Regular and repeated rainfall across the Delta has had a significant impact on this year’s rice crop. Many rice fields in Arkansas are, or already were, up against the final recommended time to apply preflood nitrogen (N). Further complicating the situation has been the rain and wind in previous weeks that also prevented preflood “cleanup” herbicide applications that needed to go out ahead of fertilizer applications. With the latest round of wet weather to hit the state, preflood N management in rice just became almost unmanageable.
When preflood N is applied onto dry soil to rice at the 4- to 5-leaf stage and a flood is applied timely, plants take up at least 60 percent of the total N applied. This N uptake typically occurs over three weeks with about 10 percent uptake in week one, 20 percent in week two, and 30 percent in week three (to total 60 percent). In general, the period from the optimum time to apply preflood N until internode elongation (IE) is about three weeks. From the final recommended time to apply preflood N to IE is about two weeks. However, these timings are also based on plant development when rice has received a flood and N fertilization at the 4- to 5-leaf stage — delaying the flood causes rice to develop more slowly. Keep in mind that we can only make up a small amount of yield with N applied at midseason.
If you have any time to spare, it is always best to attempt to apply preflood N onto dry soil. Applying N onto muddy soil or into standing water is far less favorable and much less efficient method of N fertilization.
In order of preference, based on yield response and N efficiency, here are options for applying preflood N based on field situations:
1. Field is dry: Apply NBPT-treated urea onto dry soil and establish the permanent flood in a timely manner to incorporate N below the soil surface. This is always the most preferred option.
2. Field is muddy: Apply NBPT-treated urea onto muddy soil and attempt to let the soil dry if you have time. If a significant rainfall event occurs to re-wet the field then begin flooding; otherwise let the soil dry before establishing the flood. If you’re applying N to mud we do not know how much N will be lost, but if you feel the need to increase the N rate, do so only slightly (10 to 20 pounds N higher). This increase may or may not provide much benefit, but it’s less likely to hurt in this case. Watch the crop closely and apply extra N if N deficiency occurs.
3. Field has standing water: Get the water off the field if at all possible. Many no longer have time to get the water off and let the soil dry. In this case, “spoon-feeding” N into the flood in small quantities every five to seven days for three to four weeks is the best option. A small quantity means 45 pounds N per acre (100 pounds urea per acre). If you have a short time to IE, maybe applying N for three weeks at 45, 60, and 60 pounds N per acre will be better. However, smaller quantities applied more frequently are your best option in this situation. Do not, for any reason, apply the entire recommended preflood N rate in one application into standing water.
Preferred “worst-case” management would be to apply N to muddy/wet soil as rice reaches the end of the optimum window according to the DD50 program. Attempt to dry the soil out and if a significant rainfall occurs, start flooding. Realize that some N is lost in this case and be prepared to monitor the crop closely and apply additional N later if the rice looks like it needs it.
Previous research has shown that N applied onto dry soil has the most yield benefit. Applying urea onto muddy soil can result in a 20 percent yield loss. However, applying ammonium sulfate or urea + NBPT onto muddy soil and letting the soil dry can reduce the yield loss to only 10 percent. In this research, N was applied just prior to permanent flood at the 4- to 5-leaf stage.
Past the 4- to 5-leaf stage, potential yield losses could become more dramatic. However, many factors influence how much flexibility we have in our N fertilization timing, including cultivar, length of maturity, native soil N, soil type, etc. If native soil N is high, then the effect is reduced. If it is a longer season cultivar then there is a greater window before midseason. In any case, don’t let it get too late before applying N. Use of the DD50 Rice Management Program can help to time management decisions in these situations (http://DD50.uaex.edu).