The composition of an herbicide and its interaction with plant physiology both determine the way it kills different types of weeds. Herbicides kill by simple contact or by entering the vascular system of the plant. The effect of weed killers does not stop with the weed, however; within the past 50 years herbicides' pervasive presence in the environment has caused increasing concern for their effects on human and animal life.
Contact Herbicide Effects
Photosynthesis inhibitors invade the plant's chloroplasts; cells that process sunlight and produce chlorophyll, the substance central to plants' digestive systems. Photosynthesis inhibitors cause yellowing around leaf edges or along veins where chloroplasts move. Older leaves begin turning brown and die first. Atrazine and cyanazine are photosynthesis inhibitors.
Cell Membrane Disrupters are contact weed killers that form oxygen compounds on plant tissue when activated by sunlight, oxidizing the cell membranes on leaves. Plants become limp, then turn yellow and brown and die. Paraquat and fomesafen are two of this group of powerful weed killers.
Systemic Herbicide Effects
Amino Acid Synthesis Inhibitors stop production of specific amino acids necessary to plant survival and growth. Glyphosate is a widely used non-selective amino acid synthesis inhibitor. Symptoms may take up to two weeks to appear. They affect the entire plant, stunting its growth until it dies; leaves turn yellow from the growing ends and may have red or purple streaks.
Growth regulators disrupt hormone balance and plant's use of proteins. They cause discolored and misshapen leaves and bent or brittle stems. 2,4-D, dicamba and triclopyr are systemic growth regulators that move throughout the plant.
Lipid synthesis inhibitors prevent fatty acids from forming to create the lipids necessary for generating plant tissue. Lipid Synthesis Inhibitors are selective herbicides like fenoxaprop and sethoxydim. They do not affect broad-leaved plants, only grasses. New grass blades are yellow and can be easily pulled out of the stalk, or whorl. Lipid synthesis inhibitors can take up to two weeks to kill grassy weeds.
Pigment inhibitors travel from the soil up through plant roots. Clomozone and norflurazon move through the plant to the leaves, turning them white. Susceptible weeds emerge as white seedlings and die because they are unable to generate photosynthesis.
Seedling growth inhibitors are chemicals that interfere with root or shoot growth of new plants. Effects include stunted, crinkled shoots and stunted, discolored shoots that do not emerge from the ground. Benefin, Pendimethalin, Trifluralin and Metolachlor are popular seedling inhibitors and are classified as pre-emergent herbicides.
The Environmental Working Group reports that tests across the U.S. Midwest, Louisiana and Maryland revealed that the Environmental Protection Agency's Lifetime Health Advisory (LHA) level for cyanazine was exceeded at least once during a four-month period in 18 of 29 communities. Water-borne atrazine exceeded the Minimum Contaminant Level (MCL) for 40 percent of the population in the study. Herbicides were found in 28 or 29 of the cities sampled. The persistence of herbicides in the ground leads to ground water and watershed pollution and overuse of the chemicals leads to damage to neighboring plants and crops. Research on natural herbicides, as of 2010, like the work being done on corn gluten meal at Iowa State University and development of no-till farming, promise to contribute to an environment tainted by fewer chemical herbicides.