Mesotrione is a new herbicide being developed for the selective pre- and post-emergence control of a wide range of broad-leaved and grass weeds in maize (Zea mays). It is a member of the benzoylcyclohexane-1,3-dione family of herbicides, which are chemically derived from a natural phytotoxin obtained from the Californian bottlebrush plant, Callistemon citrinus. The compound acts by competitive inhibition of the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD), a component of the biochemical pathway that converts tyrosine to plastoquinone and alpha-tocopherol. Mesotrione is an extremely potent inhibitor of HPPD from Arabidopsis thaliana, with a Ki value of c 6-18 pM. It is rapidly taken up by weed species following foliar application, and is distributed within the plants by both acropetal and basipetal movement. Maize is tolerant to mesotrione as a consequence of selective metabolism by the crop plant. Slower uptake of mesotrione, relative to susceptible weed species, may also contribute to its utility as a selective herbicide for use in maize.

The influence of temperature and relative humidity on the activity of acifluorfen, fomesafen, lactofen, and acifluorfen plus bentazon on prickly sida, pitted and entireleaf morningglory, and common cocklebur was evaluated in a growth chamber. Reduced control of all species was observed at 50% relative humidity as compared to 85% relative humidity when temperatures were higher (32/55 C day/night). Similar response to relative humidity was observed at the lower temperature (25/15 C) when treatments were applied 14 days after emergence (DAE). Changes in temperature at the same relative humidity did not alter herbicidal activity. Delaying application timing from 7 to 14 DAE decreased control by all herbicides except lactofen applied at high relative humidity, which controlled prickly sida at both 7 and 14 DAE.

Field studies were conducted in 1987 and 1988 to evaluate the vegetative response of soybeans to early postemergence applications of lactofen and to measure crop recovery. Soybean injury 4 d after treatment (DAT) averaged 29 and 34% with 0.22 and 0.44 kg ai ha−1of lactofen, respectively. Severity of leaf injury depended on leaf size at the time of lactofen application. Leaf area of soybean trifoliates one through three were reduced 30 to 45% at 12 DAT but recovered by 26 DAT. Later emerging trifoliates were not affected. Fresh weights of lactofen-treated soybeans were less than those of untreated plants 4, 8, and 16 DAT, and dry weights were less at 16 and 26 DAT. Lactofen reduced stomatal conductance of expanded leaves but had no effect on unexpanded leaves. Soybean yields did not differ with lactofen treatment.