The development of Clearfield rice, which is tolerant to imidazolinone herbicides, is the most important advance in rice weed management in recent years. This technology allows rice producers to use certain imidazolinone herbicides to selectively control red rice in some commercial rice fields. The introduction of new herbicides for use on Clearfield rice or in rotation with Clearfield rice is needed to secure the long-term future of Clearfield technology.
Newpath (imazethapyr) was the first imidazolinone herbicide registered for use on Clearfield rice. While effective on most annual grasses, including red rice, Newpath has several limitations. Newpath does not control several broadleaf weeds, most notably hemp sesbania and jointvetch. Growers can only apply Newpath twice in one growing season, so it cannot be used to control weeds escaping initial control measures. Newpath is most effective on grasses that have not tillered. Higher Newpath rates and additional applications do not consistently control large grasses. Several herbicides used for weed control in conventional rice can be used in Clearfield to improve control of annual grasses and, if needed, to serve as rescue treatments. However, these conventional herbicides do not control red rice.
The gene conferring tolerance to imidazolinone herbicides can be transferred from Clearfield rice to red rice. Consequently, it is critical that any red rice escapes be controlled. Unfortunately, there are often too many red rice escapes to be removed manually. Additional herbicides with red rice activity are needed to control red rice escapes. Most red rice escapes are not detected until late in the season. As a result, an ideal herbicide would possess excellent postemergence activity with limited or no residual activity. Residual control is of little benefit in flooded fields and could potentially limit future crop rotation.
Since the introduction of Clearfield rice, Clearpath (imazethapyr plus quinclorac) and Beyond (imazamox) have been added to the arsenal of products used to control weeds specifically in Clearfield rice. Of these two, Beyond is the only product that represents new chemistry capable of controlling red rice. Clearpath contains Newpath, Facet
Clearpath is a commercial package mix from BASF containing Newpath and Facet. Research has not indicated any synergistic results by mixing Newpath with Facet, which means both herbicides control the weeds they normally would. Clearpath may be more economical than purchasing Newpath and Facet separately. Facet improves postemergence and residual activity on barnyardgrass and signalgrass. More importantly, Facet controls sesbania and jointvetches and improves eclipta control.
Growers buying Clearfield rice are required to make two Newpath applications when using Clearfield rice. The first application should be made to soil before rice emergence or at the 1-2 leaf stage, which is preferable. The second Newpath application needs to be applied before permanent flood is established and before grasses reach the 4-5 leaf stage (usually 10 to 14 days after the first application). Clearpath can be substituted for Newpath in either application. The best sesbania control is usually observed when Clearpath is used in the second application. However, if sesbania is taller than 2 inches, then Newpath tank mixes with other herbicides may be more effective. Beyond controls red rice escapes
Beyond (imazamox) is the second imidazolinone herbicide to be registered for use on Clearfield rice. Beyond appears to have excellent postemergence activity on red rice, barnyardgrass and Amazon sprangletop and is more effective at controlling larger grasses than Newpath. There has been some interest from growers and consultants in replacing one of the Newpath applications with Beyond. At this time, Beyond can only be applied following two applications of Newpath. While the excellent postemergence activity makes earlier Beyond applications attractive, it must be remembered that reducing the potential for outcrossing by removing red rice escapes is the primary purpose of Beyond. If Beyond were used in place of Newpath, there would be no effective program for controlling red rice escapes.
The label states that Beyond can be applied after the second Newpath application through 14 days past panicle initiation. Research suggests that timing is critical. If applied too early, coverage is an issue so applications need to be made after red rice is at least as tall as commercial rice. Even though applications can be made up to 14 days after panicle initiation in commercial rice, it is important that the application is made before the early boot stage of red rice. Beyond also controls Amazon sprangletop and barnyardgrass when applications are made before early boot stages. Research suggests that as with Newpath, increasing Beyond’s rate doesn’t ensure adequate control when applications are made after the optimum timing.
Until the introduction of Beyond, there was no effective way to manage red rice escapes. It is critical that every attempt be made to eliminate red rice from Clearfield rice to delay the development of red rice resistant to imidazolinone herbicides.
The best strategy for managing weeds in Clearfield rice is to make two timely Newpath applications followed by good water management. Under moderate to light red rice infestations, two timely Newpath applications followed by good water management will likely be all that is needed for red rice control. When annual grasses and red rice have escaped initial control attempts, Beyond can be used to remove these weeds from the field.
Bill Williams, Associate Professor, Northeast Research Station, St. Joseph, La.
(This article was published in the winter 2006 issue of Louisiana Agriculture.)