3A: Resistant Weeds

Glyphosate-resistant marestail in a soybean field.

Before the introduction of genetically engineered-herbicide resistant (GE-HR) technology, corn and soybean farmers typically applied herbicides once or twice in the early spring. The majority of the overall pounds applied were sprayed prior to planting, or around the time of crop emergence (one to two weeks after planting).

Now, with the extensive planting of GE-HR corn, soybeans, alfalfa, rapeseed, and sugar beets in the Midwest, herbicides can be applied throughout much of the growing season without harming the crops growing from GE-HR seeds.

As a result, several species of weeds have developed resistance to glyphosate-based herbicides (GBHs). Dozens of weeds have become resistant to multiple herbicides and are often referred to in the media as super-weeds. Indeed, like the antibiotic-resistant “superbugs” we hear about in stories about hard-to-combat infections, some super-weeds are resistant to all, or most viable, herbicide-based control alternatives.

The International Survey of Herbicide Resistant Weeds is the definitive source of information on weeds that are resistant to a specific herbicide, or multiple herbicides, and when and where resistant weed phenotypes were first confirmed.

No one knows for sure how many acres will be infested with glyphosate-resistant weeds in the 2017 crop season, but the total reach of resistant weeds will likely exceed 120 million acres, as evident in Figure 3A.1 below.

The best publicly available data on the spread of glyphosate-resistant weeds comes from annual surveys conducted by Stratus Research. In a January 25, 2013 Blog, Stratus Vice President Kent Fraser released data from surveys conducted in 2010, 2011, and 2012. In this three-year period, the acres infested with glyphosate-resistant weeds almost doubled, rising from 32.6 million in 2010 to 61.2 million in 2012.

In the 2013 blog, Stratus reported that nearly half (49%) of farmers surveyed in 2012 said they had glyphosate-resistant weeds on their farm, up from 34% in 2011. The problem was most advanced in the south – in 2012, 92% of Georgia growers was dealing with resistant weeds.

A growing portion of these growers are contending with two or more resistant weeds — 27% in 2012, up from just 12% in 2010.

Figure 3A.1 graphs Stratus data on the acreage infested with glyphosate resistant weeds. Data for 2010-2012 comes from the Fraser blog, and the 84 million-acre data point for 2014 is from an April 1, 2015 Dow AgroSciences press release, and is in turn based on the results of the Stratus survey conducted in 2014.

The gaps in this data for 2013 and 2015-2017 were filled by projections based on the rate of change for the previous years, assuming that since 2014, the rate of increase has begun to slow down since such a large portion of the GE cropland base already harbors glyphosate-resistant weeds.

Moreover, the added costs to manage resistant weeds and associated collateral damage are expanding at an ever-faster pace because: (a) the number of acres infested with two or more resistant weed species is growing rapidly, and (b) the severity of infestations is rising on many acres (i.e., the number of resistant weeds/acre).

University weed scientists warn growers that in general, they can expect their herbicide costs to go up $25 per acre on fields with one glyphosate resistant weed, and $50 per acre or more on fields with two or more resistant weeds.

The sometimes rapid movement of resistant weeds across a farming region is evident in the below series of maps developed by a weed scientist at North Dakota State University. Click here to view the maps in a separate window.

These maps were produced by Jeff Stachler with the North Dakota State University and colleagues at the University of Minnesota, to illustrate the spread of glyphosate-resistant weeds in the two-state region.

Early Warnings Were Ignored

From prior to the planting of the first GE-HR crops in 1996, academic weed scientists were warning that the technology is uniquely capable of promoting the emergence and spread of resistant weeds.

But the industry, and especially Monsanto scientists and corporate officials, argued that Roundup and other GBHs had been used effectively since 1974, and without triggering more than a few, isolated pockets of resistant weeds.

But it was not long (only about 3 years) before the experts worried about resistance in the wake of GE-HR technology were proven right, as clear in the timelines below showing milestones in the emergence of herbicide-resistant weeds.  Click on the green hotspot icons to pull up each of the cited references.

The Herbicide Treadmill

The following resistance-promoting features of HR cropping systems are now widely recognized:

From 1996-2005, farmers used glyphosate-based herbicides such as Roundup almost exclusively, promoting weed resistance due to heavy selection pressure on weed populations.

  • Predominant or exclusive reliance on herbicides for weed control;
  • Heavy, if not sole, reliance on a single herbicide (glyphosate from 1996 through about 2005);
  • Reduced interest in and adoption of prevention-based, non-chemical control strategies;
  • Extended spray season;
  • Multiple applications, and
  • Applications at higher rates per acre.

These factors lead to the emergence of weed phenotypes resistant to one or more herbicides (Mortensen et al., 2012), in a process dubbed the “transgenic treadmill” (Binimelis et al., 2009).

In the section on How to Reduce Risks, we describe the institutional failures by the industry and EPA that enabled the rapid spread of resistant weeds.

Essential remedial actions on the farm, in industry, and by regulators are also described, if and when the nation decides it is time to deal with pervasive, fast-moving slippage in the efficacy of GE-HR based weed management systems, and the collateral damage and adverse birth outcomes that will accompany more intensive use of multiple herbicides.

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