Perhaps you missed it? Back in July, the commercials that once plagued our evening television commercial space stopped. Have you noticed? There was a last-chance type message to get in on the class-action Roundup lawsuit before it ended. In the settlement, Bayer agreed to pay out $10 Billion in settlements in addition to several suits that are still tied up in appeals. This also resulted in the active ingredient in Roundup being pulled from the ready-to-use (RTU) products and other homeowner versions of the popular herbicide. Currently, Roundup Brush Killer is the only formulation that I have seen on the shelves still having glyphosate as an ingredient. In my training last week, we were made aware that this will only last as long as the current supply holds out.

So why has this happened? That is the million-dollar question. This is a very interesting situation in which the courts have decided the science more so than the regulatory agencies around the globe (Schreiber, 2024). Not that we should all blindly trust the regulatory agencies. However, this controversy has taken one of the most benign products, in terms of toxicology, we have ever had away from us as home gardeners (Brosnan et al., 2024).

There are a couple of ways that regulatory agencies around the world define a herbicide’s toxicity. There is acute toxicity and chronic toxicity (Vollmer, 2022). Acute toxicity can be expressed in terms of LD₅₀ or the dose required to kill 50% of a population (Breeden et al., 2015). The LD₅₀ for glyphosate is 5,600 mg/kg of body weight. For comparison sake, the LD₅₀ of caffeine is 192 mg/kg (Vollmer, 2022). In this case, the bigger the number, the less toxic a herbicide is to us.

Chronic toxicity is expressed as the mg/kg of body weight a person can be continually exposed to without experiencing an adverse effect or the No Observable Adverse Effects Level (NOAEL) (Vollmer, 2022). The NOAEL is then given two ten-fold safety factors, one for interspecies variability and one for within-species variability, to establish the Acceptable Daily Intake (ADI). The ADI for glyphosate is 100 times lower than the actual NOAEL (Vollmer, 2022). To take this discussion one step further, there is only one agency in the entire world that has classified glyphosate as “Probably Carcinogenic to Humans”. This is the International Agency for Research on Cancer.

The problem here is that IARC did a hazard assessment as opposed to a risk assessment (Schreiber, 2024; Vollmer, 2022). A hazard assessment, simply put, assesses if the compound can cause an adverse issue regardless of concentration. A risk assessment, in turn, assesses the actual risk of that exposure to occur. The result was IARC classifying glyphosate as a type 2A Probable Human Carcinogen. Other notable compounds that have made the type 2A list are red meat, shift work, and hot beverages. To date, no other regulatory agency in the world has classified glyphosate as such (GLP, 2024).

 Why is this important to us? The formulation for Roundup has changed and given the information above, not necessarily for the better. The new formulation, Roundup Weed and Grass Killer, has replaced glyphosate with the active ingredients triclopyr, fluazifop, and diquat. While there are multiple active ingredients available under the Roundup label, this is what has replaced what we would have deemed the original.

Leaves of muscadine grapes affected by synthetic auxin herbicide. This is very similar to what we may see with the active ingredient triclopyr. Left to right: healthy leaf to progressively worse symptoms in the youngest leaves on the right. Muscadines at the Beaufort County Center, Gene Fox, 8/14/2024.

The problem with these is just simply that they are not glyphosate. I forgot to mention that Roundup chemically adheres to soil particles, as such, it is basically non-mobile in the environment with essentially no residual activity (Gannon, 2019). Diquat is a contact herbicide that controls broadleaf and grassy weeds. The acute toxicity is much greater than glyphosate ranging between 30 and 400 mg/kg (Brosnan et al., 2024). Diquat essentially has no residual activity as it is readily adsorbed to soil particles and unavailable for plant uptake. Fluazifop (pronounced flu-az-a-fop) is a selective grassy weed killer having no effect on broadleaf weeds. The LD₅₀ ranges from 2721 to 4096 mg/kg. Fluazifop is adsorbed to grass leaves but also has soil residual activity for 14 days after application. This means you cannot plant any plants in the treated area for 14 days (Brosnan, et al., 2024). Triclopyr is an auxin mimic with intermediate toxicity (LD₅₀ 713 mg/kg). It is not adsorbed to soil particles and has a half-life of up to 46 days meaning it is residual in the soil for at least 14 days (Brosnan et al., 2024). It is most often used to treat woody vegetation and unwanted vines like poison ivy in forestry products such as Garlon. This active ingredient kills fish and has been associated with groundwater contamination according to the Garlon 4 Ultra herbicide label.

All of this means that the Roundup we are used to using has changed. We need to be very careful applying around our fruit trees, muscadines, vegetables, and ornamentals. This is not the product we have grown to use for nearly all of our weed-killing needs, this is different. We need to be aware and know how and where to use this and other new products under the Roundup label. Please read the label and understand the differences between these new products and the old products.

If you are having an issue in your home garden or landscape, send your questions to Gene Fox, Consumer Horticulture Agent with the North Carolina Cooperative Extension Service, please email Gene at gene_fox@ncsu.edu or call at (252)946-0111. Check out our Facebook page, Beaufort County Master Gardeners, for The Plant of the Week and Finding Info Friday! We are having an Extension Master Gardener Volunteer training starting in September. Visit our website and look under How To Become a Master Gardener for more information. Until then, Happy Gardening!

Table 1. Roundup-branded herbicides available to consumers. Reproduced from Brosnan et al., 2024.

Table 2. LD₅₀ values of common substances. Reproduced from Vollmer., 2022.

Table 3. Just a few of the more commonly encountered substances that are classified by IARC as Group 2A Probable Carcinogens. Anon, 2024.

Citations:

Anon. 2024. “International Agency for Research on Cancer List of Classifications.” IARC Monographs on the Identification of Carcinogenic Hazards to Humans. Retrieved August 13, 2024.

Breeden, Gregory K., James T. Brosnan, Greg Armel, and Joseph Thomas. 2015. “Safety of Herbicides Compared to Other Commonly Used Chemicals.” Turfgrass Science at the UT Institute of Agriculture – W270. Retrieved August 13, 2024.

Brosnan, Jim, Natalie Bumgarner, Rebecca Bowling, Greg Breeden, and Celeste Scott. 2024. “Update on Roundup-Branded Herbicides for Consumers.” University of Tennessee Institute of Agriculture. Retrieved August 13, 2024.

Gannon, Travis. 2019.“Optimizing Pesticide Applications – And Dealing with Myths Around Pesticide Use.” Presented at the ACRE Squared, September.

Schreiber, Kayleen. 2024. “Infographic: Global Regulatory and Health Research Agencies on Whether Glyphosate Causes Cancer.” Genetic Literacy Project. Retrieved August 13, 2024.

Vollmer, Kurt. 2022. “Understanding Glyphosate and Other Pesticides (FS-1193).” Agriculture and Food Systems – FS-1193. Retrieved August 13, 2024.