The influence of environmental factors on germination and emergence of horseweed was examined in growth chamber experiments. Germination was highest (61%) under 24/20 C day/night temperature under light. Horseweed seed germination was observed under both light (13 h photoperiod) and complete darkness (24 h), but germination under continuous darkness was only 0 to 15% compared with 0 to 61% under light. All other experiments were conducted under 24/20 C and 13-h light conditions. Germination was 19 to 36% over a pH range from 4 to 10, with a trend toward higher germination under neutral-to-alkaline conditions. Horseweed germination was > 20% at < 40 mM NaCl concentration and lowest (4%) at 160 mM NaCl. These data suggest that even at high soil salinity conditions, horseweed can germinate. Germination of horseweed decreased from 25% to 2% as osmotic potential increased from 0 (distilled water) to −0.8 MPa, indicating that germination can still occur under moderate water stress conditions. Horseweed seedling emergence was at its maximum on the soil surface, and no seedlings emerged from seeds placed at a depth of 0.5 cm or higher.

A tall waterhemp population from Missisippi was suspected to be resistant to glyphosate. Glyphosate dose response experiments resulted in GR50(dose required to reduce plant growth by 50%) values of 1.28 and 0.28 kg ae ha−1glyphosate for the glyphosate-resistant (GR) and -susceptible (GS) populations, respectively, indicating a five-fold resistance. The absorption pattern of14C-glyphosate between the GR and GS populations was similar up to 24 h after treatment (HAT). Thereafter, the susceptible population absorbed more glyphosate (55 and 49% of applied) compared to the resistant population (41 and 40% of applied) by 48 and 72 HAT, respectively. Treatment of a single leaf in individual plants with glyphosate at 0.84 kg ha−1, in the form of 10 1-µl droplets, provided greater control (85 vs. 29%) and shoot fresh weight reduction (73 vs. 34% of nontreated control) of the GS plants compared to the GR plants, possibly indicating a reduced movement of glyphosate in the GR plants. The amount of14C-glyphosate that translocated out of the treated leaves of GR plants (20% of absorbed at 24 HAT and 23% of absorbed at 48 HAT) was significantly lower than the GS plants (31% of absorbed at 24 HAT and 32% of absorbed at 48 HAT). A potential difference in shikimate accumulation between GR and GS populations at different concentrations of glyphosate was also studied in vitro. The IC50(glyphosate concentration required to cause shikimate accumulation at 50% of peak levels measured) values for the GR and GS populations were 480 and 140 µM of glyphosate, respectively, resulting in more shikimate accumulation in the GS than the GR population. Sequence analysis of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), the target site of glyphosate, from GR and GS plants identified a consistent single nucleotide polymorphism (T/C, thymine/cytosine) between GR/GS plants, resulting in a proline to serine amino acid substitution at position 106 in the GR population. The GR and GS plants contained equal genomic copy number ofEPSPS, which was positively correlated withEPSPSgene expression. Thus, glyphosate resistance in the tall waterhemp population from Mississippi is due to both altered target site and nontarget site mechanisms. This is the first report of an alteredEPSPS-based resistance in a dicot weed species that has evolved resistance to glyphosate.

Metabolomics and biochemical assays were employed to identify physiological perturbations induced by a commercial formulation of glyphosate in susceptible (S) and resistant (R) biotypes of Amaranthus palmeri. At 8 h after treatment (HAT), compared to the respective water-treated control, cellular metabolism of both biotypes were similarly perturbed by glyphosate, resulting in abundance of most metabolites including shikimic acid, amino acids, organic acids and sugars. However, by 80 HAT the metabolite pool of glyphosate-treated R-biotype was similar to that of the control S- and R-biotypes, indicating a potential physiological recovery. Furthermore, the glyphosate-treated R-biotype had lower reactive oxygen species (ROS) damage, higher ROS scavenging activity, and higher levels of potential antioxidant compounds derived from the phenylpropanoid pathway. Thus, metabolomics, in conjunction with biochemical assays, indicate that glyphosate-induced metabolic perturbations are not limited to the shikimate pathway, and the oxidant quenching efficiency could potentially complement the glyphosate resistance in this R-biotype.

Field studies were conducted in 2005 and 2006 to determine the most effective chemical options within three individual herbicide-based burndown programs, glyphosate, paraquat and glufosinate, for controlling glyphosate-resistant horseweed in Mississippi. Burndown treatments were applied April 5, 2005 and March 15, 2006 to horseweed plants 15 to 30 cm in height. Glyphosate at 0.86 kg ae/ha alone provided 60 to 65% horseweed control 4 wk after treatment (WAT). Control 4 WAT ranged from 73 to 74% when the glyphosate rate was increased to 1.25 kg/ha. Glyphosate at 0.86 kg/ha applied in combination with 2,4-D at 0.84 kg ae/ha or dicamba at 0.28 ae/ha maximized control of horseweed (≥ 90%) 4 WAT and soybean yield. Horseweed control 4 WAT with paraquat alone at 0.84 kg ai/ha ranged from 55 to 63% and control did not improve by increasing the rate to 0.98 kg/ha. Addition of 2,4-D or dicamba to paraquat maximized horseweed control both years (78 to 89%), whereas soybean yield was maximized with addition of dicamba or metribuzin at 0.42 kg ai/ha. Glufosinate applied alone at 0.47 kg ai/ha resulted in at least 88% control of horseweed and maximized soybean yield. Results indicate that effective management of glyphosate-resistant horseweed can be obtained in glyphosate-resistant soybean in glyphosate-, paraquat-, and glufosinate-based preplant weed control programs.