A preemergence followed by postemergence herbicide program is likely to be the most reliable and effective under a wide range of conditions in corn or soybeans. However, preemergence or postemergence only herbicide programs can work under the proper conditions in both crops. Introduction Corn growers have shifted toward greater use of postemergence herbicides in recent years, mainly due to the increasing restrictions on use of atrazine and the release of several new postemergence products for corn. New herbicide resistant corn hybrids that allow the use of Liberty, Roundup or imidazolinone herbicides like Lightning are also increasing the use of postemergence herbicides. When weeds are killed after establishment rather than at germination or early emergence, the chances that weeds may interfere with crop growth are increased.
Weeds compete with a crop for water, nutrients and light.
This interference with crop growth often reduces yield, and may also causes secondary problems such as reduced grain quality, slower dry-down and harvest difficulties or losses. The larger or faster growing the weed, the greater the chances of crop interference. The key to successful weed management is controlling weeds before they can reduce crop yield. Some herbicides, such as Roundup, can control larger weeds. The pending availability of Roundup for use on resistant corn hybrids will enable growers to control weeds larger than 6 to 8 inches in height.