Now that you’ve learned about insects and agroecosystems, we turn to plant “pests”, or weeds. Similarly to insect control, there have been numerous chemicals developed to eradicate certain weeds in agriculture. These are called herbicides, and in this module, you’ll learn about the history of weed control, herbicides and their impacts on the environment, animals, and human health. But first:
What is a weed?
A weed is often defined as a plant that is growing where it is not wanted, and in competition with cultivated plants. What exactly does it mean for a plant to be “in competition”? We’ve created an example below.
Example #1: If they were growing side-by-side, would these two plants be in competition with one another?
Corn Plant (5 cm) Bean Plant (5 cm)
For the corn plant on the left, it wouldn’t matter if the bean plant is present, as it is not shading out the corn plant.
Example #2: What if this fully grown, 30 cm corn plant was growing alongside the 5 cm bean seedling above? What about if the bean plant was 30 cm growing alongside the 30 cm corn plant?
Corn Plant (30 cm) Bean Plant (30 cm)
For the taller, 30 cm corn plant, it does matter which size the bean seedling is- if it’s 5 cm, it won’t shade out the corn, but if it’s 30 cm, they will be in competition. Luckily, fully-grown bean plants usually are shorter than fully-grown corn plants, so they do not compete. In fact, they actually help each other and are excellent companion plants!
Given this demonstration, it is evident that the point at which competition is initiated is a critical issue- whichever plant has the initial advantage is the one that will dominate in competition. For plants Competition refers to the negative effects caused by the presence of other plants, usually by reducing the availability of resources such as sunlight, water, or nutrients. Weed control through history has been based on this fact of ecological plant competition theory. Slash and burn agriculture, can be considered in this light. From the initial burning of the slash that resulted from clearing the fallow to the repeated annual burning during the planting phase, the whole point of fire is to provide the crops with a head start over the other plants in the system.
Some Notes About the History and Impact of Herbicides
(Adapted from “The Ecology of Agroecosystems” by John Vandermeer, 2011)
During World War II the U.S. and British militaries had begun experimenting with herbicides. British forces used two herbicides, 2,4,D and cacodylic acid, to destroy food crops in areas known to be strongholds of the extensive left-wing revolutionary movement. For much of the history of herbicide application, both military and agriculture, these two herbicides and some relatives were a mainstay of vegetation destruction. During wartime, by far the most important use of herbicides was the destruction of crops.
When the US increased its attack on Vietnam, the Air Force began experimenting with various cocktails to defoliate forests. Guerilla nationalist fighters would attack from forested cover and disappear. A cocktail made from 2,4D and 2,4,5,T came to be known as “Agent Orange”, and was sprayed over thousands of hectares of Vietnamese forests. Agent Orange contains Dioxin, one of the most potent carcinogens known. US Veterans of the Vietnam war continue to suffer from what is now admitted to be the consequences of exposure to Dioxin, including elevated rates of several kinds of cancers.
In the 1970s, another “miracle” herbicide, atrazine was developed. It had two important properties: it was extremely cheap to produce, and had a low breakdown rate in the environment. Atrazine became the most widespread herbicide in grain production in the United States and was exported around the world to control broadleaf weeds. Recently, a raging controversy has gathered steam surrounding atrazine apparent effect as a hormone mimic in nature. For unanticipated chemical reasons, atrazine acts biologically in a fashion similar to estrogen. Consequently, feminization of wild animals, especially noted in amphibians and reptiles, has sounded an alarm. It also has numerous effects in humans- the same gene that can cause feminization is linked to breast cancer.
In these videos you will learn more about the effects of Atrazine (watch at least one):
Film 1: Atrazine and the Environment
Film 2: Tyrone Hayes: The toxic baby
The newest herbicide, with similar controversy, is Monsanto’s Roundup. The active ingredient in Roundup is glyphosate, which is a critical enzyme inhibitor. It is translocated into the plant tissue and is thus an incredibly effective biocide, like fire in that it kills all plant material, but better than fire because it kills all the tissue, including residual rootstock. Crop seeds are then genetically modified to resist glyphosate, which are sold with glyphosate herbicides, as a “roundup-ready” package. With these roundup-ready crops, one can plant the crop and then spray roundup over the entire field, killing all plants except the crop itself. This package is very expensive, and you cannot save seeds, so farmers must buy it year after year. While atrazine and related compounds have an indirect effect, through feminization and carcinogenicity. Roundup has a direct effect, killing the frogs that it meets and whatever else will eventually become known. Recent studies at the University of Pittsburgh demonstrated that roundup directly kills amphibians.
As you can see the history of chemical herbicides is related to death, poisoning and environmental degradation. If you must use them, see this guide for safe herbicide applications. However, its use is not compatible with agroecology. In the next two modules you will learn agroecological practices that will help you to design your farm to avoid competency between plants.