Fall application of herbicides is effective for controlling perennial weeds, as they begin to store carbohydrates produced through photosynthesis in the roots. Applied herbicides are translocated to the roots of the weeds as the carbohydrates are being stored, thus killing the whole plant.
Although conservation farming reduces soil erosion and increases soil health, the increased soil cover from plant residues reduces soil-herbicide contact. Figure 1, shows the fate of herbicide after it has been applied in the field. Another approach is to use pre-emergence herbicides that have soil residual activity. For the fall-applied herbicides and the pre-emergence herbicides to be effective, soil contact and even distribution is crucial.
Herbicide applied on field surfaces that have high plant residue can be intercepted or absorbed before they even reach the soil. The relationship between the herbicide interception by crop residues is proportional to the amount of crop residue cover. The ability of the herbicide to wash-off the stubble depends on the characteristics of the herbicide and the environmental conditions at the time of application.
Increased soil cover also results in increased organic matter, thus increasing the potential of the soil to bind herbicides tightly. This not only reduces the herbicide available to kill weeds, but it might also increase the persistence of the chemical in the soil, which can affect subsequent crops.
As producers decide on their weed control programs, it is important to know the amount of plant residue cover in the different fields, in addition to knowing the weeds that are to be controlled by the applied herbicide.
Other important considerations include:
Reading the herbicide label on how to handle high residue situations
The solubility of the herbicide
How readily the herbicide can bind to the soil components
The rate of chemical breakdown after application
Expected environmental conditions at time of application