What ensures a pesticide remains effective against weeds over successive applications?

The residual effect of a pesticide is crucial in ensuring that it remains effective against weeds over successive applications. This refers to the pesticide's ability to remain active in the environment for a period of time after application, continuing to suppress or kill weeds even after the initial application has dried or been partially degraded.

When a pesticide has a strong residual effect, it forms a longer-lasting barrier or activity in the soil or on the foliage, which allows it to combat new weed germination and growth that occurs after the initial treatment. This persistence not only helps reduce the frequency of applications needed to maintain control over weeds but also minimizes the chance of weed resistance developing, as it consistently manages the weed population.

In contrast, while concentration, application rate, and toxicity level are important factors in the initial effectiveness of a pesticide, they do not specifically guarantee that it will remain effective over time. Concentration refers to the strength of the pesticide at the point of application, application rate is about how much is used, and toxicity level indicates how harmful the substance is to organisms. Therefore, the residual effect is the key factor for long-term effectiveness against weeds.