Herbicide systems consisting of PRE, early POST, and late POST options arranged factorially were compared for control of sicklepod in narrow-row soybean with modified acetolactate synthase (ALS) (E.C.4.1.3.18). Other weeds present included common cocklebur and mixed infestations of entireleaf, ivyleaf, pitted, and tall morningglories. PRE options were alachlor or alachlor plus metribuzin plus chlorimuron. Early POST options included chlorimuron, chlorimuron plus thifensulfuron, and no herbicide applied 3 wk after planting. Late POST options were chlorimuron and no herbicide applied 5 wk after planting. POST herbicides were more effective than PRE herbicides on all weeds. Chlorimuron and chlorimuron plus thifensulfuron applied early POST were equally effective on these weeds and usually more effective than chlorimuron applied late POST. There was no advantage of two POST applications compared with a single early POST application. Greatest net returns were obtained in systems using only early POST herbicides. There was no economic advantage from using metribuzin plus chlorimuron PRE in systems that included an early POST herbicide.

Tomatoes (Lycopersicon esculentum ‘Roma VF’) and four weeds were grown in various combinations in field plots in 1973 and 1974. Season-long interference by jimsonweed (Datura stramonium L.), tall morningglory [Ipomoea purpurea (L.) Roth], and common cocklebur (Xanthium pensylvanicum Wallr.) at densities of 11, 43, and 86/m2 reduced tomato yields in 1973. In 1974, tomato yields were reduced by these three broadleaf weeds at densities of 2.7, 5.4, 8.1, and 11/m2. Season-long interference by large crabgrass [Digitaria sanguinalis (L.) Scop.] reduced tomato yield at densities of 55, 215, and 430/m2 in 1973 and 11, 22, 33, and 55/m2 in 1974. The fresh weight of tomato shoots decreased with all weed densities in both years. Total weed shoot weight increased with density and individual weed weights decreased with increasing densities. Tomato fruit quality, as measured by soluble solids, acidity, and color, was not influenced by the various weeds and densities.

Competition of weeds was characterized by determining the distance down the soybean row that a weed affects soybean biomass and yield. Field studies were conducted for 2 yr to compare competitive effects of common cocklebur, johnsongrass, Palmer amaranth, sicklepod, and tall morningglory on ‘Forrest’ and ‘Centennial’ soybeans. The weeds did not significantly reduce soybean biomass for 6 weeks after emergence. Palmer amaranth, common cocklebur, and tall morningglory had the greatest biomass by 6 weeks after emergence. However, only competition from common cocklebur and Palmer amaranth measurably reduced soybean biomass during the growing season. Biomass of Forrest and Centennial soybeans was reduced when these cultivars were growing within 12.5 and 50 cm of common cocklebur, respectively. Johnsongrass, sicklepod, and tall morningglory grew more slowly than the other weeds and had no measurable competitive effects on soybean biomass. Soybean competition reduced biomass of all weeds 90 to 97%. Soybean cultivar influenced the level and duration of competitiveness depending on the weed species present. Biomass of both soybean cultivars was reduced when they were growing within 50 cm of Palmer amaranth. Soybean seed yield was reduced when soybeans were growing within 25 cm of common cocklebur and Palmer amaranth and also when they were growing within 12.5 cm of tall morningglory. Sicklepod had no effect on soybean seed yield.

The effect of sequential hexane, ethyl acetate, and aqueous methanol extracts of ‘Regal’ sweetpotato periderm on seed germination of sweetpotato, proso millet, and seven weed species was studied. The hexane extract, which contained the nonpolar components of the periderm tissue, was least inhibitory. It inhibited velvetleaf, proso millet, black nightshade, and redroot pigweed germination, and maximum inhibition was 56% for black nightshade at 200 mg of periderm extracted ml–1. The ethyl acetate fraction was inhibitory to proso millet, velvetleaf, black nightshade, goosegrass, tall morningglory, coffee senna, and redroot pigweed. The estimated I50 for ethyl acetate ranged from 17 mg periderm extracted ml–1 for black nightshade to 201 mg ml–1 for coffee senna. Sweetpotato, tall morningglory, and eclipta germination was not inhibited by this extract at the concentrations tested. The aqueous methanol extract was much more inhibitory than the hexane or ethyl acetate extracts, and there was considerable variation between species in response to this extract The I50 estimates for the aqueous methanol extract were 0.5, 0.6, 2.8, 4.4, 5.1, 9.6, 15.7, 21.0, and 25.8 mg ml–1 for velvetleaf, proso millet, black nightshade, goosegrass, sweetpotato, tall morningglory, eclipta, coffee senna, and pigweed, respectively.

Thiazopyr at 0.14, 0.28, and 0.42 kg ai/ha and pendimethalin at 1.1 kg ai/ha applied preemergence (PRE) were compared as components in weed management systems for no-till cotton. Mid- and late-season control of mixtures of large crabgrass, goosegrass, and fall panicum by thiazopyr at 0.28 kg/ha was 89 to 97% and 11 to 50%, respectively, compared with 11 to 38% midseason and 0 to 5% late-season control by pendimethalin. Thiazopyr at 0.42 kg/ha and pendimethalin controlled broadleaf signalgrass 44 and 0%, respectively, late in the season. Adding fluometuron PRE at 1.7 kg ai/ha had little to no effect on large crabgrass, goosegrass, and fall panicum control but increased broadleaf signalgrass control 47 to 79 percentage points compared with thiazopyr or pendimethalin alone. Late-season control of annual grasses by thiazopyr or pendimethalin plus fluometuron PRE followed by methazole plus MSMA early postemergence (POST)-directed and cyanazine plus MSMA late POST-directed was at least 95% at two locations and 80% at the third location. Common lambsquarters was controlled 54 and 95% in systems without and with fluometuron PRE, respectively. Acceptable control of ivyleaf, pitted, and tall morningglories at all locations and smooth pigweed at two of three locations was achieved only in systems with POST-directed herbicides. Adding POST-directed herbicides to systems with thiazopyr or pendimethalin plus fluometuron PRE increased cotton yield at two of three locations. Treatments had no effect on fiber quality.

The effects of diluent volume and calcium concentration on glyphosate [N-(phosphonomethyl] glycine] phytotoxicity were evaluated by adding 0.0, 0.0025, 0.005, 0.01, 0.02, and 0.04 M CaCl2 to 1.68 kg/ha glyphosate and applying at 130, 190, 375, and 750 L/ha to tall morningglory [Ipomoea purpurea (L.) Roth] plants that were 10 to 12 cm tall. Thirty days after treatment, glyphosate phytotoxicity was reduced by calcium at diluent volumes of 375 and 750 L/ha. Addition of a sulfonine red dye to the spray solution showed that at 130 L/ha, runoff from tall morningglory leaves was negligible. At 375 and 750 L/ha different volumes, ½ to ¾ of the spray solution, as measured by dye retention, ran off the leaf surface. At 750 L/ha, the water and dye collected at the leaf margins and the spray pattern on the leaf surface was not uniform.

Broadleaf weed control with DPX-PE350 at 56, 84, 110, 140, and 170 g ae ha−1 applied postemergence was evaluated. DPX-PE350 was less effective and higher rates were required when applied to three- to six-leaf weeds compared to one- to two-leaf weeds. Listed in order of increasing susceptibility are sicklepod, tall morningglory, pitted morningglory = palmleaf morningglory, and entireleaf morningglory = hemp sesbania = velvetleaf.

The combination of glyphosate and 2,4-D at various rates was evaluated for controlling existing weeds at planting in no-till corn and soybeans. Herbicide combinations in soybeans also included paraquat plus 2,4-D, linuron, or diuron. Standard treatments included glyphosate (0.6 and 1.1 kg ae/ha) and paraquat (0.3 and 0.6 kg ai/ha), and 2,4-D (0.6 kg ae/ha) alone. For corn, the addition of 2,4-D to glyphosate did not improve weed control, although the addition of 2,4-D to paraquat did improve horseweed control. Corn yield with the herbicide combinations was higher than that for the nonselective herbicides alone. Although initial weed control was good in soybeans, weed regrowth in all paraquat alone treatments was substantial, especially with horseweed. The addition of 2,4-D to paraquat improved control of horseweed and tall morningglory. The addition of linuron or diuron to paraquat improved horseweed and common ragweed control, whereas the addition of 2,4-D to glyphosate improved the control of tall morningglory but not the other weed species. Generally, after 4 weeks, all glyphosate treatments provided better horseweed control than all paraquat treatments. Paraquat plus either linuron or diuron and glyphosate alone or in combination with 2,4-D gave the highest soybean yields.

Translocation of 14C-glyphosate [N-(phosphonomethyl) glycine] in tall morningglory [Ipomoea purpurea (L.) Roth., ‘Heavenly Blue’] was compared with movement of 14C-photosynthate. The flow of photosynthate was manipulated by creating assimilate sinks in mature leaves and cotyledons using combinations of stem girdling, leaf shading, and localized cytokinin application. Comparisons of 14C distribution patterns indicated minor differences between glyphosate and assimilate sink partitioning when 14C-glyphosate or 14CO2 were applied to a single leaf. Labeled glyphosate moved upward through stem girdles in greater proportions than 14C from photosynthate, suggesting a greater capacity for glyphosate to transfer from symplast to apoplast. When 14C-glyphosate was applied to the stem, 14C still moved symplastically to natural and artificial sinks, but apoplastic movement into all transpiring tissues above the site of application greatly increased. Results support the classification of glyphosate as an ambimobile rather than a phloem-mobile herbicide.

The initial metabolism of acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)acetamide] in tolerant and susceptible plants was examined to determine if metabolism is a factor in the selective phytotoxicity of this herbicide. Six crop and ten weed species were used. In every case acetochlor was converted initially to thioether conjugates. In thirteen of the species, initial conjugation was with glutathione (GSH). The homoglutathione (hGSH) conjugate was formed in three legumes. Tolerant plant seedlings metabolized acetochlor more readily than susceptible plant seedlings. Based on these results, acetochlor detoxification is a primary factor in the selective phytotoxicity of this herbicide.