Seeding Rate Work in Furrow-Irrigated Rice

Seeding Rate Work in Furrow-Irrigated Rice

justin chlapecka
Dr. Justin Chlapecka
Assistant Research Professor/
Rice Extension Specialist
University of Missouri

We recently wrapped up the “meeting season” circuit, where it seems like we speak about every other day on various research topics that we’ve been looking at throughout the year(s). While meeting season is a great avenue to exchange ideas with other researchers, farmers, consultants, and retailers, I believe most of us are sitting around looking at the calendar and sitting on go. Recent bouts of dry weather and 75 degree+ temperatures through February and March have my internal clock thinking it’s a lot later than it actually is.

The meeting circuit this winter included various destinations in Missouri, Arkansas, and even California. The No. 1 topic coming out of the Missouri rice research program is the possibility of increasing yield potential in furrow-irrigated rice through variable rate seeding technology. Small plot research for two years in a row has consistently shown that increasing seeding rate beyond current recommendations in non-flooded portions of the field can increase yield potential.

While significantly increasing the field’s overall seeding rate is likely not economically beneficial, especially with $6.50 per bushel rice, shifting seed to where it can have a greater positive impact on yield is something that we will continue to look at in the next few years. The same seeding rate studies that have shown a positive relationship between seeding rate and yield beyond the current seeding rate recommendations have also suggested that it is much more difficult to get the same response in flooded portions of the field, i.e. the bottom of the field.

Pretty much every cultivar we have tested, whether it be hybrid or inbred variety, appears to have no negative response when decreasing seeding rates by 10% to 20% from recommended. I have spoken with several farmers and consultants over the past few weeks who plan to try out a field or two with a 10% to 20% decrease at the bottom of the field and a 10% to 20% increase at the top of the field, leaving the middle third at the recommended or average rate. I would caution individuals to not drop the seeding rate by any more than 20% due to the potential for a weak or patchy stand in areas. Keep in mind that you know your field better than anyone when deciding whether seeding rate could be reduced and still obtain an adequate stand.

This year should be a good indicator of what, if any, seeding rate tweaks need to be made in furrow-irrigated rice. We plan to do several larger-scale tests ourselves, in cooperation with the University of Arkansas, to see if the small plot data will translate to production-sized fields. If you’ve got any questions or would like to participate, don’t hesitate to give me a shout. As always, God bless and eat MO rice!

Water Seeding Rice

ronnie levy
Dr. Ronnie Levy
Extension Rice Specialist
Louisiana State University

Extreme wet conditions or red rice weed suppression are conditions that encourage water seeding. The most common water-seeding method is the pinpoint flood system. After seeding, the field is drained briefly. The initial drain period is only long enough to allow the radicle to penetrate the soil (peg down) and anchor the seedling. A three- to five-day drain period is usually sufficient under normal conditions.

The field is then permanently flooded until rice nears maturity (an exception is mid-season drainage to alleviate straighthead). In this system, rice seedlings emerge through the floodwater, and seedlings must be above the water surface by at least the four-leaf rice stage. Before this stage, seedlings normally have sufficient stored food and available oxygen to survive.

Atmospheric oxygen and other gases are necessary for the plant to grow and develop. The pinpoint flood system is an excellent means of suppressing red rice emergence from seeds in the soil because oxygen necessary for red rice germination is not available as long as the field is maintained in a flooded (or saturated) condition.

Seedbed preparation is somewhat different when water seeding is used compared with dry seeding. With water seeding, the seedbed is left in a rougher condition than for dry seeding. A flood is established, and rice is seeded within three to four days. This will reduce potential weed problems and provide a more favorable oxygen situation at the soil/water interface. Low oxygen levels are often a problem where floodwater is held for a long time before seeding.

A preferable alternative to a rough seedbed is preparation of a seedbed similar to that for drill seeding. The seedbed is firmed with a grooving implement, resulting in a seedbed with grooves (one to two inches deep) on seven- to 10-inch spacings. In some situations, a field cultivator can achieve the desired grooves.

Conservation tillage has gained acceptance in many rice-growing areas. No-till and reduced-tillage systems, such as fall- and spring-stale seedbeds, have been shown to significantly improve the quality of floodwater being removed from rice fields by reducing sediment losses.  Research has established the advantages and disadvantages of reduced-tillage rice production, and it has identified stand establishment and early season plant density as critical components of managing a reduced-tillage rice production system. The stale seedbed system, however, offers one important benefit; during wet springs it improves the likelihood of timely planting. Time, money, and labor are conserved by controlling preplant vegetation with a burndown herbicide rather than waiting for the seedbed to dry for planting when excessive rainfall occurs.

Inadequate stand establishment is a common problem in water-seeded and conservation tillage rice, especially in a pinpoint flood system. The seeding rate when water seeding is 80 pounds to 120 pounds compared to 50 pounds to 80 pounds when drill seeding. Where seed depredation by blackbirds is potentially high, use a higher seeding rate and consider using a bird-repellent seed treatment.

Water Right, Water Light

Jarrod Hardke, University of Arkansas
Professor/Rice Extension
University of Arkansas System Division of Agriculture

Alternate wetting and drying (AWD) can be a useful tool to reduce water use and greenhouse gas emissions. As a result, a number of programs are incentivizing this practice. In cascade flooded systems, benefits can be realized, but it’s harder to implement effectively. Full benefits and reduced risk are most often realized in fields using multiple inlet rice irrigation (MIRI) or zero grade.

To use AWD properly, we need to avoid: 1) having to reflood too soon after establishment of initial flood and preflood nitrogen (N) application; 2) drought stress at ½” internode elongation; and 3) drought stress at flowering and grain fill.

The first big risk is N loss — it’s not the loss of the initial flood that causes N loss, but rather the reflooding event. We want to maintain the initial flood for around three weeks and avoid having to reflood until closer to four weeks; and be re-flooded prior to ½” IE.  By four weeks, rice has fully taken up preflood N.

Then after ½” IE, we have a window of a few weeks for another drying event prior to flowering. Drought stress at ½” can impact grain and panicle formation. Drought stress from flowering through grain fill can impact pollination and complete grain fill.

So ultimately, there are two reasonably safe windows to accomplish drying events for flooded fields. The first between initial flood and ½” IE and the second between ½” IE and flowering.

Note that a “drying” event isn’t really trying to get the soil dry; it can still be tacky to muddy. If you use a Pani pipe to monitor water levels, we only want to see the water depth fall about two inches below the soil surface in the upper side of the paddy (bottom still flooded). Soil type and other factors affect this recommendation, but for general management and crop insurance purposes, a two-inch depth should be used.

Some type of tool, whether Pani pipe or soil moisture sensor, should be used to monitor changes in soil moisture in the field. Flying blind for these practices can make realizing their full benefit much more difficult.

Weed control in an AWD system should not generally differ from your regular program. If following these guidelines, soil will remain flooded to mostly saturated, which will minimize the chances for additional weeds to emerge. Overdrying the soil, however, could result in additional escapes.

When used properly, AWD maintains maximum yield potential while reducing water use and pumping costs (win-win). If attempting AWD on levee fields, it is best to use MIRI rather than to cascade flood. For help with implementing these practices, contact your local county Extension agent as well as irrigation and agronomy specialists to maximize your benefit and minimize risk.

Insect Management

Luis Espino
Dr. Luis Espino 
Rice Farming Systems Advisor
University of California Cooperative Extension

Arthropods have not been very problematic in California the past few years. However, do not let your guard down and let the bugs catch you unprepared. The tadpole shrimp is our key pest. Shrimp tend to be a problem in the same fields year after year because their eggs remain in the soil from one season to the next. Make sure to scout fields soon after seeding, maybe even before seeding, especially fields that take more than a few days to flood. Tadpole shrimp eggs hatch very soon after the field is flooded, and the young shrimp grow quickly. Seeding into a field with shrimp present, even small shrimp, is asking for trouble. Small shrimp (about half an inch or less) can injure rice, just not as much as larger shrimp. Also, do not just rely on the muddiness of the water. When they are small, shrimp do not disturb the soil much; therefore, you might not see muddy water even though shrimp are present.

Rice seed midge tubes in an affected field. Notice the tube attached to the damaged seed in the middle of the picture.

Another arthropod that can be problematic in California is the armyworm. Many fields had significant yield losses during the outbreak of 2015. We have not seen a similar outbreak since, but in some years, worm numbers have been high. I have been monitoring armyworm moth populations with pheromone traps across the rice area since 2018. Last year we had the highest moth numbers we have ever seen, but that did not translate into high worm numbers in the field. In fact, the worm pressure was very low. While the traps do not predict worm numbers, they can help us improve the timing of scouting. We know that we will see the highest worm numbers one to two weeks after the moth numbers peak. With Intrepid now fully registered for use in rice, we have a good tool that we can use to control armyworm populations if they get out of hand. You can sign up for my armyworm updates on the University of California rice website.

The last pest that can be of concern is rice seed midge. In the past three years, we have seen some fields suffer stand reduction due to midge. This is a difficult pest to manage given its sporadic nature. Ian Grettenberger, UC Davis Extension entomologist, has been doing insecticide trials for midge control. His work shows that pyrethroids are not very effective against midge, but at this point, it is the only tool we have. Like tadpole shrimp, rice seed midge can be worse in fields that take a long time to flood. Also, late-planted fields are at more at risk. Scout your field right at seeding and for the next few days, looking for the silken tubes rice seed midge form on the soil surface and inspecting seed looking for injury. If there is enough injury to reduce the stand significantly, use an insecticide. A brief drain will also work; however, take into consideration any effects the drain may have in weed control and fertility.

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