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

Vroege bepaling van de effectiviteit van ALS-remmende herbiciden

MLHD, de methode om lage herbicide doseringen op maat en met controle op effectiviteit toe te passen is specifiek ontwikkeld voor fotosynthese-remmers. Telers ervaren de beperkte toepasbaarheid van de methodiek soms als bezwaar. Via onderzoek worden de mogelijkheden om MLHD te verbreden naar middelen met andere werkingsmechanismen verkend. In dit artikel wordt een overzicht gegeven van dergelijk onderzoek gericht op ALS(AcetoLactaatsynthase)-remmende herbicide

Kritisch doseren van herbiciden volgens MLHD

De minimum effectieve dosering van een herbicide wordt bepaald door een complex van factoren zoals onkruidsoorten, onkruidgrootte, gewasstadium, weersomstandigheden, spuittechniek, formulering en hulpstoffen. In dit artikel wordt nader ingegaan op het Minimum Letale Herbicide Docering (MLHD) concept. MLHD is de laatste jaren ontwikkeld met als doel (vooral via sensingtechnieken) herbicidengebruik rationeler en duurzamer te make

Are pre-spraying growing conditions a major determinant of herbicide efficacy?

To evaluate the effect of pre-spraying growing conditions on herbicide efficacy, two years of experimentation were conducted in which Persicaria maculosa plants were exposed to different light intensities for 1¿4 days before metribuzin treatment. Specific leaf area, rather than plant growth rate or plant size, was the only parameter that correlated well with herbicide efficacy in both years of experimentation. The negative relationship between the ED50 and the specific leaf area indicates that leaf characteristics might be an important determinant of herbicide efficacy, for instance through an effect on herbicide uptake. In the third year of experimentation this hypothesis was further investigated by raising six cohorts of weed plants at a 1-week interval and thus exposing them to a range of weather conditions. Clear relationships between uptake and herbicide efficacy were found for a combination of four plant species (Solanum nigrum, Senecio vulgaris, Chenopodium album and Brassica napus) and two herbicides (phenmedipham and bentazone). For phenmedipham, uptake was negatively correlated with global radiation and positively correlated with relative humidity. For the herbicide bentazone the opposite was found. These results were not species-specific. This study shows the importance of the sensitivity of herbicide × species combinations and indicates that pre-spraying weather information is relevant for the development of reduced dose rate recommendations

Can photosynthesis-related parameters be used to establish the activity of acetolactate synthase-inhibiting herbicides on weeds?

The application of the acetolactate synthase (ALS)¿inhibiting herbicide metsulfuron on greenhouse- and field-grown black nightshade and greenhouse-grown ladysthumb resulted in progressive inhibition of the level of carbon dioxide (CO2) fixation, the relative quantum efficiency of electron transport through photosystem I (PSI) and II (PSII), and the leaf chlorophyll content. Photosynthetic-related measurements, measured 2 to 4 d after treatment (DAT) at photon flux densities of 400 to 500 ¿mol m¿2 s¿1, provided valuable information before the visual symptoms that first appeared at 7 to 10 DAT with the herbicide. Measurements of the quantum efficiency for electron transport by photosystem II and the loss in leaf chlorophyll content appeared to be two of the most practical parameters to use when designing an early detection method to assess the toxicity of metsulfuron. The use of chlorophyll fluorescence would require a comparison of steady-state PSII measurements for control and treated plants, which could be realized by either measuring in time (before/after application) or space (treated/untreated patch