Field experiments were conducted on the Eastern Shore of Virginia from 1999 to 2001 to evaluate the effects of tank mixture applications of isopropylamine or trimethylsulfonium salts of glyphosate with two liquid formulations of manganese (Mn lignin or Mn chelate) on spray solution pH and weed control in glyphosate-resistant soybean. Additions of manganese to herbicide solutions resulted in a reduction in the acidifying effects of the herbicides as well as in the control of common lambsquarters, large crabgrass, morningglory spp., and smooth pigweed. Reduced control caused by manganese could be overcome with higher rates of the herbicides on some species, but reduced control of common lambsquarters was seen when manganese was included with any herbicide application rate. For most species, Mn chelate caused a greater reduction in control than did Mn lignin. Although manganese caused significant decreases in weed control, soybean yield was not influenced by glyphosate salt, application rate, or manganese. Reduced weed control caused by the addition of manganese to herbicide solutions may be due to the complexing of the herbicide formulations, which could result in the formation of insoluble salt complexes that are not readily absorbed through the plant cuticle, resulting in decreased glyphosate phytotoxicity.

Studies were conducted in 2000 and 2001 to investigate responses of glyphosate-resistant cotton, glyphosate-resistant soybean, and selected weed species to postemergence applications of isopropylamine (Ipa) and diammonium (Dia) salts of glyphosate at selected rates ranging from 0.42 to 3.36 kg ae/ha. No differences were detected between either glyphosate salts or application timings for cotton injury, cotton lint yield, micronaire, fiber length, fiber strength, or fiber uniformity. In a weed-free soybean study, no differences in soybean injury occurred between early-postemergence treatments of the two glyphosate salts. Injury from late-postemergence treatments did not exceed 12% with glyphosate-Ipa or 9% with glyphosate-Dia at 3.36 kg/ha. Soybean treated with glyphosate-Ipa yielded 3,050 kg/ha, whereas soybean treated with glyphosate-Dia yielded 2,880 kg/ha, when averaged over glyphosate rate and application timing. In a soybean study that included weed control as a variable, weed control at 14 d after treatment (DAT), and soybean yield was independent of glyphosate salt. Control of common ragweed, ivyleaf morningglory, pitted morningglory, and large crabgrass at 28 DAT was similar at 0.84 kg/ha of either glyphosate salt.

Field studies were conducted from 1998 to 2000 to evaluate the effects of the trimethylsulfonium (Tms) salt of glyphosate on glyphosate-resistant soybean at Belleville, IL. Glyphosate-Tms and glyphosate-isopropylamine (Ipa) at 1,120, 1,680, 2,240, 3,360, and 4,480 g ai/ha were applied at the V4 and R1 growth stages of glyphosate-resistant soybean. Glyphosate-Tms and glyphosate-Ipa caused greater chlorosis when applied at the R1 growth stage when compared with the V4 growth stage, and chlorosis increased with rate. Chlorosis ranged from 0 to 20% depending on the year. In 1998, glyphosate-Ipa at 2,240 and 3,360 g/ha applied at the R1 growth stage caused 4 to 5% more chlorosis than glyphosate-Tms at the same rates. In addition to chlorosis, glyphosate-Tms caused bleaching (white speckling) of soybean leaves, with bleaching increasing as glyphosate-Tms rate increased. Glyphosate-Ipa caused no bleaching, regardless of rate. Glyphosate-Tms and -Ipa caused no visible height reduction at 14 and 28 d after treatment in any year. In 1998 and 1999, glyphosate-Tms and -Ipa, at the highest rate applied at the R1 growth stage, increased days to maturity of soybean. Despite the injury and delay in maturity caused by glyphosate-Tms and -Ipa, there was no difference in grain yield across years because of glyphosate salt, rate, or application timing.

Field trials were conducted in Virginia to evaluate trumpetcreeper control with glyphosate and conventional herbicide systems in double-crop glyphosate-resistant soybean. When compared with the untreated control, none of the conventional herbicide systems evaluated in these trials provided any reductions in the trumpetcreeper stem density at 1 yr after treatment (YAT). Glyphosate systems generally provided much higher levels of trumpetcreeper stem reduction than conventional herbicide systems. By 1 YAT, densities of trumpetcreeper that received conventional herbicide treatments ranged from 97 to 141% of the initial population, whereas densities of trumpetcreeper that received preemergence (PRE) followed by postemergence (POST) or POST-only applications of the isopropylamine (IPA) or diammonium (DIA) salts of glyphosate ranged from 30 to 66% of the initial population. However, PRE applications of the IPA or DIA salts of glyphosate generally provided lower levels of trumpetcreeper stem reduction than PRE followed by POST or POST-only applications. These results indicate that glyphosate applications made later in the growing season will provide greater trumpetcreeper stem reductions than those made at or close to soybean planting in double-crop soybean production systems. Glyphosate systems did not provide higher soybean yields than the untreated control that contained trumpetcreeper only when densities averaged 6 trumpetcreeper stems/m2 in 2000, but 12 of the 16 glyphosate-containing treatments provided higher soybean yields than the untreated control that contained trumpetcreeper only when densities averaged 13 stems/m2 in 2001.

Three field studies were conducted during 1998 to 2002 at Stoneville, MS, to examine the efficacy of glufosinate and glyphosate on redvine and trumpetcreeper control in glufosinate- and glyphosate-resistant soybean. Glyphosate at 2.52 kg ae/ha applied approximately 3 wk before planting soybean reduced trumpetcreeper density (45 to 52%) but not redvine compared with no glyphosate in both glufosinate- and glyphosate-resistant soybean. However, glyphosate applied preplant reduced biomass of both species in glufosinate-resistant soybean. Glyphosate early postemergence (EPOST) followed by (fb) late postemergence (LPOST) had no effect on redvine density but reduced trumpetcreeper density (70%) compared with the no-herbicide control. There were no differences in densities and biomass of redvine and trumpetcreeper and soybean yield among isopropylamine, diammonium, and aminomethanamide dihydrogen tetraoxosulfate salts of glyphosate. Overall, trumpetcreeper is more susceptible to glyphosate than redvine. Glufosinate EPOST with or without acifluorfen or glufosinate EPOST fb LPOST had no effect on densities of redvine and trumpetcreeper but reduced biomass 45 to 76% and 35 to 58%, respectively, compared with the nontreated control. These results show that glyphosate preplant and POST in-crop applications can reduce trumpetcreeper density but not redvine, and glufosinate POST applications can suppress growth of both species.

Field studies were conducted over 2 yr to evaluate weed control, yield, and net returns of glyphosate-resistant soybean using total postemergence (5 wk) (POST) herbicide systems with glyphosate–isopropylamine (Ipa) or glyphosate–trimethylsulfonium (Tms) alone, tank mixed with fomesafen, or in sequential treatments with bentazon, fomesafen, Ipa, or Tms. Soybean early-season injury ranged from 0 to 28% across the test. Although Ipa did not injure soybean, glyphosate–Tms early postemergence (3 wk) (EPOST) injured soybean from 7 to 17% depending on the rate. Glyphosate–Tms mixed with fomesafen EPOST injured soybean from 20 to 28%. Red morningglory control by Ipa and Tms at 0.8 kg ae/ha was no more than 88%. Sequential applications of Tms or Ipa controlled red morningglory 78% or less. Fomesafen improved red morningglory control by Ipa and Tms. Bentazon did not affect the control of red morningglory by these herbicides. Sicklepod, smooth pigweed, and large crabgrass control was 81, 93, and 79%, respectively, or greater for all herbicide treatments. By midseason, narrow-row soybeans had canopied, and competition from weeds was minimal. Overall, the net returns were reflective of soybean yield, and maximum net returns were recorded for treatments with reduced herbicide inputs. Conversely, sequential application of herbicides as EPOST followed by POST treatments resulted in lower net returns because of increased herbicide and application costs.

Activator adjuvants for herbicides increase herbicide activity and encompass a wide variety of surfactant chemistry. Activator adjuvants may be included in the product formulation by the manufacturer or tank-mixed by the herbicide applicator. Activator adjuvant efficacy is a function of not only the adjuvant but also the herbicide, the particular weed species, and environmental conditions. Proposed modes of activator action include reduction of spray solution surface tension to enhance contact area, solibilization of the leaf cuticle, emulsifier action, increased spray retention, protection of the herbicide in the spray solution, promotion of rainfastness, acting as a cosolvent or copenetrant, modification of spray deposition on plant foliage, and enhanced movement on the foliage surface to areas of greater absorption.

Epicuticular wax is a major barrier to the movement of foliar-applied herbicides into plant leaves. Many researchers have evaluated the penetration of herbicides in plants using similar methods. However, the methods used to remove unabsorbed herbicide from the leaf surface have been inconsistent among scientists. Laboratory experiments were conducted to evaluate the effectiveness of various compounds as a leaf wash for removing unabsorbed 14C-labeled herbicides from shattercane and velvetleaf leaves. Product B and Product C provided poor results as a leaf wash compared with water, chloroform, methanol, and Product A. Product A increased the amount of 14C-bentazon removed from velvetleaf by 26% compared with the amount removed by chloroform, methanol, or water. Removal of 14C-nicosulfuron was not different between chloroform, Product A, methanol, or water. Although water was the best treatment for the two salts of 14C-glyphosate, Product A was as good or better than chloroform and methanol.