3: Herbicide Use


Pesticides include any compound used to control pests, including insecticides, fungicides, and herbicides.

The term “pesticide” encompasses all synthetic chemicals and natural products applied by farmers in an effort to minimize losses in crop yield and/or quality to pests.

Pesticides include:

  • Herbicides applied to manage weeds,
  • Insecticides, and Bacillus thuriengenisis (Bt) toxins produced by genetically engineered (GE) crops, for insect control,
  • Fungicides sprayed to combat plant diseases,
  • Nematicides targeting nematodes,
  • Plant growth regulators, and
  • Several other types of products.

Pesticide use across the Midwest has increased dramatically in the last decade as a result of the planting of genetically engineered (GE) crops, the spread of weeds resistant to the commonly-used glyphosate herbicide (also known as Roundup), and the rise of insects resistant to the Bt toxins produced by GE-Bt corn.

Tracking trends in overall pesticide use has become more complicated in the GE crop era for a variety of reasons, including:

Big changes in herbicide use, pest pressure, insecticide use, and a new reliance on seed treatments have made tracking overall pesticide use much more complicated.

First, big changes in herbicide use.

The vast majority of GE crops express one or more genes conferring resistance to herbicides that otherwise would harm the crop, along with any weeds growing in the same field.

Excessive reliance on just one herbicide – glyphosate, the active ingredient in Roundup and many other glyphosate-based herbicides (GBHs) – has triggered the emergence and spread of over a dozen glyphosate-resistant weeds. This has, in turn, forced farmers to spray additional herbicides, often at higher rates and, in many fields, more than once in a season.

Second, changes in insect pest pressure and insecticide use, and in recent years, net increases in overall insecticide/Bt toxin use on a per acre basis.

GE-Bt corn and cotton crops produce their own insecticide within plant tissues, increasing the weight of insecticides on a per acre basis as much as, and sometimes even more than, the reduction in conventional insecticide use.

For details on several of the most widely planted Bt corn and cotton varieties, see Table 3.1 below, which is derived from the 2012 paper “The Impact of GE Crops on Pesticide Use in the U.S.: The First Sixteen Years” in the peer-reviewed journal Environmental Sciences Europe.  This table shows that the use of Bt traits actually can mean an overall increase in insecticidal compounds in fields planted to GE-Bt crops.

Third, a big jump in reliance on seed treatments.

For a host of reasons, today’s GE crops are more vulnerable than past non-GE plant varieties to early-season insect and plant disease damage. To keep crop losses to a minimum, the vast majority of GE seed is coated prior to planting with 1 to 3 insecticides and another 1 to 3 fungicides.

Each biotechnology/seed/pesticide company has its own, proprietary seed treatment technology.  These often change from year to year and can differ across the country as a function of the severity of various soil-borne insects and plant pathogens.

The big three ag-biotech companies all have proprietary seed-treatment techniques.

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Throughout this section, we will introduce analytical concepts, tools, and data sources useful in tracking overall changes in pesticide use, with special focus on the recent and dramatic changes in herbicide use.

But first, additional information is provided on the impacts of GE technology on herbicide, and overall pesticide use, in the two most widely grown crops in the Midwest – corn and soybeans.

A classic Midwestern farm scene, corn (foreground), soybeans (background) and scenic rural vistas.  Photo: Flickr CC

Impacts of GE Crop Technology

Genetically engineered (GE), herbicide-resistant (HR) crops have been planted on nearly three-quarters of the cropland in the Midwest for over a decade, triggering the emergence and spread of nearly a dozen glyphosate-resistant weeds.

Tougher-to-kill weeds now infest around two-thirds of the ~170 million acres producing GE corn and soybeans in the U.S.  Many fields are infested with two different species of resistant weeds, and some harbor three.

The presence of weeds resistant to glyphosate and/or other herbicides forces farmers to:

  • Spray more often,
  • Apply herbicides at higher rates per acre,
  • Make applications later in the season to target weeds that survive earlier control efforts, thereby extending the time period when drift and volatilization can lead to human exposures and damage to nearby vegetation, and
  • Augment standard weed management programs through practices like tillage or by spraying additional herbicides targeting specific, hard-to-control weeds.

GE corn and soybean varieties have recently been approved that are resistant to multiple herbicides, including combinations of glyphosate, glufosinate, 2,4-D, dicamba, isoxaflutole, and ACCase inhibitors.

There are two groups of herbicides classified as ACCase inhibitors, known as the “fop” and “dim” families due to their common suffixes.  Some GE crops are resistant to the “fop” herbicides, which  include Assure II (quizalofop), Fusilade (fluazifop), and Hoelon (diclofop).

The intensity of herbicide use in a given area is a function of five key variables –

  1. How many different herbicide active ingredients are sprayed in a given production cycle.
  2. How many times each individual herbicide is applied.
  3. Herbicide rates of application (typically measured in pounds of active ingredient per acre).
  4. The number and diversity of weed species that are controlled by each herbicide.
  5. How long each herbicide application adequately controls target weeds.

The first three of these five variables determine whether, and to what extent, herbicide use impairs environmental quality or poses a public health risk. The later two variables are critical for farmers, and determine both weed management system costs and efficacy, and whether more or less herbicide use will be required in the future.

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