Herbicides are the type of pesticide designed to control or kill weeds or other unwanted vegetation, and account for by far the largest share of overall pesticide use.
On farms, some are applied before a crop is planted, and incorporated in the soil. This class of pre-plant herbicides exerts residual control, by blocking the germination of weeds, or killing weeds as they first begin to grow.
Another class of herbicides are applied after crops have started to grow, and fall into the category of post-emergence herbicides.
A few herbicides are also applied late in the crop season to kill mother plants, so that grain, beans, or other crops will dry quicker and more evenly, so they can be harvested sooner than otherwise would be possible.
Such pre-harvest, desiccation uses of herbicides account for the lion’s share of dietary exposure to herbicides, and are a growing cause of concern, especially in the case of glyphosate.
While farmers and ranchers apply most of the volume of herbicides used in any given year or region, other uses and methods of applying herbicides around homes, parks, schools, businesses, sidewalks, rights of way, and in landscaping account for a disproportional share of high-exposure episodes.
Why? Because on farms and ranches, the vast majority of herbicides are applied in hi-tech sprayers in which the operator sits in a steel-glass cab that includes an efficient air filtration system. Commonsense measures like wearing gloves when filling the sprays tank, and exercising caution to avoid spills, keep exposure levels relatively low.
Most non-agricultural herbicide uses, on the other hand, are made with handheld or backpack sprayers, or ATV or small truck mounted sprayers, whereby the applicator holds a wand and directs the spray toward the area in need of treatment. The applicator also has to walk or rise in and around the treated area to ensure even coverage, leading to an unavoidable amount of dermal exposure.
Such handheld equipment is also prone to slow leaks, and spills sometimes happen during the filling of spray tanks. Applying herbicides on a windy day dramatically increases dermal exposure levels.
While herbicide use is typically measured in pounds of active ingredient applied, virtually all herbicides as sold to farmers, homeowners, or other weed managers as formulated products that contain both active ingredients and a number of so-called inert ingredients.
Inert ingredients include surfactants and adjuvants that alter the properties of formulated herbicides to make them more effective, more stable during storage, and/or more compatible with other agricultural chemicals and fertilizers that are sometimes included in tank mixes, in order to reduce application costs.
For more information on the types of herbicides currently available, the many different ways, places, and times they are applied, and an overview of factors influencing herbicide efficacy, see the “Introduction to Weeds and Herbicides” on the University of Pennsylvania Extension website.
Brief Overview of Herbicide Use
In 2017, the EPA released its latest in a series of reports entitled “Pesticide Industry Sales and Usage: 2008-2012 Market Estimates.” In 2012 according to the EPA, 678 million pounds of herbicide active ingredient was applied in the U.S. — enough to spray about 2 pounds on every harvested acre of cropland nationally.
The 678 million pounds of herbicides applied in 2012 accounted for 57% of total U.S. pesticide use (1,182 million pounds). Insecticides and fungicides accounted for only 5% and 9% respectively, while fumigant pounds applied were 37% of total use.
Agricultural operations accounted for about 91% of total herbicide use in 2012 according to EPA data, and 89% of total pesticide use. About 5% of herbicide pounds applied were done by homeowners, landscapers, and groundskeepers, and industry and government agencies applied the remaining 4%, mostly to control weeds along roads, power and pipelines, and around industrial facilities.