a muddy or flooded soil.
Research in Louisiana has shown that urea will break down faster and gaseous losses of nitrogen will begin faster when urea is applied onto a muddy soil making the application less efficient (Figure 2). However, treating urea with a urease inhibitor will provide some protection when nitrogen is applied on a moist soil (Figure 2). Research has also shown that dropping urea fertilizer into a standing flood early in the season can greatly reduce nitrogen use efficiency (Figure 2).
Following research-based nitrogen fertilizer best management practices in rice production make rice one of the most efficient cropping systems in Louisiana when it comes to nitrogen use. Dedicated agronomic and fertility research in Louisiana over the past decade has increased the efficient nutrient use of all fertilizer nutrients and has helped make rice production in Louisiana sustainable for future generations.
Dustin Harrell holds the Mosaic Company Professorship at the H.Rouse Caffey Rice Research Station in Crowley; Manoch Kongchum is an assistant professor at the station; and Nutifafa Adotey is a postdoctoral researcher at the station.
(This article appears in the fall 2017 issue of Louisiana Agriculture.)
In delayed-flood rice production, urea fertilizer is most efficient when applied onto a dry soil (H. Rouse Caffey Rice Research Station). Photo by Dustin Harrell
Urea fertilizers are most efficient when applied onto a dry soil. Darker green vegetation indicates rice that has taken up more nitrogen fertilizer because it was applied more efficiently (H. Rouse Caffey Rice Research Station). Photo by Dustin Harrell
Figure 1. Ammonia volatilization loss from urea and urea treated with a urease inhibitor over time on a Crowley silt loam soil at the H. Rouse Caffey Rice Research Station.
Figure 2. Nitrogen use efficiency of urea and urea treated with a urease inhibitor when applied onto a dry, moist and flooded soil at the H. Rouse Caffey Rice Research Station. Different letters on top of the bars indicate significant differences.