Herbicides control or kill unwanted pest plants, known as weeds. Different herbicides work in different ways. Farmers use herbicides to target weeds within crops--including corn, wheat and soybeans. Farmers do not rely on herbicides as the only weed-control method for crops, but consider them as part of a good weed-control management program.
Herbicides kill or suppress the invasion of weeds into crops. Weeds present the number one pest problem in American food production. Crops must compete with weeds for water, light, minerals, space and nutrients. Many types of crops release allelopathic chemicals, which are toxic to grain crops. When weeds are harvested along with the crop, they reduce the quality of the end product. The weed-infested crop often suffers reduced crops yields and increased production costs, according to 24d.org, an industry task force on 2,4-D research.
Farmers must choose between two types of herbicides based upon the desired outcome of the application. Non-selective herbicides provide a total kill on all vegetation. All plants in contact with a non-selective herbicide will be killed or stunted. Non-selective herbicides may be used to chemically remove weeds from a field prior to cultivation.
Selective herbicides kill specific types of plants without harming other plants. For example, a specific herbicide may kill broadleaf plants, but doesn't harm corn. Selective herbicides are first chosen based on the type of crop, then on the type of weed controlled.
The timing of application depends on both the crop and the desired emergency control. Herbicides are either pre-emergent or post-emergent. Pre-emergent herbicides are applied before the weed seeds germinate and emerge from the soil. Once applied, most pre-emergents require irrigation to incorporate them into the soil. The pre-emergent chemicals stop the germination process.
Post-emergent herbicides are applied directly to the crop and weeds after the plants reach a specific height or growth stage.
Post-emergent crop herbicides function in two ways: by contact or systemically. Contact post-emergent herbicides kill the plant upon contact. The first effects of the herbicide are seen in the withered and dying foliage of the weed. The plant is killed from the outside. Systemic herbicides are absorbed into the plant and translocated into all systems, including the roots. Systemics function in a variety of ways by affecting the growth regulation, hormones or photosynthesis of the plant. The plant often begins to curve downward, with leaves curling under.
Non-selective herbicides provide the broadest weed control, yet they damage crops. Herbicide-resistant crops are genetically modified to resist non-selective herbicides, such as glyphosate and glufosinate. The crops contain genes that allow for the degradation of the active ingredients in the herbicide. Instead of focusing on making the herbicide selective, the crop plant is cultivated to be herbicide resistant.