We can use this information to make better decisions regarding the risk of crop injury when selecting or applying herbicides. Herbicide Selectivity Most field crop herbicides are selective, which means that the herbicide controls certain weeds without harming the crop. For most herbicides, this selectivity is based on the crop's natural ability to metabolize or breakdown the herbicide into non-toxic chemicals before it damages the plant. There are other cases where herbicide selectivity is based on placement. One example of selective placement is when Prowl is applied preemergence in corn.
Prowl is sprayed on the soil surface so it is positioned above level where the corn roots develop.
This protects the corn roots from Prowl damage. Occasionally, herbicide selectivity is obtained because the herbicide does not bind to the target site in the plant. For instance, the postemergence grass herbicides like Assure II bind to and inhibit the ACCase enzyme in grasses, which kills them. However, these same herbicides do not bind to the insensitive form of the ACCase enzyme that exists in soybeans and other broadleaf plants. As a result, soybeans are rarely affected by these postemergence grass herbicides.