Use of synthetic auxin herbicides has increased across the midwestern United States after adoption of synthetic auxin-resistant soybean traits, in addition to extensive use of these herbicides in corn. Off-target movement of synthetic auxin herbicides such as dicamba can lead to severe injury to sensitive plants nearby. Previous research has documented effects of glyphosate on spray-solution pH and volatility of several dicamba formulations, but our understanding of the relationships between glyphosate and dicamba formulations commonly used in corn and for 2,4-D remains limited. The objectives of this research were to (1) investigate the roles of synthetic auxin herbicide formulation, glyphosate, and spray additives on spray solution pH; (2) assess the impact of synthetic auxin herbicide rate on solution pH; and (3) assess the influence of glyphosate and application time of year on dicamba and 2,4-D volatility using soybean as bioindicators in low-tunnel field volatility experiments. Addition of glyphosate to a synthetic auxin herbicide decreased solution pH below 5.0 for four of the seven herbicides tested (range of initial pH of water source, 7.45–7.70). Solution pH of most treatments was lower at a higher application rate (4× the labeled POST rate) than the 1× rate. Among all treatment factors, inclusion of glyphosate was the most important affecting spray solution pH; however, the addition of glyphosate did not influence area under the injury over distance stairs (P = 0.366) in low-tunnel field volatility experiments. Greater soybean injury in field experiments was associated with high air temperatures (maximum, >29 C) and low wind speeds (mean, 0.3–1.5 m s−1) during the 48 h after treatment application. The two dicamba formulations (diglycolamine with VaporGrip® and sodium salts) resulted in similar levels of soybean injury for applications that occurred later in the growing season. Greater soybean injury was observed after dicamba than after 2,4-D treatments.

Effects of ammonium and methylammonium on foliar uptake of 14C-thifensulfuron, in the presence of 0.2% nonionic surfactant by vol, into velvetleaf were investigated. Total 14C uptake into velvetleaf 24 h after treatment was only 15% without addition of nitrogen, but addition of 0.32 M NH4Cl, urea ammonium nitrate, or CH3NH2·HCl increased total 14C uptake to 31, 38, or 59%, respectively. Total 14C uptake 24 h after treatment was increased 3- and 4-fold by addition of 0.08 and 0.16 M CH3NH2·HCl, respectively, but was not increased further by greater CH3NH2·HCl concentrations. Although the increase in 14C uptake was followed by an increase in total 14C translocated, the relative amount of 14C translocated (expressed as a percentage of total 14C uptake) was not affected by nitrogen additives. Enhancement of uptake of 14C-thifensulfuron by CH3NH2·HCl was partially reversed by addition of 0.16 M CaCl2 to the application solution.

Experiments were conducted in the greenhouse and field to investigate differential tolerance among tall, pitted, red, entireleaf, and ivyleaf morningglory species to DPX-PE350. Tall morningglory in the greenhouse was more tolerant of DPX-PE350 at 7 and 14 g ai ha-1 applied POST than were other species of morningglory. Increased tolerance of tall morningglory also was observed under field conditions with DPX-PE350 applied PRE and POST. Adjuvants did not increase biomass reduction of tall morningglory by DPX-PE350 applied POST.