By: Stu Ellis
For years, hundreds of agronomists and soil scientists have convinced farmers that reduced tillage not only is good for the soil and crops, but is more economical than multiple tillage operations. And they have been successful, because tillage has declined radically and sediment loss through erosion has been significantly reduced. But now the US farmer and his machine shed devoid of any tillage equipment now has a mess on his hands. Without a way to mechanically clear away certain weeds, and with an increasing number of weeds that are immune to glyphosate, many farm fields now have perennial problems with weeds that are becoming resistant to glyphosate and other herbicides. Ooooops.
The problem began innocently enough, since centuries of agriculture were rooted in the theory that soil inversion between crops was good for the soil and the ensuing crop. All of our grandfathers were applauded if corn stalks were buried out of sight. Following World War II the chemical industry began to produce herbicides that helped with weed control along with conventional tillage, which interrupted weed growth. But along the way the weeds got smart, and their genetic changes allowed germination to be triggered by sunlight or being near the surface of the soil as tillage churned them upward, says CAST, the Council for Agricultural Science and Technology. In the latest CAST issue paper authors say weeds not only adapted to tillage but have adapted to herbicide chemistry, and a variety of actions need to be implemented from USDA on down.
The creation of the Soil Conservation Service was designed to reduce soil erosion and water runoff, which developed into conservation tillage that allows at least 30% of the soil surface to be covered by plant material such as corn stalks. That was distilled into no-till, and refined to mean that no more than 25% of the soil surface could be disturbed for either planting or nutrient injection. That kept weed seeds near the surface for future germination. The evolution of cropping systems created glyphosate-resistant crops through genetic engineering and the elimination of the need for tillage and cultivation equipment. In the process, weeds have not only become tolerant of tillage practices, but tolerant of the new chemistry. And the CAST researchers report, “In addition, federal policies and programs in many instances play a significant role in the election of crops and conservation programs that are implemented. These governmental programs in turn can play a major role in weed selection and weed management practices chosen in response to these weed populations.”
The Natural Resources Conservation Service, successor to the Soil Conservation Service, and the soil and water conservation districts worked national and locally to assist farmers in the implementation of conservation practices for soil and water. They have been focused in recent years on the prevention of tillage on highly erodible land. Using the tools provided by the Conservation Reserve Program and the Conservation Security Program, agencies provided incentives to take land out of production, which would encourage continued weed growth.
Less tillage, more herbicides, and more policies have lead to increased instances of herbicide resistance by weeds. 197 species of weeds are now reported to have resistance to some type of herbicide, with a third found in the US. Most of those have ALS resistance, Glyphosate resistance was being recorded prior to the development of GR crops in the mid-1990s. Its attractiveness stemmed from the herbicide, the job it did on non-resistance weeds, improvements to the soil, and the simplicity of using only one herbicide. CAST says, “Grower adoption of conservation tillage. With the evolution of HR weeds and the resultant inability to maintain weed control, however, the continued inclusion of conservation tillage systems is threatened.” And the CAST authors add, “Glyphosate-resistant weeds pose the greatest threat to conservation tillage since its adoption and in some instances have caused farmers to change their conservation tillage practices or, in a few more limited situations, have eliminated conservation tillage where it once thrived.”
Producers are now faced with the conflict of having to use tillage or soil conservation practices that allow herbicide resistant weeds to thrive. CAST reports that the close collaboration of FSA, SWCDs, and NRCS has developed sound conservation programs to save the soil and keep water from erosion. But, disagreement is inevitable believes CAST and as commodity prices rise many farmers may abandon their conservation efforts in an effort to plant more land into lucrative crops. Currently, organizational policies do allow specific efforts to eradicate weeds in no-till and other conservation-designated area, and reintroduces the soil conservation practices.
Diversity of herbicide use is the key to controlling weeds that are becoming tolerant of primary herbicides and overtaking crops being planted into areas that are non-productive because of weeds. Rotation of herbicides can be a valuable practice in an effort to avoid problems with the increasing number of herbicide tolerant weeds, but there are even problems with that practice. The CAST weed specialists outlined a trio of measures they said were abandoned when glyphosate arrived:
• herbicide tank-mixtures,
• rotation of alternative herbicides with glyphosate, and
• preemergence-applied residual herbicides.
“The changes in herbicide use were attributable to the initial effectiveness of glyphosate, the marketing message for the technology, and the belief by many that glyphosate resistance in weeds would never be a major concern. Rotation of herbicides, however, specifically when the mechanisms of herbicide action (MOA) are considered, is an important tactic to mitigate and manage herbicide resistant weed populations. If only rotation of herbicide modes of action is practiced as the tactic to manage herbicide resistant weed populations, however, the evolution of herbicide resistant weed populations will only be delayed.”
CAST endorsed mechanical weed control strategies as part of an integrated weed control program, particularly if no other herbicide approach is feasible. While the researchers say there are many cultural strategies, including: variable planting time and crop seeding rate; crop rotation sequence; planting configuration; choice of crop cultivar; nutrient management optimization; and cover crops, mulches, and intercrop/relay crop systems. But they say the adoption of these is fair to poor. And the risks that must be considered include weed management strategies along with mechanical control options.
With the growing resistance of weeds to many herbicides, the CAST researchers are calling for a national set of policies to be implemented:
• Herbicide-resistant weeds pose one of the most significant threats to soil conservation since the inception of the USDA NRCS.
• Some weed species have resistance to herbicides such that they have forced growers to include or intensify tillage if they are to remain economically viable in their farming operation. In most soil conservation tillage situations, however, the objectives of conservation tillage can still be met even in the presence of HR weeds.
• The NRCS and NACD must realize and support the value of developing integrated management programs, perhaps including tillage, for the management of GR weeds. Additionally, the NRCS and NACD should assist in determining when and how to implement tillage to complement conservation tillage systems, thus having minimal impacts on soil quality and the environment.
• The NRCS and NACD should work to qualify and promote HR weed best management practices (BMPs) in the suite of existing conservation programs such as the EQIP.
• The NRCS and NACD should strongly encourage HR weed BMPs to be high-priority practices qualifying for land stewardship programs.
• Stronger educational programs are needed that demonstrate how HR weeds can be best managed without losing the tremendous conservation gains attained in recent decades.
• More research is needed on how to best meet the needs of HR weed management, while at the same time meeting soil conservation compliance goals.
The resistance of weeds is growing to many herbicides and many farmers have abandoned practices that either use tank mixes of different herbicides, pre-emergent herbicides, or rotation of different herbicides. Agricultural policies have been to move away from mechanical weed control as part of the effort toward conservation tillage. However, tillage and cultivation need to be part of the program to eradicate herbicide resistant weeds.