‘Wilmington’ and ‘Pensacola’ cultivars of bahiagrass (Paspalum notatum Flügge) were found to exhibit a differential level of tolerance to certain triazine herbicides. Pensacola was more susceptible to postemergence treatments of atrazine [2 - chloro - 4 - (ethylamino) - 6 - (isopropylamino) -s - triazine], simazine [2 - chloro - 4,6 - bis (ethylamino) - s-triazine], and cyanazine {2 - [ [4 - chloro - 6 - (ethylamino) -s - triazin - 2 - yl] amino] - 2 - methylpropionitrile} than Wilmington. Injury to Pensacola was noted as early as 2 weeks after the application of 2.4 kg/ha atrazine and cyanazine, whereas similar injury to Wilmington was not observed until later than 4 weeks after the treatment date. Results of 14C-atrazine treatments indicated that Wilmington bahiagrass accumulated approximately twice as much 14C-atrazine: peptide conjugate as the Pensacola cultivar.

Comparative growth, development, and resource allocation of triazine - susceptible and triazine - resistant biotypes of common groundsel (Senecio vulgaris L.), grown under two light regimes, were studied over a 7 - week period. Dry-matter production, height, number of leaves, and leaf area of the susceptible biotype were greater than those of the resistant biotype at all harvests under both light regimes. Root/shoot ratios were lower in the resistant than in the susceptible biotype under high light. Lower values for these parameters in resistant plants are due to lowered photosynthetic capacity, which limits growth and lowers relative root production. Net assimilation rate (NAR) was lower in resistant than susceptible plants when grown under high light, but mean leaf area ratio over a harvest interval (L) of resistant plants was higher than that of susceptible plants under high light. Relative growth rate (RGR) was higher for susceptible plants before day 42 and higher for resistant plants after day 42 under high light. A lower plastochron index (PI) of resistant plants up to 5 weeks old, relative to susceptible plants of the same age, suggests that, when grown under high light, resistant plants are developmentally at least 2 days behind susceptible plants. Shading lowered dry-weight production, height, number of leaves, leaf area, NAR, RGR, PI and root/shoot ratios of both biotypes. Values for these growth parameters for susceptible plants were similar to or larger than for resistant plants when grown under low light. In both biotypes, resource allocation patterns and L shifted towards increased shoot and leaf production in response to shading.

Field experiments were conducted in 2001, 2002, and 2003 to evaluate PRE applications of mesotrione at 150, 230, and 310 g ai/ha alone, and in mixtures with S-metolachlor at 1,070 g ai/ha and atrazine at 560 and 1,120 g ai/ha in corn. Corn injury was 11 to 18% with all treatments in 2002 when 3.2 cm of rainfall occurred within 10 d after PRE applications, but no injury was observed in 2001 and 2003 when rainfall was 0 and 1.1 cm within 10 d after PRE applications, respectively. Rainfall following PRE herbicide applications also influenced weed control, where weed control was generally poor with all herbicide treatments in 2001. Mesotrione at 150 g/ha controlled common lambsquarters and smooth pigweed at least 95% in 2002 and 2003, but control was 70% or less in 2001. PRE mesotrione at rates of 230 or 310 g/ha controlled common ragweed at least 83% in 2002 and 2003, but control exceeded 88% with mixtures of mesotrione at rates greater than 150 g/ha plus S-metolachlor plus atrazine at 560 g/ha. Morningglory species (ivyleaf morningglory, pitted morningglory, and tall morningglory) were not consistently controlled by mesotrione alone. In 2002 and 2003, mixtures of all mesotrione rates plus S-metolachlor plus atrazine at 1,120 g/ha controlled morningglory species at least 90%. Corn treated with mesotrione at any rate plus S-metolachlor plus atrazine at 1,120 g/ha consistently produced high yields. It is concluded that control with this three-way mixture would be most consistent with a minimum rate of mesotrione at 230 g/ha and atrazine at 1,120 g/ha.

Current plant bioassays included in the guidelines for testing pesticides do not include the measurement of reproduction endpoints. A bioassay, based on reduction of flowering of cress was developed to detect soil residues of hexazinone and simazine at levels of 0.02 and 0.10 ppm, respectively. The endpoint used in the described bioassay is the percentage of plant viability that implies that the tested plants have reached the flowering stage. It was found that sensitivity of cress is lower in soils containing higher organic matter.