Weeds impact every farm field, every year, everywhere around the world. 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 like weeds.
Herbicides are the only type of pesticide for which use is rising in the U.S. and worldwide, with no end in sight. Why? Because of the widespread adoption of genetically-engineered crops (also known as GMOs, or Genetically Modified Organisms), and the spread in their wake of herbicide-resistant weeds.
GMO corn, soybeans, and cotton engineered to resist post-emergent (during the growing season) applications of herbicides including glyphosate (e.g. Roundup), 2,4-D, and dicamba are the fuel pushing the herbicide treadmill into ever-higher gears.
Before herbicide-resistant GMO crops, farmers could only spray fields with broad-spectrum herbicides at the beginning of the year to kill weeds before planting, or after the crop was harvested.
Broad-spectrum herbicides are capable of killing a wide range of plants, including both weeds and agronomic crops, leading to the limits on when such herbicides can be applied. GMO technology extends by several weeks to a few months the time period during which broad-spectrum herbicides can be sprayed on actively growing crops, killing weeds but leaving the farmer’s cash crop unharmed.
Annual Spray Days
Herbicide use across the Midwest has increased dramatically in the last decade as a result of the spread of dozens of weeds resistant to the most commonly-used herbicide glyphosate (e.g. Bayer/Monsanto’s Roundup).
Excessive reliance on just one herbicide – glyphosate – 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.
The intensity of herbicide use in a given area is a function of five key variables –
- How many different herbicide active ingredients are sprayed in a given crop year.
- How many times each individual herbicide is applied.
- Herbicide rates of application (typically measured in pounds of active ingredient per acre).
- The number and diversity of weed species that are controlled by each herbicide.
- 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 latter 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 future crop years.
Other changes in Midwestern farming systems adopted in the quest for higher per acre yields have made crops more vulnerable to a wide array of pests. The three longstanding trends that make crops less healthy and/or more vulnerable to pests are incrementally high seeding rates (i.e. growing more plants per acre), increasing the amount of nitrogen and other plant nutrients applied at one time (nutrient spikes can fuel pest spikes, as well as crop growth), and the near-total dominance of just two crops (corn and soybeans) and absence of beneficial crop rotations.
In addition to the mammoth weed management challenges now unfolding across the Midwest, these corn-soybean farming system changes have led to:
- The widespread coating of seeds with multiple fungicide and insecticide seed treatments;
- A troubling rise in fungicide spray applications on corn; and
- Spread of insects resistant to the Bt toxins produced by GE-Bt corn, coupled with insecticide applications made as a tactic to forestall the spread of Bt-resistance insects.