Broad-leaved dock (Rumex obtusifolius) is a persistent weed in pasture fields. In spring, new shoots grow rapidly from thick, deep taproots. The plant dies back in winter, but the taproots remain alive underground. For long-term control, it is essential to destroy the taproots before the plants flower and set seeds in mid-summer.
Farmers use selective herbicides to treat the pastures without harming the grass crop.
Below is information on four selective herbicides, W, X, Y, and Z.
| Selective herbicide | Effect on broad-leaved dock | Weeds controlled | Cost of herbicide |
|---|---|---|---|
| W | Destroys taproot and prevents seed production | Broad-leaved dock and several other broad-leaved weeds | High |
| X | Destroys taproot and prevents seed production | Only broad-leaved dock | Medium |
| Y | Destroys taproot and prevents seed production | Only broad-leaved dock | Medium |
| Z | Kills foliage but leaves taproot intact | Broad-leaved dock and several other broad-leaved weeds | Low |
A farmer has a pasture field containing grass (the crop), broad-leaved docks, and several other weed species (such as dandelions and nettles).
Evaluate the information on selective herbicides W, X, Y, and Z. State which selective herbicide would be the best choice to treat this pasture field, suggest the best time of year to apply it, and explain your choice.