According to the Ohio State University, plant hormones act at low concentrations to control the metabolic and developmental process during plant growth. There are five known plant hormones auxins, cytokinins, ethylene, abscisic acid and gibberellins. Synthetic hormone herbicide or weed killer inhibits the hormone auxin, which in high concentrations causes poor growth and plant death. Although there are many benefits with using synthetic plant hormones as an herbicide, there are some risks.
Indole-3-acetic acid is the only naturally occurring auxin hormone in plants. According to the Ohio State University, synthetic auxins are common in commercial agriculture for controlling fruit set and growth after pruning on fruit trees. However, synthetic auxins inhibitors in the form of 2,4-D and MCPA, when applied in high dosages, promote poor plant growth and plant death. Weeds with broad-based leaves are the most common characteristic that auxins used as herbicides target. However, according to Bio Topics, weeds are defined as a plant that is competing for nutrients over crops or selected plants, which mean that auxin inhibitors used as herbicides can also effect sought after plant growth.
Auxin-based herbicides, first discovered in the 1940s, have become an effective method for weed control. Auxins specifically target broadleaved plants, which make it effective for killing particular weeds with a low risk of killing non-target crops. However, weed resistance to the hormone has been a cause for concern among plant growers. According to the University of Florida, 24 known weeds have developed cross-resistance to four of the most common auxin inhibitors over 60 years of use. Although resistance to auxin inhibitors is a cause for concern, the University of Florida states that the low percentage of resistance reported are good results for a synthetic herbicide used over the course of six decades.
A study done by the Environmental Protection Agency (Reference 4) found that acute exposure to the synthetic auxin inhibitor, NAA, posed no serious health risk to aquatic animals, birds, or other organisms that consume plants. The hormone inhibitor targets specific plant growth characteristics and has little to no effect on non-targeted life forms. However, according to the North Carolina State University (Reference 5), use of auxin inhibitors near greenhouses should be avoided because the chemical can damage greenhouse plants. To prevent damage to plants, never use auxin inhibitors around non-weed broadleaved plants, or plants that are susceptible to the herbicide.