By Trent Roberts, Rick Norman and Jarrod Hardke
Preflood nitrogen (N) applications in rice are very important for setting yield potential in rice as two of the three yield components (tiller number and seeds per panicle) are set by the preflood N. The efficiency of preflood N uptake in rice is influenced by many factors including N source, timing of application, rate of application and water management following application. One of the most unique aspects of direct-seeded, delayed-flood rice production systems (which are widely used across the Mid-south) is that they can be one of the most efficient (>85% N use efficiency) or inefficient (<30% NUE) production systems depending on how the producer manages the preflood N. Our research has shown that an optimum preflood N rate can maximize grain yield and milling potential with significantly less (20 lb N/acre) total N applied and the additional savings of an aerial application at midseason. Optimum preflood N application is recommended only for pureline cultivars, but preflood N management for hybrids should follow the same general rules of thumb to ensure that N efficiency is maximized for those cultivars as well. The following guidelines should be used for effective optimum preflood N management in rice:
- Determine season total N rate using N-STaR or cultivar/soil texture/previous crop
- The field is not a sandy (permeable) soil, but a silt-loam or clayey-textured soil.
- The field can be flooded in five to seven days.
- The preflood urea is treated with a recommended urease inhibitor that includes NBPT or ammonium sulfate is used as the N source. (refer to FSA2169)
- A two to four inch flood can be maintained for a minimum of three weeks following the preflood-N application.
- The preflood-N fertilizer rate must be applied uniformly across the field – no streaking.
A recently developed factsheet “FSA2187 Optimum Preflood Nitrogen Applications in Rice” (https://www.uaex.edu/publications/pdf/FSA2187.pdf) has been released that covers all the basics of implementing this production practice. Additionally, the recently revised “FSA2169 Nitrogen Fertilizer Additives” (https://www.uaex.edu/publications/PDF/FSA-2169.pdf) can provide insight into the selection and effective use of urease inhibitors for preflood N management.
Some potential drawbacks to optimum preflood N are whether or not the rate was sufficient and was the rate managed correctly. The recent development and implementation of the Greenseeker response index for rice allows producers to “check” the sufficiency of their N program post-flood in a timely manner that can allow corrections to be made and no yield penalty will occur. Subsequent blog articles will focus on how to use the Greenseeker response index effectively in rice, but the key thing that cannot be missed is the incorporation of a high N check plot in the field. The best approach is to flag a 5 x 5 foot square area prior to flooding and add an additional ¼ to 1/3 cup of urea over the top of the preflood N rate which was applied to the field. This will allow you to take measurements post-flood and determine if additional N is needed to maximize rice grain and milling yield potential.
As always, if you have any questions or there is any way that we can help please let us know!