Types of herbicides
- Translocated herbicides move to the site of action via the transport mechanisms within the plant; the xylem and phloem. The xylem transports water and nutrients from the soil to growth sites and the phloem transports products of photosynthesis (for instance, sugars) to growth and storage sites. It may take up to two weeks for symptoms to develop on the target weeds depending on herbicide rate, conditions and species.
- Contact herbicides have limited movement within the plant, so complete coverage of the target is critical. Compared to translocated herbicides (for example, glyphosate), contact herbicides (for example, paraquat, oxyfluorfen, diquat and bromoxynil) tend to show symptoms rapidly, usually within 24 hours.
- Selective herbicides will kill target weeds and not desired plants (the crop or pasture) when applied at a specified application rate.
- Non-selective herbicides (also called knockdown herbicides) such as glyphosate or paraquat will damage most plants.
- Residual herbicides remain active in the soil for an extended period of time (months) and can act on successive weed germinations.
- Non-residual herbicides, such as the non-selective paraquat and glyphosate, have little or no soil activity and are quickly deactivated in the soil. They are either broken down or bound to soil particles, becoming less available to growing plants. They also may have little or no ability to be absorbed by roots.
- Post-emergent and pre-emergent are terms that refer to the target and timing of herbicide application. Post-emergent refers to foliar application of the herbicide after the target weeds have emerged from the soil, while pre-emergent refers to application of the herbicide to the soil before the weeds have emerged.
- Herbicide mixtures and sequential applications involve the application of more than one herbicide, usually to increase the spectrum of weed species controlled but also for resistance management. A mixture involves the application of multiple products in a single application. Where herbicides are antagonistic and cannot be mixed together in a single tank, they are applied sequentially.