Greenhouse studies were conducted to examine the influence of temperature and relative humidity (RH) on the foliar activity of mesotrione on five weed species. Regression analysis was performed allowing for comparison of estimated GR50 (herbicide dose to inhibit growth by 50%) values for each weed response at either temperature (18 or 32 C) or RH level (30 or 85%). Temperature and relative humidity did not influence the response of ivyleaf morningglory, common cocklebur, and velvetleaf to mesotrione markedly. An increase in temperature or relative humidity increased the efficacy of mesotrione on common cocklebur and velvetleaf up to threefold. Conversely, common waterhemp and large crabgrass were six- and sevenfold more susceptible at 18 C than at 32 C, respectively. Common waterhemp and large crabgrass were four- and twofold more susceptible to mesotrione at 85% compared with 30% RH, respectively. The influence of temperature and RH on the efficacy of mesotrione in foliar applications is species dependent and may be an important consideration for field applications.

Field experiments were conducted in 1999 and 2000 to determine the influence of mesotrione postemergence application rate, application timing, and addition of atrazine on corn injury, weed control, and corn grain yield. Corn injury in the form of leaf bleaching ranged from 0 to 15% at 7 d after treatment (DAT). In general, most of the bleaching injury rapidly dissipated with slight (≤ 8%) to no corn injury observed at 28 DAT. Control of common cocklebur with mesotrione at 14 DAT ranged from 79 to 98% for all treatments over both years. Applying mesotrione at 140 g/ha, at the early postemergence (EPOST) timing, or in combination with atrazine provided the greatest control of common cocklebur at 14 DAT. Application rate of mesotrione was the only factor that was significant in both years for control of common cocklebur later in the season at 56 DAT. Control of ivyleaf morningglory with mesostrione at 14 DAT ranged from 60 to 90% for all treatments in both years. Control of ivyleaf morningglory at 14 DAT was enhanced by the addition of atrazine to mesotrione. Control of ivyleaf morningglory at 56 DAT was greater with mid-postemergence and late postemergence than with EPOST applications, and was generally enhanced by the addition of atrazine. Yellow nutsedge control with mesotrione was inconsistent, ranging from 40 to 87% at 14 DAT for all treatments over both years. The addition of atrazine to mesotrione increased yellow nutsedge control from 47 to 87% at 14 DAT in 2000. Increasing the rate of mesotrione from 70 to 140 g/ha, as well as the addition of atrazine, improved control of yellow nutsedge at 56 DAT. Corn grain yield was not affected by corn injury or weed control as there were no significant differences in grain yield between herbicide-treated plots and handweeded plots.