Illinois Field Crop Applicators Practice Test

At which stage in the lifecycle are annual weeds most susceptible to postemergence herbicides?

1. Seedling

2. Rosette stage

3. Prior to bolting

4. Both seedling and rosette stage

The correct answer is: Both seedling and rosette stage

Annual weeds are generally most susceptible to postemergence herbicides during the seedling and rosette stages because these stages represent the period of rapid growth and development. At the seedling stage, the plants are small and their physiological processes, such as nutrient uptake and cell division, are highly active, which allows them to absorb herbicides more effectively. During the rosette stage, many annual weeds have developed a relatively large leaf area and are still in a vegetative state, making them vulnerable to herbicide application. At this point, the plants have a significant leaf surface for herbicide absorption, and their growth processes remain vigorous. In contrast, once annual weeds progress beyond these stages, such as entering the flowering or bolting stages, they often become more challenging to control. Their energy is directed towards reproduction and they might develop tolerance to certain herbicides. Thus, targeting weeds during both the seedling and rosette stages maximizes the effectiveness of postemergence herbicide applications.