What are the two main categories of herbicides based on the timing of application?

The main categories of herbicides based on the timing of application are pre-emergence and post-emergence. Pre-emergence herbicides are applied before the target weeds germinate, providing a preventive measure that interrupts the growth process right at the start. This timing is critical because it targets weed seeds in the soil before they have the opportunity to sprout and compete with crops for resources.

On the other hand, post-emergence herbicides are applied after the weeds have already emerged. This type of herbicide targets actively growing weeds, making it effective against those that have already established themselves in the field. Understanding the timing of application is essential for effective weed management and maximizing the herbicide's effectiveness, as each type serves a specific purpose in controlling weed populations at different stages of their life cycle.

The other categories, such as selective, non-selective, systemic, and contact, focus on the mode of action or the physical form of the herbicide rather than the timing of application.