What type of herbicide is best to kill thistle plants in an already growing pasture forage crop?

• A. Soil-translocated-persistent-nonselective herbicide
• B. Foliar-translocated-nonpersistent-selective herbicide
• C. Plant growth regulator applied over the entire field
• D. Contact herbicide

Answer: (B)

The choice of a foliar-translocated-nonpersistent-selective herbicide is well-suited for controlling thistle plants in a pasture forage crop because such herbicides are designed to be absorbed by the leaves and then translocated throughout the plant. This means that they can effectively disrupt the growth processes of the thistle without adversely affecting the surrounding forage crops that are more resilient to this specific herbicide.

Selective herbicides target specific types of plants, in this case, the thistle, while leaving desirable forage species unharmed. This selective action is crucial in a pasture setting, where preserving the health and yield of the forage is essential for livestock production. Nonpersistent indicates that the herbicide breaks down within a shorter period after application, reducing the likelihood of long-term residual effects in the soil or on subsequent plantings, which is important for maintaining the ecological balance in pasturelands.

Soil-translocated-persistent-nonselective herbicides would indiscriminately kill all vegetation, which could devastate the pasture forage crop. Plant growth regulators control plant growth but may not effectively eradicate a weed like thistle. Contact herbicides act only on the parts of the plant they physically touch, which may not provide thorough control since thistles can have extensive root systems