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Does Herbicide Use Encourage “Superweeds”?

When Roundup® entered the market in the early 1970s, it seemed to be an herbicide dream come true. Inexpensive, effective and non-persistent in the environment, glyphosate, the main ingredient in Roundup®, gave most users few things to complain about. Roundup® was cheaper than many other herbicides, so farmers could use it throughout the growing season with little economic impact. Farmers also enjoyed weed-free fields, and with the introduction of Roundup Ready crops in the 1990s, spraying weeds in developing fields became easier. National Park Service staff even use glyphosate to kill invasive weeds in the forest because it effectively controls vegetation and has low mammalian toxicity.

Glyphosate, a “PPO inhibitor,” works by blocking the pathway for aromatic amino acid production, which inhibits the production of necessary plant proteins, and results in the death of the plant.  Mammals do not have the same protein synthesis pathway, and therefore are not affected by this chemistry, although other organisms – bacteria, fungi, and other microorganisms – can be negatively impacted by glyphosate.

Unfortunately the honeymoon with glyphosate ended in 2002 in Macon County, Georgia, with the first reports of glyphosate-resistant Palmer amaranth. According to North Carolina weed scientist Alan York, glyphosate-resistant Palmer amaranth has overtaken acreage in Georgia, South Carolina and North Carolina. Palmer amaranth is one of six weed species in the southern U.S. that have been reported to have resistance to glyphosate.

How did herbicide resistance happen in the first place? After the first reports of resistance in Georgia, groups in Europe and North America pointed to the Roundup Ready® technology as the source of the world’s new “superweed.” However, according to a scientific review on gene flow of transgenic plants by Stephen Chandler of Australia and Jim Dunwell of the UK (2008), gene flow from transgenic plants can only occur between plants within a species, and between very close relatives (such as sorghum and johnsongrass; or wheat and jointed goat-grass). While this is uncommon, crop rotation does help to prevent potential out-crossing from crop plants to nearby weeds.

Most herbicide-resistant weeds develop by selection pressure resulting from repeated use of a single herbicide, or herbicides with the same mode of action. The principle behind herbicide resistance is the same as it is for any type of living organism; bacteria develop resistance to antibiotics after periods of uninterrupted use. Insects and pathogens develop resistance to pesticides that are used at the exclusion of other integrated pest management practices. Any herbicide that is used in the absence of other weed control practices has the potential to promote resistance.

To reduce the chance of herbicide-resistant weeds, Purdue University weed scientist Bill Johnson recommends rotating herbicides with different modes of action. In a five-year study published in Weed Science, Purdue University weed science researcher Bill Johnson found that use of a variety of herbicides in addition to Roundup® significantly reduced marestail, a common glyphosate-resistant weed in Indiana.

“Go after weeds with two different herbicides,” Johnson says in a Science Daily article. “We want to minimize the number of weeds resistant to Roundup. To do that, you want to minimize the exposure that a weed population has to Roundup. If you diversity a little bit, you’ll extend the life of the technology.”

Crop rotation between different crop species also serves to keep weed populations down. Extension specialists in Oklahoma, for instance, discovered that rotating wheat with canola significantly reduced weeds and gave wheat growers a cleaner crop.

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