Sunday, June 16, 2024

Specialist Speaking: Weed Control

ss-wayDr. M.O. “MO” Way
Rice Research Entomologist

Serious Weed Pests

Weeds typically are the most serious and consistent pests of rice production. Failure to control weeds in rice leads to significant and often devastating yield losses.

Last year, I observed firsthand the total loss of rice production in organic fields infested with hemp sesbania and rough jointvetch. These weed problems were exacerbated by damage to rice from fall armyworm, which delayed canopy closure and released the growth and rapid development of tremendous populations of these legume weeds forcing farmers to totally abandon harvest of large, infested portions of fields.

I could not believe the obvious huge populations of weed seed banks in the soil. For instance, the Penn State Extension Service estimates each barnyardgrass plant can produce over 7,000 seeds and a heavily infested field of barnyardgrass contains over 2,000 lb/acre of seed!

Probably the first thing you should do for your weed management program is make sure you have a clean seedbed before planting. This can be accomplished using burndown herbicides. Tillage also works, but can bury seeds on the top of the soil causing seeds to enter dormancy and become problematic in the future. Controlling weeds on your levees and margins surrounding fields can help reduce your weed seed bank. Then there is the old method of roguing problem weeds before they head. I know this is not an advanced technological method, but for spot areas, pulling weeds can be very effective.

Scouting fields is very important for effective weed control. You have to know your weeds and where they are located in your field. Knowing your weeds includes being able to identify them in the seedling as well as later stages of growth. The LSU AgCenter has a great tool/key for identifying seedling weeds of rice – “Schematic Diagram of Seedling Weeds of Rice.” Here is the link information to this helpful publication: for-seedling-weed-identification-in-rice.aspx.

If you can’t identify a particular weed, you can take a smartphone picture of the weed. Make sure you take the picture with the sun behind you and light illuminating the weed. Then email the pic to your research/Extension scientist. Or remove the weed (roots and all), place in a bucket or plastic bag with water and bring to your rice research/Extension center for ID. Don’t put the bucket in the back of your pick-up where wind and sun can dry out the plants. Try to select plants that possess reproductive structures that are often necessary for correct identification.

Dr. Suhas Vyavhare stands in front of a bad patch of hemp sesbania in an organic rice field. The farmer could not harvest the infested areas.
Dr. Suhas Vyavhare stands in front of a bad patch of hemp sesbania in an organic rice field. The farmer could not harvest the infested areas.

Another option for unknown young weeds is to carefully remove the weeds from the field, put them in a pot with rice field soil and grow them to a stage more easily identifiable. Or, you can bring the weeds to your rice research/Extension center, and a scientist can grow the weeds in a greenhouse to a stage more easily identified.

There are many effective herbicides to manage your weeds. I don’t have the expertise or space to provide detailed information on these options. But, I do know controlling weeds when they are young is critical to effective weed management. Again, scouting fields early in the season is absolutely crucial to selecting the proper herbicide(s), rates and timings. Soil-applied herbicides with residual activity require a flush or timely rain for “activation.” This means water is needed to get the herbicide into the soil where germinating weed seeds can be exposed to the herbicide.

For preflood applications of herbicides, many farmers who did not treat their seed with an insecticide add a pyrethroid to the herbicide to control rice water weevil. This has been an effective treatment and obviously saves money on application costs.

Finally, if you are planting a Clearfield variety, make sure you follow the stewardship requirements/guidelines regarding the use of this technology. Google search BASF-2015-CL-Rice-Stewardship- Guidelines.pdf to view the “2015 Clearfield Rice Stewardship Guide.”



ss-atwellSam Atwell
Agronomy Specialist

Picking a Winner

Most farmers understand they must farm all year round. Sure, they are working on different tasks each day, but like any sports team they must continue honing all of their skills even in the off season. Weed control is one of those tasks that they evaluate at harvest and begin planning for before the next season begins. For the most part, Missouri rice farmers do a good job controlling our ample supply of weeds. We had a few more spots with sprangletop and sedge escapes than in past years. An increase in flat, umbrella and yellow nutsedge may be on the increase because of less competition due to low seeding rates of rice that we are planting. Most growers want to start clean and stay clean.

For 2015, our rice weed control program has already passed from the planning stage to the action stage. Gentlemen, start your sprayer engines. Of course, there are many ways to attack weeds in your rice fields, but which should you choose, when and why? I love the word “choice” because your choices develop into consequences: good, not so good or bad. This is why I have such respect for American growers that blend together very complex choices to produce high-quality economic crops. First, growers must choose their rice production system and then build a weed control program that will work well in that system. The choice of system (zero-grade, water-seeded, precision-graded, drilled, furrow irrigated) depends on factors such as soil type, topography, grade, field size, Clearfield, variety, water supply, weed species and weed pressure. So, like a coach, growers go about the business of putting a group of weed control tools together to win and feel more comfortable when they start ahead and stay ahead.

They can choose either fall tillage with pre-plant residual herbicides, or a strong burndown, plus, or followed by, an effective pre-emergent herbicide. Either system, along with a healthy uniform crop, should take them into the early post stage well ahead of the weeds. Attack again with early post herbicides, followed by a mid-season post herbicide program that should eliminate new weeds. This, with an added residual along with a permeant flood, will allow them to coast weed-free into harvest. This can be accomplished with the Clearfield or conventional system. If the defensive weeds try to make a late-season run, the (coach) grower has many specialist herbicides on the bench to go in and take out a stubborn weed. Correct choices with good consequences can only happen with correct weed identification. Match the right herbicide with the right weeds to form a winning team.

There are many herbicide recommendations listed in the “University of Arkansas MP 44” from which Missouri growers can choose to win. The Rice Weed Control Team (choices and combinations) include tillage; burndown herbicides: 12; pre-emergence herbicides: 8; early post-emergence herbicides: 26; Mid-season herbicides: 18; Clearfield herbicides: 4 + 26; water and late post herbicides: 15.



Dr. Jason Bond
Weed Science Specialist

Burndown is a Must

Little-to-no tillage occurs in the spring in most Mississippi rice fields because of the heavy soil textures common in the rice-producing counties. Therefore, burndown herbicide applications are extremely important. As rice planting dates have moved earlier into the spring, growers and consultants should give attention to preplant intervals for burndown herbicides. Glyphosate plus 2,4-D is a popular, broad-spectrum burndown treatment for rice, but 2,4-D should not be applied within 28 days of planting. A spring burndown program for glyphosate-resistant Italian ryegrass should begin with clethodim (12 to 16 ounces of Select Max or 6 to 8 ounces of 2-lb clethodim formulation) applied not less than 30 days before planting. The only other reliable postemergence herbicide option for Italian ryegrass control is paraquat. Large ryegrass (12 to 24 inches) will require two applications of paraquat (4 pints of Gramoxone SL or 2.67 pints of 3-lb paraquat) spaced 10 to 14 days apart.

Barnyardgrass is resistant to five herbicide modes of action in six states across the rice-producing area of the United States. Barnyardgrass management requires a thorough, multi-faceted approach. A good barnyardgrass control program would begin with making certain that all barnyardgrass is controlled at planting. Start clean. This cannot be underrated as a control tactic. Use an at-planting application of a residual herbicide. Like Palmer amaranth in row crops, barnyardgrass is much easier to control before it comes out of the ground. Over-the-top programs should contain mixtures of postemergence and residual herbicides with multiple modes of action. Mixtures containing different modes of action are also important to reduce the selection pressure on the limited chemistries available in rice that are active against barnyardgrass.

Regardless of the herbicide mixture, timing is critical. Well-timed, early season applications always provide the most consistent control. Too often, barnyardgrass is sprayed in the three- to four-leaf stage (or tillering) rather than the one- to two-leaf stage. Barnyardgrass cannot be sprayed too early. Finally, strive for 100 percent control before flood. Do not wait for barnyardgrass to emerge to decide how to control it in rice. Use all the tools available, i.e., effective burndown, residual herbicides, well-timed postemergence applications, tank mixtures and the flood.

DustinHarrellDr. Justin Harrell
Extension Rice Specialist

Begin Weed-Free and Stay Weed-Free

There are many crop management decisions that are made every year that have a direct impact on yield at the end of the year. Weed control decisions have to rank up there on top, especially early in the season when rice seedling competition with weeds can dramatically reduce grain yields.

A good weed management program in rice begins with a weed-free seedbed. This can be done with tillage in our conventionally prepared seedbeds or with a good burndown application in our fall or spring stale-seedbed systems. Several herbicide choices or herbicide combinations are available for burndown applications. Many have a plant-back restriction like 2,4-D, Valor and Leadoff, while others, like glyphosate, do not. A complete list of common burndown herbicides and their plant-back restrictions was put together by Dr. Eric Webster and can be found in the “Rice Varieties and Management Tips” publication (

From this point, herbicide programs can begin to deviate due to seed technology used, the weed spectrum at a particular location and personal preference. I like to use a preemergence herbicide application at this time. Most preemergence herbicides require water for activation, so flushing is often needed after application. Don’t waste a good herbicide application because you fail to activate the herbicide. Over the next few weeks, use postemergence and other herbicides as needed, according to the emerging weed spectrum. It is important to stay weed-free going into permanent flood.

Most “new” herbicides over the past few years have been the premixed herbicides where two herbicides are packaged together to increase the weed spectrum and sold under a different trade name. These are great, but are not really “new” herbicides with new modes of action. However, this may change in a couple of years.

Dr. Webster has been testing an experimental herbicide with the active ingredient benzobicyclon. The mode of action of this herbicide is a photosynthesis inhibitor (HPPD inhibitor). The herbicide causes very little to no injury to rice. In Dr. Webster’s research with the compound, he found that it provided very good control of many aquatic and some perennial weeds. He also found that it had good activity on sprangletop, which will definitely give it a place in Louisiana rice production. He also noted that this herbicide actually had some activity on cattails. This in itself is exciting since we really do not have anything to help us control cattails, which tend to be a big problem in rice fields rotated with crawfish production. The one knock of the herbicide is that it is very slow before you see the herbicide activity. I am not sure what the trade name of this herbicide will be, but, hopefully, we will see it on the market in a couple of years.

Another herbicide that will be available in a few years is Provisia (quizalop-p-ethyl), which will be used in conjunction with the new BASF Provisia rice seed technology. The Provisia herbicide will be a postemergence grass herbicide, which will not have any residual control. The great thing about the Provisia technology is that it will be able to control Newpath-resistant outcrosses and weedy rice. Therefore, we can clean up those fields with the Provisia technology where we have basically lost the ability to use the Clearfield technology. The ability to rotate both the Clearfield and Provisia systems will prolong the viability of both technologies.

The future looks very exciting when it comes to weed control in rice! But for now, just remember, if you want to maximize your rice yield in 2015, it is important to begin weed-free and stay weed-free.


Dr. Luis Espino
Rice Farming Systems Advisor

Herbicide resistance

I would like to focus on how to detect and confirm the occurrence of herbicide resistance. Herbicide resistance is a serious problem in California rice. However, not every herbicide failure can be attributed to herbicide resistance. Other factors can be the cause of control failures. Among the most common are weather, incorrect rate, poor coverage or application timing, skips and spray equipment malfunction.

When weed control fails, it is important to determine the cause. And when the cause is herbicide resistance, herbicide programs need to be adjusted. Resistance occurs after the same herbicides have been used repeatedly at the same site for several years. You will notice a gradual decline in the efficacy of the herbicide to control weeds that were once susceptible. When herbicide resistance is the problem, you will find healthy plants alongside dead ones of the same species after treatment; surviving weeds form discrete patches that consistently survive the herbicide treatment.

The Rice Weeds program at the Rice Experiment Station (RES) in Biggs conducts herbicide resistance testing for the major rice herbicides used in California. Results of these tests help growers improve their weed control programs and also help the rice industry keep track of resistance issues. If you suspect herbicide resistance, collect seeds of the target weed and bring them to the RES to be tested.

Follow these guidelines:

Don’t wait until harvest to collect the seed. By then, most weeds have shattered their seeds. If you collect after harvest, you may collect seeds from weeds that have emerged late and thus have not been exposed to the herbicide. The objective is to collect seed from plants that have survived the herbicide action.

Collect seeds when they are mature and dislodge easily from the seedhead. In general, sprangletop matures the earliest, between rice panicle initiation and heading. Early watergrass, barnyardgrass, smallflower umbrellasedge and ricefield bulrush usually follow, maturing sometime before rice heading until maturity. Late watergrass matures last, at about the same time that early rice varieties (M-205, M-206) mature.

Collect seeds, not seedheads. Gently shake the seedhead inside a paper bag. Seeds that shatter are mature and will readily germinate. If seedheads are collected, seeds might not be mature or might have shattered already. Collect seeds from areas of the field where you are certain the herbicide application in question was appropriate. Avoid field borders, tractor tire tracks, skips or areas where you suspect the herbicide was not sprayed correctly or not sprayed at all.

Make sure to collect enough seed. In order to have conclusive results, several replications of herbicide resistance testing are needed. When not enough seed is provided, replications may not be possible. For small-sized seed weed species such as sprangletop, smallflower umbrellasedge or ricefield bulrush, collect seeds from at least 20 mature seedheads at each location. For barnyardgrass, early and late watergrass, collect from at least 30 mature seedheads.

ss-hardkeDr. Jarrod T. Hardke
Rice Extension Agronomist
University of Arkansas, Cooperative Extension Service

Always start at the beginning

There’s an old saying that goes “start at the beginning, and when you get to the end, stop.” I think that saying can be applied to rice weed control – only you need to start before you begin. A solid pre-emergence herbicide program is crucial to start the season on the right note. The past two years and their cool, wet conditions brought unprecedented herbicide activation and residual activity. The levels of control we observed in those seasons shouldn’t become a regular expectation.

Apply a pre-emergence herbicide at planting and fully prepare the field. If no rainfall is received soon after application, a flush may be needed to ensure proper activation. Flushing is an added expense, but the most expensive herbicide application is one that doesn’t work. If you don’t activate a herbicide you applied, then it won’t work for you. Even with proper application and activation, residual activity shouldn’t be expected for more than a few weeks. Early planted rice that endures cool conditions may not grow off quickly and fields could require an additional herbicide application well before we’re ready for flood establishment. If that happens, herbicide selection should focus on weeds present and utilize chemistry that will provide additional residual activity. This may enable us to get to flood without another herbicide application, but environmental conditions will largely dictate that situation.

A lack of good herbicide activity prior to flood establishment may lead to a flush of weeds as the flood is being established and a salvage application may be necessary. There are big differences of opinion as to what a “salvage” application is. A salvage application is one that cleans up weeds we missed controlling earlier, but they need to be relatively small – two to three inches. Larger weeds can still get away from us even with high rates of appropriate herbicides. Knee-high weeds aren’t salvage candidates – they’re misses. Attempts to control them are what are referred to as “revenge” applications. Spraying them may make you feel better, but it isn’t economical.

Spray early and activate all the way through. Proper selection of herbicides will be critical to consistent, efficient weed control throughout the season. Recipe for early season weed management success: use burndown and residual herbicides (activate!); flush, if needed; and use multiple herbicide modes of action.

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