A new method for assessing herbicide damage in weeds

Herbicide application is one of the most widely used weed management tools. Whilst herbicides are effective at killing plants [weeds], their application can lead to variable levels of control and uptake rates, as well as off-target damage, and as such assessments of their effectiveness are needed. The effectiveness of herbicide application is typically assessed using visual estimations of the level of damage present, either to individual leaves or whole plants, using a series of pre-determined categories (e.g. <25% leaf burn) over time. Such visual assessments lack accuracy and only provide information on the effectiveness of the herbicide applied. In many instances the weed is not killed by the herbicide application and thus information is also needed on how it responds to being damaged, but not killed. Here, we outline the use of several tools to increase rigor into assessments of herbicide effectiveness. First, the leaf area is determined using leaf area software to differentiate between healthy (green) and damaged (brown) areas of the leaf. Then each leaf is assessed to determine the changes occurring within the leaf to support the visual changes observed (i.e. damage). These tools assess the chlorophyll content, photochemical index, and stomatal conductance of the leaf. Results from a trial on four weed species showed that these tools provide greater accuracy for assessing herbicide damage, especially when combined, as well as how leaf function changes in relation to herbicide damage. Whilst the use of these tools may not be suitable for many land managers, the use of digital photos and leaf area software is very effective and should be considered as an alternative to visual assessments