Use plant growth stage and DD50 to gauge when best to make N applications.
• By Jarrod Hardke •
Proper nitrogen (N) management is all about timing. Whether we’re talking about preflood N, midseason N or boot N, we need to be on time to maximize yield and the economic return of the application.
For all rice cultivars, whether varieties or hybrids, the same general recommendation applies for preflood N management. We want to apply N and flood up around the five-leaf stage but no later than the “final N date” based on DD50 (heat unit) accumulation. This final date differs for each cultivar because each one reaches growth stages at different times, and this is why use of the DD50 Rice Management Program is so important (https://dd50.uaex.edu).
For example, for rice emerged April 20, the final N application timing for Diamond may be May 29 while the final timing for CL153 may be May 23.
Nitrogen applications later in the season are also critical but can have a more general window of application. Research on varieties from 2012-2018 helped us paint a solid picture of the optimum timing for midseason N application.
The recommendation is to wait to apply midseason N (46 lb N/acre) until at least four weeks after preflood N incorporation by the flood AND after beginning internode elongation. Often this timing will coincide with roughly ½-inch internode elongation, but both requirements need to be met prior to midseason N application.
The data clearly shows that it takes at least three weeks for preflood N to be taken up by the rice plant and sometimes as long as four weeks. Once the rice plant has effectively taken up the preflood N, it is then ready to maximize uptake of midseason N. If midseason N is applied too early (overlap in timing where the rice plant is still taking up preflood N), much of the midseason N is lost before the plant can effectively take it up.
The window of application to maximize yield is approximately two weeks long — a much larger window of opportunity than previously thought. So midseason N should be applied four to six weeks after preflood N incorporation but before you reach late boot (flag leaf fully emerged).
Midseason N applications are not recommended on hybrid cultivars unless an N deficiency is noted since the preflood N rates set for hybrids are sufficient to supply the plant until a late boot application can be made.
Instead, it is recommended to apply N to hybrids at late boot to beginning heading — that is, from the time you can see the flag leaf collar to when the first panicles begin to emerge. Applying 30 lb N/acre at this time results in small increases in grain yield and milling yield resulting in a positive net return over the cost of application.
Application made earlier than the late boot application window may still have benefits but also may carry the negative risk of increasing plant height and lodging potential.
Properly managed N fertilization goes a long way toward maximizing yield, milling and economic return. Focus on your N timing in 2020 to maximize overall rice crop success.
Dr. Jarrod Hardke is Extension rice agronomist with the University of Arkansas. He may be reached at email@example.com.