On the optimization of low dosage application systems : Improvement of dose advice and early detection of herbicidal effects

Application of herbicides at rates below the recommended label dose has received considerable attention in recent years as it is a means of reducing overall herbicide use. To minimize the risk of inadequate weed control in these situations, the Minimum Lethal Herbicide Dose (MLHD) technology, which was specifically developed for the use of low rates of photosynthesis inhibiting herbicides, relies on an appropriate calculation of the optimum herbicide dose and on an early detection method of herbicidal effects to evaluate the efficacy of the treatment shortly after application. The study described in this thesis was undertaken to further improve the prediction of the optimum dose of photosynthesis-inhibiting herbicides and to explore the opportunities of expanding the MLHD-technology toacetolactate synthase (ALS) inhibiting herbicides.

The first part of the study examined the influence of pre-spraying weather factors on the efficacy of photosynthesis-inhibiting herbicides. The results put forward that pre-spraying weather conditions have a considerable effect on herbicide efficacy and that this effect is mainly through an effect on herbicide uptake. Herbicide-specific correlations between uptake and individual weather parameters were found. These correlations were non-species specific and indicate that inclusion of pre-spraying weather conditions in advisory systems that recommend on the use of reduced herbicide dose rates is recommendable.

In the second part of the study theextension of the MLHD technology to acetolactate synthase (ALS) inhibiting herbicides, particularly the development of an early detection method of herbicidal efficacy, was explored.Application of the ALS inhibiting herbicides metsulfuron-methyl on Solanum nigrum and Polygonum persicaria plants resulted in a progressive reduction in CO ₂ fixation and the relative quantum efficiency of photosystem I (Φ _PSI ) and photosystem II (Φ _PSII ) electron transport, emerging a couple of days after herbicide application. Ф _PSII turned up to be one of the most promising and practical parameters to use when designing an early detection method for sensing the toxicity of metsulfuron-methyl. The loss of Ф _PSII was light-dependent and could be earliest detected at the base of the youngest leaf of treated plants.