Derek M. Scroggs and Paul R. Vidrine
With weeds being a major pest in production agriculture, most growers have come to rely on herbicides to prevent negative effects on yield. Producers may choose from many herbicides, depending on the crop they are growing and the weed species affecting the crop. Valor (flumioxazin) is a broad-spectrum herbicide that controls most broadleaf weeds while offering suppression of some annual grasses. Valor is in the N-phenyl-phtalimide herbicide family, and is a lethal inhibitor of the enzyme protoporphyrinogen oxidase (PPO), which is in the pigment synthesis pathway. This PPO inhibition starts a reaction in the cell that ultimately causes cell membrane disruption and results in plant death.
Valor has a unique use in cotton and soybean weed control. In cotton, Valor can be applied 30 days before planting or directed beneath the crop before the canopy closes. In soybeans, Valor can be used as a pre-plant burndown with no time restrictions, as a pre-emergence application or directed beneath the crop. Because Valor is less effective when controlling grass species, total weed control can be achieved by including the herbicide glyphosate in the spray mixture or in a sequential application. The use of Valor in Southern row crops has increased in recent years, and this addition has added an important weed control measure for many Louisiana growers. Valor as a Burndown Herbicide
Research was conducted in 2003 at the Dean Lee Research Station near Alexandria, La., using Valor as a pre-plant burndown treatment either alone or tank-mixed with glyphosate as Roundup Weathermax before planting cotton. Visual weed control assessments were made 32 days after treatment and revealed an increase in weed control from the addition of Valor to Roundup Weathermax. Treatments with Roundup Weathermax at 22 ounces per acre controlled pitted morningglory 67 percent, palmer amaranth 75 percent, smellmellon 67 percent, hemp sesbania 70 percent and hophornbeam copperleaf 70 percent (Figure 1). Control of these weeds increased approximately 21 percent to 29 percent, depending on the weed species evaluated, with the addition of Valor at 2.0 ounces per acre. Cotton was planted after this evaluation, and no injury was seen from treatments containing Valor. Valor Pre-emergence in Soybean
Studies conducted in 2003 focused on the use of Valor pre-application followed by a post-emergence treatment with Roundup Weathermax in comparison with sequential post-emergence applications of Roundup Weathermax in Roundup Ready soybean. Results indicated excellent control of pitted morningglory (96 percent), smellmellon (96 percent), hophornbeam copperleaf (96 percent), browntop millet (95 percent), barnyardgrass (95 percent) and spotted spurge (96 percent) with the Valor/Roundup Weathermax treatment when visual weed control was evaluated 51 days after treatment (Figure 2). With two applications of Roundup Weathermax control was less – pitted morningglory (77 percent), smellmellon (67 percent), hophornbeam copperleaf (83 percent), browntop millet (73 percent), barnyardgrass (25 percent) and spotted spurge (78 percent). In 2004, studies conducted with the same Valor/Roundup Weathermax treatment resulted in excellent control of pitted morningglory, smellmellon, hophornbeam copperleaf, spurred anoda, barnyardgrass and browntop millet, with control averaging approximately 96 percent 70 days after treatment. In both studies, no soybean injury was observed and optimum yield was achieved. Valor Layby in Cotton
Valor has recently been given a 2 (ee) label for application to 16-inch cotton and a minimum of 2 inches of bark visible on the lower stem of the plant. Also, with the currently available Roundup Ready cotton varieties, glyphosate must be directed beneath the crop after the fifth-true-leaf stage, and contact to stem or foliage must be avoided or yield loss may result. Research in 2003 evaluated the co-application of Valor and Roundup Weathermax at standard layby application timing. At 15 days after treatment, this co-application demonstrated adequate control of pitted morningglory (89 percent), prickly sida (93 percent) and hemp sesbania (96 percent). This was higher than control exhibited by Roundup Weathermax alone, which controlled pitted morningglory at 63 percent, prickly sida at 68 percent and hemp sesbania at 59 percent (Figure 3). An additional study in 2003 evaluated the combination of Valor and MSMA (monosodium acid methanearsonate) at the same application timing as above. At 16 days after treatment, results indicated 96 percent control of pitted morningglory; 98 percent control of palmer amaranth, hophornbeam copperleaf and smellmellon; and 96 percent control of browntop millet and barnyardgrass. A more-recent study conducted in 2004 also demonstrated similar results with combinations of Valor and Roundup Weathermax or MSMA as a layby treatment, further indicating the importance of Valor in cotton weed control systems. From these studies, no crop injury was observed and no negative effects on cotton yield were noticed.
When used according to herbicide labeling, Valor has been shown to be a beneficial tool growers can use to effectively control weeds. If used as a pre-plant burndown application, Valor can help achieve a clean seed bed at planting and can also minimize early-season weed competition. When used in a pre-emergence application in soybean, Valor can add a residual weed control component and be an effective initial treatment that can help delay or decrease post-emergence applications of glyphosate in Roundup Ready soybean. As a layby treatment in cotton, Valor can aid not only as a contact herbicide but also as a residual component in late-season weed control and can act as a good tank-mix partner with glyphosate in order to maintain a clean crop until harvest. Valor has demonstrated excellent versatility in multiple situations and if implemented in an appropriate manner can be effectively used to increase overall weed control in Louisiana crops.
(This article was published in the summer 2006 issue of Louisiana Agriculture.)