Agriculture’s Greatest Threat

Weed Resistance. The concept has been communicated and threatened ad nauseam the last five years. When many farmers’ go-to chemical, glyphosate, became ineffective at controlling some of the most noxious pests, the issue transformed from coffee shop talk to “code red” all across the industry.  

When a weed species mutates and sits on the verge of taking over fields, the industry has responded by formulating new genetically modified crops that tolerate the next herbicide technology in line. While this has been an effective option up to the present day, we are reaching a critical point in the battle of weed control.  Our options are shrinking at an alarming rate and some weeds are becoming tolerant to chemistry in as little as three growing seasons. By titling this article “Agriculture’s Greatest Threat” I was being a bit dramatic, but we may find it to be true. If we lose our ability to control weeds in our fields using herbicides, where does that leave us? 

Chemicals kill weeds by disrupting an important process within the plant that is needed for survival. Some inhibit nitrogen metabolization, while others stop the plant from synthesizing amino acids, and the list goes on. This is going to get “nerdy” for a bit, but it is important to understand the difference between Mode and Site of Action. The Mode of Action is the end result that the chemical achieves while the Site of Action is the method by which the end result is accomplished. For example, the Mode of Action of both “Herbicide 1” and “Herbicide 2” may be pigment inhibiting. However, they do this in different ways. “Herbicide 1” may disrupt diterpene synthesis (a critical component of phytol which is used in chlorophyll) while Herbicide 2 Inhibits HPPD (an enzyme that breaks down the amino acid tyrosine into parts the plant can use).

For a weed to become resistant, it becomes resistant to a particular Site of Action. So, a plant species that is resistant to “Herbicide 1” may still be controlled using “Herbicide 2”.  In Iowa, and other parts of the U.S., waterhemp is a target of concern. In 2013, waterhemp had evolved resistance to six different sites of action and the list continues to grow. Groups 2 (ALS – Pursuit), 4 (2,4-D and potentially Dicamba), 5 (photosynthesis – Atrazine), 9 (EPSP – Glyphosate), 14 (PPO – Cobra), and 27 (HPPD – Laudis) are no longer effective at controlling this biologically enhanced form of waterhemp. 

There are many reasons these plants are so qualified at developing resistance to our efforts at killing it. Resistance is the outcome of rare traits within the plant so what starts out as a plant abnormality becomes its competitive advantage in its chemical laden world. A contributing factor to the fast mutation is that there are potentially billions of opportunities every season for an organism to develop that random mutated gene. This is particularly concerning because once a species develops resistance, there is nothing motivating it to mutate away from that characteristic – leaving us with more and more obsolete technologies. The following example will further show why it is so vitally important to kill these weeds before they go to seed.

Tall Water Hemp

Yearly Seed Production 300,000 to 5,000,000 per plant
Seed Life Potential Up to 17 years after production
Reproduction Can’t self-pollinate = high genetic diversity
Growth Rate 50-70% higher growth rate than other weeds
Emergence Timing ~20% initially, then 80% ~ten weeks later
Resistance Resistant to 6 different groups


These pests are relentless adapters and designed to survive in the harshest conditions. Once they become established in fields, they will outcompete cash crops leaving the plow as the only viable tool to reclaim that field.  Late-season germinating plants will do everything possible to set seeds quickly to establish the next generation. As farmers, we have to be proactive with our herbicide programs knowing that keeping the status-quo will result in weed control becoming more challenging, intensive, and expensive.

I’m not a trained agronomist or chemical rep so I will abstain from giving crop management advice. What I will recommend is that you go talk to your agronomist or consultant and tell them you want to consider anything reasonable to prevent weeds from going to seed. This may include some tillage, multiple passes across the field with the sprayer using multiple site of action herbicide programs, adding diversity in your crop rotation, and the list goes on. The internet is full of information so learn this stuff and master the ideas to develop strategies that fit your operation. Be sure to talk to your advisor.  They have probably dealt with local problems and been a component of developing local solutions. The era of Glyphosate has lulled many to sleep and now as an industry it is time to wake up before it is too late.

For additional information, visit these helpful websites:
International Survey of Herbicide Resistant Weeds
Take Action Pesticide Resistance Management