Photosystem II (PS II) inhibitors halt electron flow within the photosynthetic electron transport chain, thereby leading to increased oxidative stress. As a result, their addition to mesotrione, which inhibits carotenoid biosynthesis by inhibition of the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD), is complementary. Field and greenhouse experiments were conducted in 2002 and 2003 to investigate the joint action of POST mesotrione plus PS II inhibitor herbicide combinations. The joint action of mesotrione plus PS II inhibitors was investigated across five plant species, three PS II inhibitors, and two moisture environments to determine their influence on the joint action response. Rates of mesotrione evaluated ranged from 4.4 to 87.6 g ai/ha alone and in combination with reduced rates of atrazine, bromoxynil, and metribuzin. In the field, all combinations of mesotrione at 8.8, 17.5, and 35.0 g/ha plus atrazine, bromoxynil, or metribuzin were synergistic for necrosis 6 d after treatment (DAT) on sunflower. Addition of atrazine at 280 g/ha to mesotrione at 8.8 g/ha increased velvetleaf leaf necrosis by 18 to 47%. In the greenhouse, the addition of bromoxynil at 70 g/ha to mesotrione at 17.5 g/ha increased leaf necrosis by 23 to 34% and biomass reduction by 38 to 47%. Synergism on Palmer amaranth occurred similarly under both normal and dry moisture environments at application. Plant height at application was found to influence detection of synergism on the whole-plant level.

Transformation of soybean [Glycine max (L.) Merr.] with a gene encoding a glyphosate‐tolerance 5‐enolpyruvylshikimate‐3‐phosphate synthase enzyme from Agrobacterium sp. strain CP4 resulted in the development of glyphosate‐tolerant line 40‐3‐2. Glyphosate (N‐phosphonomethyl glycine) is the active ingredient of Roundup herbicide. Line 40‐3‐2 was yield tested at 17 locations in 1992,23 locations in 1993, and 18 locations in 1994. At those locations, broadcast applications of glyphosate at various rates were made over 40‐3‐2 or its derivatives from early vegetative growth to pod fill. No significant yield reduction was observed as a result of the glyphosate treatment at any of the locations. Development of glyphosate‐tolerant soybean promises to provide the farmer with access to a new weed control system that should result in lower production costs and reliable weed control under a wide range of conditions.

Herbicide‐resistant crops like glyphosate resistant (GR) soybean [Glycine max (L.) Merr.] are gaining acceptance in U.S. cropping systems. Comparisons from cultivar performance trials suggest a yield suppression may exist with GR soybean. Yield suppressions may result from either cultivar genetic differentials, the GR gene/gene insertion process, or glyphosate. Grain yield of GR is probably not affected by glyphosate. Yield suppression due to the GR gene or its insertion process (GR effect) has not been reported. We conducted a field experiment at four Nebraska locations in 2 yr to evaluate the GR effect on soybean yield. Five backcross‐derived pairs of GR and non‐GR soybean sister lines were compared along with three high‐yield, nonherbicide‐resistant cultivars and five other herbicide‐resistant cultivars. Glyphosate resistant sister lines yielded 5% (200 kg ha−1) less than the non‐GR sisters (GR effect). Seed weight of the non‐GR sisters was greater than that of the GR sisters (in 1999) and the non‐GR sister lines were 20 mm shorter than the GR sisters. Other variables monitored were similar between the two cultivar groups. The high‐yield, nonherbicide‐resistant cultivars included for comparison yielded 5% more than the non‐GR sisters and 10% more than the GR sisters.

Studies to predict pesticide fate often lack measurements of model input parameters. Using independent data sets and understanding how soil properties affect herbicide retention and degradation may result in more accurate prediction of herbicide fate. We conducted laboratory studies to determine the influence of soil properties on atrazine adsorption and degradation. These data will be used in a separate study involving a pesticide fate model. Atrazine adsorption and desorption isotherms were constructed for six soil depths of a Hastings silty clay loam (fine, montmorillonitic, mesic Udic Argiustoll) using batch equilibration. The Freundlich adsorption constants (logKf) ranged from 0.38 (60 to 90 cm) to 2.91 (0 to 30 cm). Adsorption was higher in the low pH, high organic matter-containing surface soil compared to the lower soil depths. Multiple regression of the adsorption constants against selected soil properties indicated that organic matter content was the best single predictor of atrazine adsorption (R2= 0.98) followed by soil pH (R2= 0.82). Combining organic matter and cation exchange capacity in the model produced the lowestCpstatistic (2.33) and highestR2value (0.99). We observed hysteresis in atrazine adsorption–desorption isotherms by higher adsorption slopes (1/n)adscompared to desorption slopes (1/n)des. Soils that adsorbed more atrazine also desorbed less atrazine. Desorption correlated negatively with organic matter content and positively with soil pH. Atrazine degradation after 84 d of incubation generally decreased with increasing depth. The first-order degradation rate was highest 0 to 30 cm deep (0.0187 day−1) and lowest 270 to 300 cm deep (0.0031 day−1). Atrazine degradation was faster in soil treated annually for 12 yr than in soil with no previous atrazine history (p = 0.01).

Triazine-resistant (TR) common waterhemp was reported and confirmed in Fillmore County, NE in 1990. A survey of 81 fields was conducted to characterize the occurrence of common waterhemp and grower practices. Sampled fields fell into three categories: suspected by growers to contain resistant plants (26 fields), randomly selected (28 fields), and adjacent to fields containing TR common waterhemp (27 fields). Resistant plants were found in 64% of all fields. Resistance was confirmed in 92% of the fields suspected by growers to contain resistant plants. Adjacent fields were no more likely to contain resistant plants than randomly chosen fields. Crop rotation did not significantly affect occurrence of resistance. Resistance was associated with the grower, indicating movement of resistant seed between fields via equipment. Atrazine, bromoxynil plus atrazine, and bentazon plus atrazine provided less than 75% postemergence control of TR common waterhemp, while primisulfuron, dicamba plus atrazine, primisulfuron plus dicamba, dicamba, 2,4-D ester, and metribuzin plus bentazon gave over 85% control.