Which statement best describes a post-emergent broadleaf herbicide, and what is a common example?

• A. A pre-emergence herbicide that targets grasses (example: Atrazine).
• B. A soil sterilant used before planting (example: Metam).
• C. A contact herbicide that kills all plants on contact (example: Paraquat).
• D. A herbicide applied after weed emergence that targets broadleaf plants; example: 2,4-D.

Answer: (D)

Post-emergent broadleaf herbicides are applied after the weeds have already emerged and are selective for broadleaf plants, meaning they mainly affect broadleaf species while leaving grasses relatively unharmed. They work systemically in the plant, often taken up through the leaves and transported to growing points, disrupting growth and eventually killing the weed. A classic example is 2,4-D, which acts like a synthetic auxin hormone. This causes abnormal, unsustainable growth in broadleaf plants, leading to their death, while many grasses tolerate it, making it useful for controlling broadleaf weeds in pastures and rangelands.

Pre-emergence herbicides are applied before seeds germinate, creating a preventive barrier in the soil rather than acting on established weeds. A soil sterilant describes a tool used before planting to sterilize soil, not a post-emergent weed-control product. A non-selective or contact herbicide that kills on contact would not be described as a selective post-emergent broadleaf herbicide, since its action isn’t targeted to broadleaf species only.