Growth Regulators Table 3. Lipid Inhibitors Table 6. Seedling Root Inhibitors Table 7. Seedling Shoot Inhibitors Table 8. Photosynthesis Inhibitors Table 9. Cell Membrane Disrupters Table Pigment Inhibitors Table To be effective, herbicides must 1 adequately contact plants; 2 be absorbedby plants; 3 move within the plants to the site of action, without being deactivated; and 4 reach toxic levels at the site of action. The application method used, whether preplant incorporated, preemergence, or postemergence, determines whether the herbicide will contact germinating seedlings, roots, shoots, or leaves of plants.
The term mode of action refers to the sequence of events from absorption into plants to plant death. The mode of action of the herbicide influences how the herbicide is applied. For example, contact herbicides that disrupt cell membranes, such as acifluorfen Blazer or paraquat Gramoxone Extra , need to be applied postemergence to leaf tissue in order to be effective. Seedling growth inhibitors, such as trifluralin Treflan and alachlor Lasso , need to be applied to the soil to effectively control newly germinated seedlings.
Because the seeds of many weed species are quite small and germinate within 0. Herbicide positioning can be accomplished by mechanical incorporation or rainfall.
Once a herbicide comes in contact with the plant, absorption through the roots or shoots is very important. A herbicide that is absorbed through the roots will be taken up as long as the herbicide-treated soil remains in contact with the absorbing region near the root tips. As the roots grow to greater soil depths, herbicide uptake declines. Therefore, weeds not killed before the root tips grow out of the herbicide-treated soil are likely to survive. Many soil-applied herbicides are absorbed through plant shoots while they are still underground and may kill or injure the shoots before they emerge from the soil.
Volatile herbicides such as the thiocarbamates e. Less volatile herbicides such as the acetanilides e. Physical and environmental factors that promote rapid crop emergence reduce the length of time that a plant is in contact with a soil-applied herbicide and, therefore, reduce the possibility of crop injury.
