Which of the following is not allowed for weed control?

Strychnine is the correct answer because it is primarily a toxic alkaloid used as a pesticide for purposes other than weed control, mainly for the control of certain vertebrate pests. Unlike the other options, which are specifically herbicides designed for weed management, strychnine poses health risks to humans and non-target organisms, making it unsuitable for use in weed control.

Glyphosate, Roundup (which is a brand name for glyphosate), and 2,4-D are all well-established herbicides. Glyphosate functions as a non-selective herbicide, effectively targeting a wide array of weed species by inhibiting a specific enzyme pathway. Roundup, being a glyphosate formulation, shares these properties. Likewise, 2,4-D is a widely used selective herbicide that targets broadleaf weeds while sparing grasses. Thus, they are all permissible and commonly utilized in agricultural and landscaping applications for weed management.