As I write this column (March 19), we historically have had at least one rice field planted in Mississippi or are beginning to plant the last week of March. But this year, I fear we will not get the first rice planted until the first week of April due to prolonged wet conditions.
Speaking of wetness, the topic for this column is water management. In regard to rice production, it has probably been one of, if not the No. 1, hot topic over the past several years.
Declining aquifer levels in many rice-producing areas of the Mid-South rice belt necessitated a need for new research to determine ways to manage our water resource optimally.
Over the years, there have been many different methods developed to manage irrigation water, but it seems we have settled on three methods for the direct-seeded, delayed-flood rice-production method. The three that the Risk Management Agency has chosen to insure are traditional flood, alternate wetting and drying, and furrow irrigation. All three have advantages and disadvantages I will attempt to explain.
The traditional method of irrigation (flooding at five-leaf rice and holding until draining for harvest) is tried and true. It also offers benefits for pest management and nutrient use but can use the most water of all three systems or the least, as with zero grade.
Water use can be reduced successfully while maintaining weed and disease control benefits by incorporating multiple-inlet rice irrigation strategies. Research with MIRI on straight levee fields in Mississippi has shown it can reduce water use up to 13-acre inches.
Alternate wetting and drying, or AWD, is most like the traditional method. For old rice guys, we used to call it poor water management, but research has shown that old adage is false. Properly implemented, AWD (holding a full flood for at least three weeks after establishment) has the potential to reduce water use to near-zero grade levels on positive grade fields. Yet you maintain some flood water benefit for weed control and nitrogen management.
The final and perhaps hottest method in rice production circles is furrow-irrigated or row rice. Most research is still in its infancy on this water management strategy, but early trials suggest it can be used successfully in the proper production scenario.
In Mississippi, the strategy has shown similar water use levels to AWD in trials to date. However, with no ponding flood water on a portion of the field, we lose the benefit it provides for nutrient use, weed control and disease management. To many producers using this technique, sacrificing yield on the top of a field is outweighed by not having to construct levees in the field.
All three systems can potentially reduce water use and maintain good yields if used in the proper field situation. If anyone is considering transitioning to one of the newer systems and have questions, give us a call — we’ll be glad to aid in weighing the pros and cons and help you make an informed decision.
Shift to row rice has boosted attention to irrigation
It’s difficult to write about water management right now. It hasn’t stopped raining for what seems like two years. We need to get the water off of these fields so we can go to work, then we can start worrying about irrigation.
The surge in acres shifting to a furrow-irrigated system has brought much more attention to rice irrigation in general. As a reminder, rice is a semi-aquatic plant. That is, it likes water and will tolerate a flood, but it does not need one to grow and maximize yield potential. This fact has risen to the forefront as more adopt row rice.
In a levee-irrigated (flood) rice system, we typically flood for two reasons: weed control and nitrogen management. These two production areas, in terms of cost and importance to yield potential, have been the biggest drivers behind maintaining a permanent flood. We also get the added benefit of blast suppression by maintaining a deep flood later in the season.
Multiple-inlet rice irrigation (MIRI) using collapsible poly tubing to irrigate each paddy at the same time rather than cascading through gates is still preferred. This system has been consistently used on about one-third of rice acres for the past several years.
There is a slight learning curve, but most who try it see the benefits immediately and move most of their acres to it. In drier years, we see as much as a 40% reduction in water use.
In a furrow-irrigated or row-rice system, there may still be as much art as science for determining irrigation timing. Generally, on loamy soils it’s recommended to irrigate once every three to five days while on clay soils it’s recommended to irrigate once every five to seven days.
On both soil types, some data exists to show it is possible to wait longer than these periods between irrigations. But the data is limited, and individual situations will dictate whether longer periods are feasible. The use of soil moisture sensors can help to guide irrigation timing by monitoring water in the soil profile.
When we get rid of the water on hand, let’s be sure we work fields properly prior to planting. High and low spots in the field from ruts and minimal tillage can cause season-long issues with water management and ultimately affect yield, sometimes dramatically. When the field is prepared properly, given the weather window to do so, it makes water management far easier.
Maintaining a flood for 3 weeks after pre-flood N is key
Maintaining a constant flood on rice immediately after applying pre-flood nitrogen (N) fertilizer for a minimum of three weeks is critical to minimize N fertilizer losses to the environment. It does not matter if you are using water in your rice production system by traditional delayed flooding, alternate wetting and drying or multiple-inlet rice irrigation.
The reason for three weeks is simple: the ammonium form of nitrogen is stable under flooded, anerobic (no oxygen) conditions, and it takes four-leaf rice approximately three weeks after fertilization to take up the bulk of that applied N fertilizer.
Past research evaluating the rate of N uptake by rice after flooding indicated that 11% of applied N was taken up the first week, 27% after the second week and 65% after the third week (Wilson et al., 1989).
The take-home message from this research was that the rate of uptake of N fertilizer by rice increased with the size of the rice and its root system. You would also expect that when rice is growing faster after N application, say in warm conditions, the N uptake would be even more efficient. And in cooler conditions when rice was growing more slowly, the efficiency would trend lower.
The loss of the flood water before the three-week period would allow oxygen to convert any remaining ammonium-N to nitrate-N. This is not immediately bad because rice can take up nitrate-N.
However, once rice is reflooded and the system becomes anerobic again, the nitrate-N is lost very quickly as a gas by the denitrification process. Therefore, we always apply more N fertilizer before reflooding to make up for the predicted lost N.
How much N should you apply? Great question. Unfortunately, we do not have an exact answer, but growers and consultants can estimate that more N fertilizer would need to be applied if you lost the flood after the first week compared to a flood that was lost after the second week.
We should also consider revisiting this research with the newer, earlier maturing rice varieties and hybrids we grow today. I would expect to find that the N uptake efficiencies from these newer varieties and hybrids would be slightly higher.
Apply a timely flood to reduce seedling water stress
Most Texas rice farmers use a delayed flood type of irrigation system. Rice is drilled into moisture, or if there’s little or no moisture, drilled then flushed.
Once rice emerges, flushes (or rainfall) are applied as needed until rice begins to tiller when a permanent flood is established. It has been my experience that many farmers apply the flood too late related to emergence. Some farmers believe that an early flood inhibits tiller development, but this is not the case. Young plants are very susceptible to water stress.
In my research plots, I try to apply the flood about three weeks after emergence; I suggest you do the same. Row-rice irrigation is becoming more common in Arkansas, but only about 10% of Texas acreage is irrigated in this fashion. Researchers tell me that water savings are minimal, but the system can save time and labor in a soybean-rice rotation.
However, soybean acreage on the Upper Gulf Coast of Texas has been declining recently. For instance, in 2019, the entire state of Texas produced soybeans on only about 60,000 acres.
As you know, water is becoming increasingly expensive and scarce, so I encourage farmers to watch closely for levee leaks while capturing rainfall to save on flushes. It also makes sense that if farmers are charged for the water they actually use, they will use that water more efficiently compared to farmers who are charged a flat fee for main and ratoon crops.
Crop consultant Cliff Mock spoke to the Beaumont Center faculty this winter about water use in Brazoria County where he and his son, Wade, farm. In 2019, they used about 2.25 acre-feet of water for their main crop. This is simply amazing compared to the use of 4 to 6 acre-feet of water back when Texas rice was water-seeded. Cliff also opined that many Texas rice farmers already practice a form of alternate wetting drying because of difficult irrigation scheduling and efforts to reduce water use.
As always, I encourage you to scout for early season pests like chinch bug, fall armyworm and aphids. These early season pests can often be controlled by a timely flush or flood. Also, selected seed treatments can provide control. And please be on the lookout for the rice planthopper/delphacid. If you have suspicions or want more info, contact me at firstname.lastname@example.org or 409-239-4265.
Finally, I want to give a big shout out to the Mississippi folks who put on a great Rice Technical Working Group meeting in Orange Beach, Alabama, in February!