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

Advances in weed control technology have played an essential role in the development of the rice industry. Herbicides are critical to obtaining optimum yield and maximum profit. Before the development of selective rice herbicides, weed control involved intensive manual labor. As is the case today, maintaining an adequate flood was important to managing weeds in rice.

Red rice, a close relative of commercial rice, is the No. 1 weed that limits rice production in Louisiana. Since red rice and commercial rice share many of the same physiological properties, herbicides that control red rice can severely injure commercial rice. As a result, the most effective herbicides for controlling red rice have been developed mainly in broadleaf crops such as soybeans and cotton.

Until recently, herbicides that selectively control red rice in commercial rice have not been available. The development of genetic engineering techniques has given plant breeders the tools they need to incorporate into crops genes that convey tolerance to herbicides that normally injure crops severely. Crops developed using genetic engineering techniques are commonly referred to as herbicide-tolerant crops.

Three herbicide-tolerant systems are being developed in rice: Clearfield, Liberty Link and, to a lesser extent, Roundup Ready rice. The Liberty Link system is a transgenic technology and conveys resistance to glufosinate (Liberty). Liberty is a broad spectrum, non-selective, postemergence herbicide with no soil or residual activity. Clearfield rice is not a transgenic technology because it was developed through mutation and not gene transfer. It is tolerant of a family of herbicides known as the imidazolinones. Scepter and Pursuit are two of the most familiar imidazolinone herbicides. Roundup Ready rice is a transgenic technology that conveys resistance to glyphosate (Roundup). Monsanto, the company that owns Roundup Ready technology, is not focusing on the development of Roundup Ready rice.

Clearfield rice was registered in 2001 and fully released by BASF in 2002. Clearfield rice represents the first available herbicide technology that selectively removes red rice from commercial rice. Between 5 percent and 10 percent of Louisiana’s rice acreage was planted in Clearfield varieties in 2002. While Clearfield rice will be an excellent tool for managing red rice, it should not be viewed as a stand-alone herbicide program in most field situations.

Clearfield Research Review
Red Rice Control. So far, Newpath (imazethapyr) is the only imidazolinone herbicide registered for use in the Clearfield system. Single applications of Newpath have not maximized weed control or rice yield in most research trials. Trials that evaluated Newpath activity on red rice and other annual grasses have clearly demonstrated that two applications are required for acceptable weed control. The most consistent weed control has been observed when at least 4 ounces per acre of Newpath was applied to soil and followed by at least 4 ounces per acre of Newpath applied at the 3- to 5-leaf rice stage. All postemergence applications of Newpath should be applied with a surfactant. Producers should follow label recommendations when tank mixing with other herbicides. Note that the second Newpath application should be made at or before red rice reaches the 3- to 4-leaf stage. The soil application can be made before planting or immediately after planting.

For improved weed control, soil applications made before planting may be incorporated. If weeds are present at planting, the first Newpath application should be tank mixed with glyphosate or paraquat and applied before rice emergence. When Newpath is applied at the proper timing and adequate soil moisture is maintained by periodic flushing, the herbicide provides excellent red rice control.

Newpath has demonstrated good annual grass activity when applied alone, but more consistent barnyardgrass control has been observed with tank mixtures of Newpath plus Prowl or Facet applied before rice and weeds emerge. Newpath plus Prowl or Command has been the most consistent combination for controlling sprangletop. Newpath combinations with Facet, propanil and Arrosolo also improve the control of emerged grasses. These recommended programs should provide excellent control of annual grasses.

Newpath does not control several important broadleaf weeds such as hemp sesbania and jointvetch. Acceptable broadleaf weed control has been achieved by tank mixing Newpath with several postemergence herbicides such as Arrosolo, Stam, Facet, Basagran, Permit, Blazer, Storm or Grandstand. Tank mixing Newpath with recommended herbicides for improved weed control does not appear to affect red rice control negatively.

Water Management. Water management is critical to the success of Clearfield rice technology. After the soil application, a flush or rainfall is necessary within 48 hours to activate the herbicide. For maximum benefit from the soil application, good moisture should be maintained, and soils should not be allowed to become dry. Timely flushing will be a key component to successful control of red rice and other weeds in Clearfield rice. Permanent floods need to be established as soon as possible after the postemergence application. Research has shown that allowing soil to become excessively dry or delaying floods drastically reduces the success of Newpath programs.

Clearfield rice is labeled for dry-seeded production systems. Research indicates that Newpath can be equally effective in both broadcast and drill-seeded rice. Rice tolerance to Newpath applications in broadcast and drill-seeded rice has been evaluated extensively. Currently available varieties, CL-121 and CL-141, are sensitive to postemergence Newpath applications when applied before the 3-leaf stage of development. Research indicates that new CFX varieties available commercially in 2003 will be much more tolerant to Newpath than CL-121 or CL-141.

Outcrossing Issue. Because of similar biological and physiological properties common to red rice and Clearfield rice, there is a potential for outcrossing. If red rice is allowed to flower at the same time or in close proximity to flowering Clearfield varieties, the gene for Newpath tolerance can be transferred to red rice. Outcrossing seriously compromises the longevity of the Clearfield system. Therefore, it is extremely important that producers take the necessary steps to prevent outcrossing. It is essential that producers do not reduce Newpath rates and make a soil and postemergence application to prevent red rice escapes. It is equally important for producers to rotate Clearfield rice with other crops and use herbicides with alternative modes of action, such as Roundup Ready soybeans.

Soybeans are tolerant of Newpath and may be grown anytime after a Newpath application. Rotation to other non-Clearfield crops may result in severe injury if the plant-back intervals listed on the label are not followed. For example, corn (non-Clearfield), cotton and rice (non-Clearfield) should not be planted within 8.5, 18 and 18 months of Newpath applications, respectively. Continued use of Clearfield rice without rotation, however, will place tremendous selection pressure on red rice and certainly lead to a Clearfield red rice.

Clearfield Rice Success
Clearfield rice is not a “silver bullet.” Clearfield systems do give producers the opportunity to selectively control red rice in commercial rice. Two applications of Newpath at 4 ounces per acre are required to control weeds in the Clearfield system. The first Newpath application should be made at or near planting followed by a second application before permanent flood. The closer the second application is made to permanent flood, the better the control will be; however, when weeds escape the first application, the second Newpath application needs to be made before the 3- to 4-leaf stage of red rice and other annual grasses.

One important result of the Clearfield system is that water-seeding and muddy water practices will not be required to control red rice. This means less use of water and less release of sediment-filled water into the environment. Producers will now be able to use more agronomically and environmentally desirable seeding methods.
 
Bill J. Williams, Associate Professor, Northeast Research Station, LSU AgCenter, St. Joseph, La.; Ron Strahan, Extension Associate, LSU AgCenter, Baton Rouge, La.; and Eric P. Webster, Associate Professor, Department of Agronomy, LSU AgCenter, Baton Rouge, La.

(This article was published in the summer 2002 issue of Louisiana Agriculture.)