Superweeds out-flank glyphosate: calling for cultivators

As the world’s leading herbicide loses its punch, cultural methods and careful mechanical cultivation re-emerge as the smart choices for managing weeds.

By Betsy McCann

"What farmers lost over time in the face of intensive marketing of this 'simple chemical solution' was the need for knowledge of how their cultural practices could reduce weed pressure."

How farmers deal with weeds has changed significantly in the last 60 years, and continues as more weeds in more places defy predictions of their demise in herbicide-focused systems.

Mechanical and cultural methods increasingly gave way to chemical inputs after World War II with 2,4 D growth-altering herbicides. Farmers found they could exchange multiple, mind-numbing field passes with weed-management tools for one field pass with a sprayer for the financial trade-off of the cost of herbicides. What they lost over time in the face of intensive marketing of this “simple chemical solution” was the need for knowledge of how their cultural practices could reduce weed pressure.

Tracing the arc of the change, the locus of agricultural innovation development moved from the land to the laboratory after mid-century. In the fields, farmers increased inputs and parked traditional weed management implements in fencerows or in the back of their machinery sheds.

Chemical-based no-till farming rose to prominence in the ‘70s thanks to the efficacy and low cost of glyphosate-based herbicides, which offered broad-spectrum weed control. Initially a burndown treatment, they became over-the-top weed controls as varieties were modified genetically to tolerate the lethal chemistry. This started first in soybeans, then cotton and corn.

This seemed to be the perfect system by the late ‘90s, allowing farmers to plant many more acres than they could ever do if they depended on mechanical cultivation alone.

The mirage of continuous weedless perfection is fading, however. As predicted from the start by weed ecologists, repeated use of glyphosate on the same weed species in the same crops year after year had an unwanted result. More weed varieties are surviving the chemical that used to kill every plant that had not been genetically designed to withstand its biological monkey-wrenching.

The rise of the Superweeds

How bad is the problem? Weedscience.org reports that as of 2008 there were over 70 species of weeds in the United States with some level of resistance to typical herbicides, 16 of which are resistant to glyphosate. The latter group includes amaranth, pigweed, Italian and rigid ryegrass, hairy fleabane, horseweed, Johnsongrass and marestail (5).

Pigweed: One of the 16 weeds currently listed as resistant to glyphosate and a devastating invader in Georgia. (photo courtesy www.weedscience.org)

It’s worth noting that the USDA lists Johnsongrass, Sorghum halepense, as one of the 10 worst weeds in the world. Marestail is also a significant problem, and its resistant strains require additional chemical treatments, as outlined in this April 2009 bulletin from Ohio State University. In Georgia, an estimated 100,000 acres are severely infested with glyphosate-resistant pigweed (6).

How does resistance occur? Just as the use of antibiotics by humans contributes to the emergence of antibiotic-resistant forms of Staphylococcus that pose public health threats, weed resistance develops in the same fashion. When repeatedly exposed to the same toxin, weeds start to respond by selecting resistant characteristics.

Popular herbicide formulations rely on glyphosate to inhibit the enzyme that synthesizes the essential amino acids tyrosine, tryptophan and phenylalanine within plants and microorganisms. These amino acids are responsible for maintaining the plants’ normal functions. Glyphosate is usually combined with other materials to increase the chemical’s efficacy by sticking to plant material and permeating waxy leaf surfaces.

Applied as a spray, glyphosate accumulates within plant structures, stunting growth and diminishing color, wrinkling leaves and causing death within a few weeks. One of the more dangerous aspects is its ability to strongly bind to soil particles. This trait limits its impact on other plants, its makers say. Over time, however glyphosate accumulate in the soil, allowing plants and variant strains to slowly increase their resistance, or immunity, to its toxicity.

Glyphosate epitomizes the herbicide promise as agriculture’s silver bullet. That silver bullet is losing its luster and no longer kills the vampire. This dims the promise of a chemical panacea from a single field pass and opens a Pandora’s box of other problems in high-input, biotechnology-based systems, as well as new attention to more sustainable and biological solutions.

A Band-aid for a headache

When herbicides fail, what is a farmer to do? Leading industry-funded research suggests the problem with herbicide-resistant weeds is not a natural consequence of herbicide technology, but site-specific management problems with timing, concentration and technique. Approaches for combatting the weeds include increasing glyphosate concentration, rotating herbicides formulations, using specific sequences of herbicides, and altering herbicide application timing.

In a 2008 report, the Center for Food Safety noted glyphosate use increased from 7.933 million pounds in 1994 (when genetically modified seeds were first introduced) to 119.071 million pounds in 2005, an increase of 1,500 percent. The increased use, combination with other herbicides (intentionally and unintentionally inside application tanks), and amplified concentration of glyphosate used on fields creates an environment ripe for breeding more potent superweeds. Further, several researchers report findings that link exposure to glyphosate (and products built around it) with human and amphibian health risks. (See the sidebar Research findings indicate human, amphibian impacts from glyphosate.)

"The consequence of contemporary herbicide use is the engineering of fields to grow stronger, more robust weeds. It is more effective in the long run to engineer the field system to grow healthy crops."

Ultimately, the increasing failure of glyphosate as a broad-range herbicide is indicative of systemic problems within the larger chemical weed control approach. Repeated applications increase labor, costs and the likelihood of the further weed-resistance.

When herbicides do not perform as farmers expect them to, the whole premise of worry-free, one-shot weed control is lost. Logic dictates that if a chemical is creating the problem of biological resistance, applying more of that chemical will make things worse. The mindset behind the development of herbicide-tolerant plants was to create plants and a controlled field environment where weeds were removed from the picture. Over time, this narrow weed-control focus created an imbalance that chemicals cannot restore.

The problem of herbicide-resistant weeds can be addressed by several practical solutions, as many organic farmers already know. Instead of creating increased dependency on an ever-widening array of herbicides, a more sustainable approach is to shift to ecologically-oriented weed management so that healthy crops in healthy soil can outperform weeds.

The consequence of contemporary herbicide use is the engineering of fields to grow stronger, more robust weeds. It is more effective in the long run to engineer the field system to grow healthy crops.

When considering the land as its own system, weed management is a systemic issue. Much as a robust human immune system works to keep infectious agents at bay, systemic approaches to weed control consider myriad factors contributing to weed growth. Instead of looking at all farms as a single system with the same chemical composition, farm-specific ecological approaches respect the ways each farm is its own system. Differences in location, cropping history, crop rotation, biodiversity and soils are all factors to consider.

Farm-specific approaches can include combining weed management with crop rotation and soil tilth improvement. Work to create an environment that optimizes crop growth at the expense of weeds. Using cover crops to suppress weed pressure lets you also sequester carbon in the soil and also lets you fix nitrogen if you can use a legume crop. Other approaches suitable in some situations include mulching, flame weeding and use of a stale seed bed.

Cultivating makes a comeback

Timely and effective cultivation, used as the final step in a well-planned weed-management program, can help farmers deal with herbicide-resistant weeds. Dismissed by the herbicide industry as out-dated, mechanical weed management practices can be an important part of out-maneuvering herbicide-resistant weeds.

Hiniker 6000 high-residue cultivator

"...soil organic matter in a robust rotation, with tillage, can actually improve more than under a chemical no-till system without cover crops."

Cultivation includes a variety of tools, from rotary hoes and tine-weeders to high-residue management systems. Timely, correct, and careful use of these tools can aerate the soil, break soil crusts and stimulate root growth in crops. By increasing soil organic matter through cover crops and crop rotation, soil tilth increases as well, helping to decrease the risk of erosion that even careful surface tillage can cause. USDA and Rodale Institute results show that soil organic matter in a robust rotation, with tillage, can actually improve more than under a chemical no-till system without cover crops.

Turning to cultivation may be a bridge to more holistic weed management for some farmers who have to find new ways to deal with glyphosate-resistant weeds to get a harvestable crop. Melvin Tucker, territory manager for Kelley Manufacturing in Georgia and South Carolina, reports an increase in wide-sweep cultivator sales of what he believes to be 100 percent in the past five years. Prior to that, sales were practically zero because of chemical herbicide use. Then a strain of pigweed found commonly in peanut and cotton fields in the South developed a resistance to glyphosate. The problem was serious enough that some farmers faced losing half of their crop, or more, to the weeds.

While he says that in his region “most farmers would rather just spray,” Tucker said the glyphosate-resistant pigweed problem was so rampant in 2008 that cultivator demand during the season exhausted the company’s supply until they returned to cultivator fabrication over winter for 2009. Tucker explained that many farmers had to resort to sharing available cultivators farm to farm.

Although the Association of Equipment Manufacturers predicted a 4.4 percent increase in cultivator sales for 2008 (one of only four farm implement types with a predicted increase), but Wayne Buck, Sales Manager for Hiniker Company, estimates that in the past few years cultivator sales have increased 10 to 12 percent. He explained one reason for the increase was the rise in weeds resistant to glyphosate.

Learning the basics

An issue faced by many farmers returning to, or initiating, mechanical weed management is finding the tool that fits their circumstances, and knowing when and how to use it. Experienced farmers say this is as much an art developed over time as a mechanical skill that can be quickly described or learned. There’s help available to learn this approach as described in the sidebar Learning the ropes of mechanical weed control.

Over a period of years, using the smartest sustainable strategies and tools for non-chemical weed management results in fewer off-farm expenses, decreases risk of undesirable herbicide impacts, improves management skills, improves soil health and resilience, as well as increases control of crop outcomes.

"Ironically, cultivators are experiencing a potential renaissance as more farmers and researchers recommend them as a companion to the herbicides that were engineered to replace them."

Steel in the field, considered for a time to be a symbol of the past, is returning along with more sophisticated farmer management of cropping systems. Before the rise of herbicides, skillful cultivation was a tool in every crop producer’s arsenal, as farmers relied more on their land and local farmer knowledge than on distant corporate laboratories for guidance. Ironically, cultivators are experiencing a potential renaissance as more farmers and researchers recommend them as a companion to the herbicides that were engineered to replace them, if not an outright replacement for chemicals as part of regenerative, sustainable systems.

For a generation of farmers raised on the idea that herbicides were the ultimate solution to the weeds, creeping levels of resistance to chemical control opens the post-chemical chapter in weed management. Agriculturalists around the world are looking for better answers than have come so far from herbicide-focused efforts. They seek productive systems based on evolving local farmer wisdom. These deal with all pests—weeds included—as part of an approach integrating soil health, biodiversity, advanced understandings of biological interactions, and just enough steel to give crops the edge they need.

Betsy McCann is a third year doctoral student in agricultural communications at Texas A&M University, serving this summer as a communications intern at the Rodale Institute.