Wednesday, June 12, 2024

Weed management in 2023: What can we expect?

Weed management in 2023: What can we expect?

Whitney Brim-Deforest
Cooperative Extension
Rice Advisor
University of California, Davis

Coming out of 2022, roughly half of all California rice fields were left fallowed last year. Others may have been fallowed for two seasons, and many of us have questions about what weed management will look like in 2023. While we do not have data on what a one- or two-year fallow does to all of our major weed species, we have some preliminary and anecdotal data that might lend some insight. 

For small-seeded weeds such as smallflower umbrella sedge, redstem and ricefield bulrush, the fallow period will likely have no effect. A good anecdotal example is from a field in Davis that was planted for a long time in rice, followed by nothing being planted for more than 10 years. Once the field went back into rice, ricefield bulrush was widely distributed throughout the field, and readily emerged. Ricefield bulrush seeds have a thick seed coat, making it easy for them to survive in the soil for a long period of time, and they have a high level of dormancy, increasing their persistence. 

Aggressive watergrass challenges California growers and researchers who seek effective control programs.

In an unpublished overwintering experiment with smallflower umbrella sedge to determine if decomposition occurred over the winter in flooded conditions, the smallflower seeds did not decompose. Seeds of smallflower umbrella sedge are also found in the 1,000s per square foot of soil, so once established, they are difficult to get rid of. 

For larger-seeded weeds including watergrass, a fallow may have an impact, but that impact will likely only be on those weed seeds near the surface of the soil. This is where they may decompose and are at increased chance of predation. 

In a preliminary study conducted last year, we found that in fields where tillage occurred in the spring post-fallow, watergrass emerged as seeds were brought up from deeper in the soil profile, where they would have remained dormant if tillage did not occur. In fields that were no-till in the spring, in at least one instance, the watergrass did not emerge at the same rate as in the tilled field. Similar results were seen with arrowhead, a perennial aquatic species. The effect of the tillage was to bring up the seeds from deeper in the soil profile, increasing emergence in the tilled area of the field. 

At this time, no anecdotal or preliminary data exists for sprangletop. However, due to the small seed size and high dormancy period, it is likely that sprangletop would not be greatly affected by a fallow period and would emerge as normal. 

Preliminary data from grower fields that were monitored over three years shows a reduction in weedy rice seed viability in the top 12 inches of soil. However, these fields have not been planted back into rice, so it is unknown whether the reduction of viable seed in the soil translated to reduced emergence. 

To summarize, it is unlikely the one- to two-year fallows will cause a great reduction in weed emergence, especially for small-seeded species. For larger-seeded species, there may be an impact, which can be increased by use of zero- or reduced tillage in the spring following a fallow period. 

Residuals on repeat

Jarrod Hardke, University of Arkansas
Professor/Rice Extension
University of Arkansas Cooperative
Extension Service

Spend money early to save money later — the end.

The truth is that our best money spent each season, and cheapest overall investment, is in our preemergence (PRE) herbicides. This turns out to be true every year. Our goal is to start clean with a good burndown at planting combined with a good PRE program to keep it clean.

Speaking of a good burndown — when in doubt, include a burndown with your at-planting PRE herbicide. Rarely will you regret having run some glyphosate in this situation, but you’ll often regret not doing it.

If we can accomplish field preparation prior to planting time, then ideally, we’ll be able to plant into a stale seedbed situation. This gives us the best chance for an optimum, uniform rice stand, and it improves our ability to minimize weed emergence and compete with the weeds that do emerge.

Dr. Jarrod Hardke says, “Our goal is to start clean with a good burndown at planting combined with a good PRE program to keep it clean.”

Command, Prowl and Bolero are our best friends in terms of keeping grass jailbreaks at bay. Facet is another one that may be a little less effective these days as a post-emergence option for grass but is still an excellent PRE option. While Prowl and Bolero need to be used no earlier than delayed PRE, getting some combination of all these products out early and overlapping them will minimize more costly post-emergence applications later.

Speaking of PREs, League is another good option to include early to work on some of our broadleaves and yellow nutsedge. Sharpen as a PRE is going to give us some annual sedge control along with pigweed and other broadleaves. If planting early in cool conditions well before these weeds typically emerge, it may have less benefit.

Rotation of weed control technologies (and crops!) continues to be an emphasis. While we now have Provisia / MaxAce to be able to rotate with our conventional rice and Clearfield / FullPage options, we have to properly rotate to steward these technologies. Including soybean in rotation will be increasingly critical.

New technologies and products are slow to arrive, if on the way at all. So don’t pin your hopes on something new coming along to fix problems.

Post-emergence applications are unavoidable, but focusing on overlapping PRE applications will keep things more cost-effective. Stay clean early and get to flood quickly to minimize weed issues and the herbicide budget in 2023.

Weed control strategies for 2023

justin chlapecka
Assistant Research Professor/
Rice Extension Specialist
University of Missouri

As meeting season is finally winding down, many of us are just a few weeks away from putting a drill into the field. As we move into Plant ‘23, a good portion of ground is ready to go after the dry fall season. While planting into a stale seedbed will be a great option in that scenario, we must think more about burndown applications in the absence of spring tillage. Some may have already applied a burndown, but starting clean is essential to having a clean rice field come August. 

My idea has always been to get a good burndown at planting, but you must be absolutely sure that what you apply is going to work, as we likely won’t get a second chance if the rice emerges very quickly. Clomazone is an essential preemergence application at planting for barnyardgrass and most other grasses that we will contend with during the season. We routinely see the best weed control where pre-emerge products are overlapped at a bare minimum of 21 days apart. In fact, somewhere around the two to two-and-a-half week time frame is ideal for overlapping residuals in a furrow-irrigated rice system. 

Dr. Justin Chlapecka says, “The best way to kill grass in a rice crop is to never see it above the soil surface. Once we see grass in the field, we are already behind the eight ball.”

The best way to kill grass in a rice crop is to never see it above the soil surface. Once we see grass in the field, we are already behind the eight ball. While there are generally less resistance issues in Missouri than further south, we do have our fair share of problems where continuous rice has been grown. 

If you suspect herbicide resistance is an issue, imazethapyr-tolerant rice is a great tool for barnyardgrass and weedy rice control. In some areas, however, resistance has developed to imazethapyr, but quizalofop-tolerant rice could be another tool in the toolbelt. Yield has lagged since this technology was released, but the newer cultivars have shown tremendous improvement and there are more in the pipeline. 

In related news, we will greatly miss having Jim Heiser as our weed scientist in the Bootheel. As we try to fill that gap, if you have any rice questions now or in-season, you can contact us via e-mail at As always, eat MO rice!

Early weed control

ronnie levy
Extension Rice Specialist
Louisiana State University

Start Clean! Rice weed control is best accomplished by using a combination of cultural, mechanical and chemical management practices. Relying on a single control practice seldom provides adequate weed management. 

The most important factor in herbicide use is the selection of a proper herbicide. Although weeds vary in their ability to compete with rice, most fields contain a complex of weeds that will reduce yield and quality if an appropriate weed management strategy is not implemented. Several herbicides are available for use as a burndown choice, and most options are applied based around glyphosate. Price is often factored in when selecting a burndown herbicide program, but in many cases, the cheapest option may not be the best for a given situation. Starting with a clean rice field helps to stay ahead of the problem. 

Pre-emergence herbicide applications are used in many drill-seeded rice fields. Immediately after rice is planted, a herbicide is applied to the soil surface. Within a 24- to 48-hour period after herbicide application, adequate rainfall (one inch or more) must occur, or the field must be flushed for herbicide activation. Many producers attempt to avoid flushing by waiting on rainfall to save money; however, to receive optimum benefit from the herbicide, it must be activated by moisture. Efficacy is reduced the longer a herbicide remains on the soil surface without activation. Poor weed control is a common side effect of waiting for rainfall because weeds continue to grow during the waiting period. 

A preemergence application can allow a rice crop to emerge and gain a competitive advantage on many weeds present in each field. Producers should consider using a preemergence application allowing for fewer post-emergence applications during the long growing season. If the history of weed pressure in a rice field is known, selecting which fields are most likely to benefit from a preemergence program can reduce weed control cost. Some herbicides cannot be applied until after rice seed has begun geminating. Always check the label.

Delayed preemergence herbicide applications are primarily, if not exclusively, used in a drill-seeded rice production system. The rice crop is planted, and four to seven days after planting, the herbicide is applied. This delay after planting allows the rice seed to begin the germination process, allowing the young seedling to get an initial growth advantage prior to herbicide application. Some herbicides cannot be applied until after rice seed has begun germinating. Always check the label.

Early, small weed control is the answer to clean fields and preventing the development of resistant weeds. Starting early will give your rice crop a competitive edge and hopefully prevent having to make a salvage application.

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