Barnyardgrass Control In Mississippi Delta Crops
populations collected following glyphosate failures in Missis- sippi have all been controlled in green- house research.
K • Barnyardgrass control with
glyphosate may be reduced by im- proper application timing, herbicide mixtures, poor spray water quality (pH or hard water), new emergence follow- ing application, or dry/wet conditions. • Do not wait for barnyardgrass to
emerge to decide how to control it. Use all the tools available, i.e., effective burndown, multiple effective herbicide modes of action, and well-timed appli- cations. Barnyardgrass is ranked in the top five of most common and troublesome
DR. JASON BOND STONEVILLE, MISS.
ey Points To Re- member:
• Barnyardgrass The possibility that barnyardgrass in
Mississippi may have evolved resist- ance to glyphosate is a definite con- cern, but there are other circumstances that may influence the level of barn- yardgrass control with glyphosate. Among the causes of poor glyphosate performance on barnyardgrass are im- proper application timing, herbicide mixtures, poor spray water quality (pH or hard water), new emergence follow- ing application, or dry/wet conditions. Herbicide mixtures can have a large
influence on barnyardgrass control with glyphosate. Where we have col- lected barnyardgrass seed following control failures with glyphosate, the ap- plications that failed always contained one or two herbicide products in addi- tion to the glyphosate. In greenhouse research, we screen barnyardgrass re- sponse with treatments containing only glyphosate. So, we have observed con- trol failures with glyphosate-based her- bicide mixtures, but the surviving populations are controlled with
variable barn- yardgrass control be addressed? 1. Use maxi-
mum glyphosate rate. Our research shows that poor barnyardg ras s control
with
glyphosate-based herbicide mixtures can almost always be improved by in- creasing
the
glyphosate rate. La- beling for Roundup PowerMax single
allows postemer- effective
gence applications at 32 ounces per acre in cotton and 44 ounces per acre in soybean. 2. Utilize multiple herbicide
modes of action. The addition of clethodim 0.047 pounds active ingredient per acre (Select Max at 6 ounces per acre) has consistently increased barnyardgrass control with glyphosate-based herbi- cide mixtures in small-plot research and in commercial field situations. 3. Include herbicides with postemer-
gence and residual activity. Every spray application should contain multiple herbicide modes of action that exhibit postemergence and residual activity on barnyardgrass. Glyphosate plus S- metolachlor is a common mixture. S- metolachlor provides good to excellent residual control of barnyardgrass. 4. Time postemergence applications
appropriately. Regardless of the herbi- cide mixture, timing is critical. Well-
timed, early-season applications al- ways provide the most consistent con- trol. Barnyardgrass is often sprayed when it is too large, and this can be a recipe for failure. Barnyardgrass can- not be sprayed too early. 5. Prevent late-season escapes. If
barnyardgrass plants are allowed to grow and produce seed late in the sea- son or after harvest, those plants will contribute to weed soil seed bank. Those seed will contribute to problems in following years.
∆ DR. JASON BOND: Research/Exten-
sion Weed Scientist, Mississippi State University
weeds in Mississippi cotton, rice, and soybean. Barnyardgrass populations resistant to acetolactate synthase- (ALS)-inhibiting herbicides (herbicide Group 2) have been identified in most rice-producing counties in the Missis- sippi Delta. Most of these populations are also cross resistant to other ALS herbicides such as Regiment and Grasp. Every year there are numerous com-
plaints about barnyardgrass control with glyphosate. At the Southern Weed Science Society meeting in January, Larry Steckel reported that glyphosate- resistant barnyardgrass had been iden- tified
in west Tennessee.
Barnyardgrass populations collected following glyphosate failures in Missis- sippi have all been controlled in green- house research. Genetic analyses indicate that glyphosate-resistant barnyardgrass may be present in Mis- sissippi. This does not mean glyphosate-resistant barnyardgrass is present in Mississippi because (1) ge- netic research has not been concluded and (2) no plants from all suspected re- sistant populations have survived glyphosate in controlled research. How- ever, it is common to hear that glyphosate performance on barnyard- grass is often poor in the Mississippi Delta.
glyphosate alone. We have observed poor barnyard-
grass control in multiple small-plot studies in the last few years. In one study, barnyardgrass control was lower 14, 21, and 28 days after application (DAT) with mixtures of glyphosate at 0.77 pounds acid equivalent per acre (Roundup PowerMax at 22 ounces per acre) plus fomesafen (Reflex, Flexstar) compared with glyphosate alone. That reduction in control was overcome by increasing the glyphosate rate to 1.16 pounds per acre (Roundup PowerMax at 33 ounces per acre). Dicamba re- duced barnyardgrass control 7 and 14 DAT when added to both rates of glyphosate. We also observed variable barnyardgrass control in a study eval- uating the influence of foliar fertilizer on glyphosate-based herbicide mix- tures. In that research, the addition of a foliar fertilizer reduced barnyardgrass control 14 DAT when added to mix- tures of glyphosate plus S-metolachlor (Dual Magnum), fomesafen, or Cobra. In our research, barnyardgrass con-
trol is usually excellent with treatments that only contain glyphosate. However, glyphosate-only applications have no practical application where glyphosate- resistant Palmer amaranth is a prob- lem. So, how can the problem of
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