Factors affecting the performance of three postemergence, grass- selective herbicides

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Virginia Polytechnic Institute and State University

A series of experiments was conducted to investigate the effects of adjuvants, spray volume, temperature, and combinations with broadleaf-selective herbicides on the toxicity of sethoxydim (2-[l-(ethoxyimino)-butyl]-5-[2-(ethylthio)-propyl]-3-hydroxy-cyclohexene-1-one), CGA-82725 (2-propynyl 2-(4-[3,5-dichloro-2-pyridinyl]oxy)phenoxy)-propanoate), and RO 13-8895 (acetone-.Q-[D-2-[p-[α,α,α-trifluoro-p-tolyl)oxy]phenoxy]propionyl]oxime) to annual grasses. The addition of an adjuvant to the spray solution resulted in significant increases in sethoxydim toxicity to large crabgrass (Digitaria sanguinalis (L.) Scop.), while only slight increases in toxicity of either CGA-82725 or RO 13-8895 were observed due to the addition of an adjuvant. There were no differences between crop oil concentrates and a nonionic wetting agent in their enhancement of phytotoxicity of the three herbicides. Spray volumes of 94, 187, and 374 L/ha did not affect the control of giant foxtail (Setaria faberi Herrm.) by CGA-82725 or RO 13-8895. At a low rate of sethoxydim, giant foxtail control was indirectly related to spray volume. This effect was eliminated at a higher rate of sethoxydim. The toxicity of sethoxydim or CGA-82725 was not affected by temperatures of 16, 24, or 32 C, but RO 13-8895 provided slightly better control of large crabgrass at 16 C than at 32 C.

Tank mixes of sethoxydim and CGA-82725 with acifluorfen (5-[2- chloro-4-(trifluoromethyl )phenoxy]-2-nitrobenzoic acid) provided control of large crabgrass at levels equivalent to those obtained by the grass-selective herbicides alone. Acifluorfen caused a slight reduction in the toxicity of 0.06 kg/ha RO 13-8895, but at higher rates of RO 13-8895 the antagonism was not observed. Bentazon (3-isopropyl-1H-2, l ,3-benzothiadiazin-4-(3H) -one 2,2-dioxide) was compatible with CGA-82725 and RO 13-8895, but significantly decreased large crabgrass control by sethoxydim. The antagonism was eliminated by applying the herbicides as individual treatments separated by 1.5 hr or more. Soybean (Glycine max (L.) Merr.) tolerance was not affected by any of the herbicide combinations which were evaluated.

Seven soybean cultivars were evaluated for differential tolerance to the grass-selective herbicides. All cultivars possessed good tolerance to rates three to ten times those recommended for field use.