Bill J. Williams, Eric P. Webster and Ron Strahan

Advances in weed control technology have played an essential role in the development of the rice industry. Herbicides are necessary for obtaining optimum yield and maximum profit. Before the development of selective rice herbicides, weed control involved intensive hand labor. Even today, maintaining an adequate flood is important to managing weeds in rice. Combined with improved cultural and fertility practices and the development of high yielding varieties, selective herbicides have dramatically increased rice yields in the last 50 years.

The development of the phenoxy herbicides, propanil and Ordram represent three of the earliest advances in rice weed control technology. The phenoxy herbicides were developed in the 1940s. The most common one used in Louisiana has been 2,4-D. Propanil was introduced in 1961, and quickly became the commercial standard for rice weed control. The commercial package mixture of propanil plus Ordram (Arrosolo) also became popular because it controlled larger annual grasses and provided some residual activity. Since the registration of Ordram in 1967, several herbicides have been registered for use in rice and are currently recommended in Louisiana. They include Blazer, Londax, Basagran, Whip 360, Prowl, Facet, Bolero and Grandstand. These herbicides have greatly enhanced weed management by providing producers with effective options for controlling specific weed complexes unique to specific areas.

Even though herbicides are used on virtually all Louisiana rice acreage, weeds still cause considerable yield and grade losses resulting in significant revenue losses for producers. New technologies are not just needed to help manage current problems but to address new problems associated with herbicide resistance and “species shift,” which means when one weed is removed, another takes its place. Herbicide resistance is rapidly becoming a serious problem. Since the first documented case of triazine resistance in the early 1970s, more than 100 weeds have evolved resistance to virtually every class of herbicide including glyphosate. Because of increased dependence on Facet, problems with sprangletop are widespread and increasing. Perennial grasses (perennial barnyardgrass, paspalums and water bermuda) are also becoming increasingly problematic for many producers in southwest Louisiana. Red rice, the No. 1 weed problem facing Louisiana rice producers, is a close relative of commercial rice. They both belong to the same genus and species, which means they are similar genetically. Consequently, no herbicides are available that selectively control red rice in commercial rice.

Development of new, more effective and economical weed control technology has great potential for substantially reducing yield losses and production costs, thereby increasing profitability. Several new technologies are being developed for use in rice.

Herbicide-Tolerant Rice: Liberty Link and Clearfield

Herbicide-tolerant rice, a weed control technology currently under development, has the potential to revolutionize weed control and the way rice is grown in Louisiana. Liberty Link and Clearfield rice are the two herbicide-tolerant systems expected to reach the market in the next couple of years. The Liberty Link system is a transgenic technology with resistance to glufosinate (Liberty). Liberty is a broad-spectrum, nonselective, postemergence herbicide with no soil or residual activity. Clearfield rice is not a transgenic technology and is tolerant of a family of herbicides known as the imidazolinones. Scepter and Pursuit are two of the more familiar imidazolinone herbicides. Initially, NewPath will be the only imidazolinone herbicide registered for use in the Clearfield system.

The importance of herbicide-tolerant rice is that it will be the first herbicide technology that can be used to selectively control red rice in commercial rice. Research has shown that two applications of Liberty or NewPath, in their respective systems, will be required to control red rice. The rates for Liberty and NewPath have not yet been firmly established. However, the rate for each application of Liberty is expected to be between 0.375 to 0.5 pounds active ingredient per acre, while the rates for each NewPath application will most likely be between 0.063 to 0.094 pounds active ingredient per acre.

Single applications of NewPath or Liberty have not increased rice yields or controlled annual grasses in most research trials, especially on clay soils. Annual grass control with Liberty is similar to that of propanil, requiring sequential applications or tank mixes with other herbicides. Single applications of Liberty are as effective as single applications of Arrosolo or Facet on small (two- to three-leaf) barnyardgrass but not as effective on larger barnyardgrass (four- to five-leaf).

Liberty following previous applications or tank-mixed with Facet, Prowl or Bolero provides excellent control of annual grass and broadleaf weeds. NewPath plus Prowl or Facet applied three to six days after planting but before rice emergence has been an excellent program for controlling annual grasses. NewPath plus Prowl has been more consistent for sprangletop control than NewPath plus Facet. NewPath plus propanil applied to two- to three-leaf rice has also consistently controlled annual grasses and several broadleaf weeds season-long.

Combinations of Liberty or NewPath with conventional herbicides do not improve red rice control. When red rice is present, two applications of Liberty or NewPath are generally required. The recommended programs for controlling red rice in the Liberty Link and Clearfield systems are also expected to control most annual grasses. The Liberty Link system will also control most of the common broadleaf weeds. The Clearfield system, however, will need help controlling several broadleaf weeds, especially hemp sesbania. Both the Liberty Link and Clearfield systems do not control sedges effectively.

The gene for Liberty or NewPath tolerance can be transferred from commercial rice to red rice if they flower in close proximity. Therefore, it will be extremely important that producers make both applications and do not cut rates. If red rice escapes control and cross-pollinates with a Liberty Link or Clearfield variety, the technology will be short-lived. It will also be important for producers to rotate these technologies. Continued use, especially since multiple applications are required, of one technology will place a tremendous selection pressure on red rice and eventually lead to herbicide-resistant red rice.

Recently Registered Herbicides: Command, Permit and Aim

A Section 18 label, which is granted on a temporary basis, allowed use of Command in rice during the 1999 and 2000 growing seasons. A Section 3 label, which is full registration, for Command use in rice is expected soon. Though Command was widely used in 1999 and 2000 in Louisiana, it can be used only on dry-seeded rice, and applications are restricted to ground equipment. Command can cause some injury to rice plants in the form of bleaching or “whitening.” This is more likely to occur when Command is preplant incorporated but may also occur with preemergence applications and even delayed preemergence applications. However, yield loss from Command injury is rare, even in the most severe cases where moderate stand thinning is observed.

Most producers have been pleased with Command’s performance. However, some were disappointed because of the need for follow-up treatments after permanent flood. One problem with Command is that residual control begins to run out about the time permanent floods are being established (four to five weeks). As a result, many of the newly germinated and emerging annual grasses are overlooked, and the need for a follow-up treatment is not recognized. Annual grass control is usually optimum when Command is applied at 0.3 to 0.4 pound active ingredient per acre on silt loam soils and 0.5 to 0.6 pounds active ingredient per acre on clay soils. The higher rates of 0.4 on silt loam soils and 0.6 on clay soils are justified when permanent floods are delayed (more than four to five weeks after application). In many cases, Command can control annual grasses until permanent flood, if the higher rates are used and floods are established within four to five weeks after application. Excessive water (high rainfall or frequent flushing) reduces the residual control of Command. If sedges or broadleaf weeds are present, a program for controlling them will be required. Propanil plus Grandstand has been one of the most common and successful follow-up treatments.

Two herbicides registered for use in rice (Permit in 1999 and Aim in 2000) will help manage broadleaf weeds and sedges. Permit is slow-acting and usually takes two to three weeks to kill weeds. However, Permit stops weed growth within a few hours of application. Research indicates that Permit has the potential to injure rice when applied preemergence, especially on silt loam soils. Therefore, Permit applications should be limited to postemergence. Permit injury to rice from postemergence applications has been minimal. The method of rice seeding (water- or dry-seeded) is not restricted on the label. However, the best fit for Permit is in dry-seeded rice because it is limited to preflood applications.

The weed spectrum for Permit is being investigated. Though Permit controls both hemp sesbania and sedges, the control of hemp sesbania taller than 8 inches or after permanent flood has been inconsistent. However, even at low rates, Permit controls sedges. Permit rates as low as 0.012 pound active ingredient per acre have controlled moderate infestations of yellow nutsedge and purple nutsedge when applied with Londax or Grandstand.

Aim is a fast-acting contact herbicide and is labeled for both dry- and water-seeded systems. Rice has demonstrated excellent tolerance to Aim. Aim controls morning-glory, hemp sesbania and several other important broadleaf weeds in rice. Aim is a contact herbicide so coverage and timing are critical. Aim’s effect is reduced with weeds taller than 4 inches.

Future Herbicides: Regiment, Clincher, RiceStar and Aura

Regiment belongs to the sulfonylurea herbicide family, which includes Londax. Regiment is slow-acting and usually takes two to three weeks to kill weeds. However, Regiment stops weed growth within a few hours of application. Because of injury potential, Regiment application to rice before the three-leaf stage will not be recommended. Regiment is active on barnyardgrass and has killed plants as tall as 15 inches, making it a good option for managing barnyardgrass escapes. Regiment has also demonstrated good activity on rice flatsedge and hemp sesbania. The main benefit of Regiment to producers in southwest Louisiana is its ability to control perennial barnyard-grass, knotgrass and brook paspalum. However, proper weed identification will be important because Regiment does not control all of the perennial grasses found in southwest Louisiana. Another strength is its ability to control alligatorweed when tank-mixed with Aim. A serious weakness of Regiment is its inability to control sprangletop and broadleaf signal grass.

Clincher, RiceStar and Aura belong to a class of herbicides commonly referred to as the graminicides, which include Select, Fusion and Poast Plus. The graminicides control only grasses, and in the case of Clincher, RiceStar and Aura, only annual grasses. The main advantage of these herbicides over other rice herbicides is that they are safe to use around broadleaf crops. Aura has been less consistent than RiceStar or Clincher in controlling annual grasses with more than two leaves. Of the three gramini-cides, Clincher has been the most consistent at controlling annual grasses after permanent flood. Research indicates that Clincher and RiceStar can potentially be tank-mixed with Regiment to control sprangletop. Because the potential for antagonism exists, rate and adjuvant selection will be critical.

No ‘Silver Bullet’

None of the new technologies is a “silver bullet.” Nevertheless, the new technologies will give producers the tools needed to control several troublesome weeds. The herbicide-tolerant systems are going to be excellent tools for controlling red rice and other important rice weeds. However, multiple applications will be required and application timing will be critical. So, in that sense the programs for the new technologies will be similar to those for propanil. One important result of the herbicide-tolerant systems is that water-seeding and particularly muddy water practices will not be required to control red rice. Producers will be able to employ more agronomically and environmentally desirable seeding methods. The conventional herbicides provide producers with the tools needed to manage resistance, species shifts and troublesome weeds without having to use one of the herbicide-tolerant technologies. Red rice will be the only weed the herbicide-tolerant systems control that cannot be controlled with conventional herbicides.

(This article appeared in the winter 2001 issue of Louisiana Agriculture.)