What if the promise of higher yields was just another marketing myth?
One of the most persistent claims used to justify the widespread adoption of genetically modified (GM) crops and their partner chemical, glyphosate, is the promise of higher yields. Bigger harvests. More food to feed the world. But what if that story isn’t holding up under scrutiny?
Despite decades of marketing, several independent studies suggest that glyphosate-resistant crops—often referred to as Roundup Ready—may actually yield less than their conventional counterparts. And the reason might be baked into the very thing they were designed to tolerate: glyphosate itself.
Lower Yields in Roundup Ready Crops?
Let’s start with the data. A University of Nebraska study found that Roundup Ready GM soybeans yielded 5% less than their closest conventional relatives—and a full 10% less than high-performing conventional lines. That’s not a rounding error. That’s a significant shortfall in productivity.
Why would this be happening?
One potential clue comes from a 2007 study by Barney Gordon, titled Manganese Nutrition of Glyphosate-Resistant and Conventional Soybeans [PDF]. His research uncovered that glyphosate—while designed to kill weeds—was also inhibiting the uptake of key micronutrients like manganese in the soybeans themselves.
Without manganese and other trace elements, plant metabolism slows down, disease resistance weakens, and yields decline. In short, even when the plant survives glyphosate, it might not thrive.
Productivity at What Cost?
This flips the whole yield narrative on its head. Glyphosate-resistant crops were never engineered to be more nutritious, more climate-resilient, or more biodiverse—they were designed to survive being sprayed with Roundup. And that comes at a cost.
In many cases, these genetically modified crops become dependent on multiple chemical inputs to counteract the very deficiencies caused by glyphosate: additional fertilizers, fungicides, and even more herbicides when superweeds develop. What’s marketed as efficiency starts to look more like a chemical treadmill.
These costs don’t end with lower yields. They ripple outward—into degraded soil health, declining nutrient density, and increasing glyphosate residues in the food we eat.
Why This Matters for New Zealand
Here in New Zealand, we’re being told that glyphosate use is under control, that it’s safe, and that it supports our food production systems. But if the very crops it enables are producing less, then what exactly are we gaining?
It’s worth noting that New Zealand does not currently permit the commercial growing of genetically modified (GM) crops. However, the proposed Gene Technology Bill, now before Parliament, could open the door to future changes. If GM crops enter our landscape, will they bring the promised productivity—or just more chemical dependence and higher residue levels?
More to the point—what might we be sacrificing next?
MPI’s recent proposal to raise allowable glyphosate residue levels (MRLs) on certain imported and local food crops has been framed as necessary for “trade harmonization.” But if those crops were never outperforming traditional varieties in the first place, why are we tolerating higher chemical residues on our food?
If the productivity claims don’t hold, and the safety claims are shaky, then perhaps it’s time we revisit the basic assumption: Is glyphosate even helping?
Rethinking the Trade-Off
The argument for glyphosate has long rested on efficiency and yield. But when we dig deeper, that story starts to crack. Evidence suggests that glyphosate may be doing more harm than good—not just to our health or environment, but to the very goal it was supposed to serve: growing more food.
Maybe it’s time to stop accepting the sales pitch and start asking better questions.
Resources & References
Before You Dismiss the Data…
It’s easy to write off claims like “glyphosate reduces yield” as just anti-GMO rhetoric — until you dig into the research yourself. Below are independent studies and expert analyses that have challenged the dominant narrative around GM crop productivity. These aren’t blog posts or opinion pieces — they’re published data, field trials, and agronomic reports. If the promise of “higher yields” is what’s keeping glyphosate on our fields and in our food, shouldn’t we take a closer look?
Research Shows Roundup Ready Soybeans Yield Less
University of Nebraska – (May 2000)
A news release summarizing two‑year trials (North Platte, Clay Center, etc.) showing Roundup Ready soybeans yielded 6% less than near-isolines and 11% less than high-yielding conventional lines
URL: https://www.nlpwessex.org/docs/soyatrialnebraska.htm
Evidence of the Magnitude and Consequences of the Roundup Ready Soybean Yield Drag
Charles Benbrook (1999)
An analysis of over 8,200 varietal trials from eight U.S. states in 1998, finding an average yield drag of 5.3–6.7% in Roundup Ready soybeans .
URL: Evidence of the Magnitude and Consequences of the Roundup Ready Soybean Yield Drag from University Based Varietal Trials in 1998
Manganese Nutrition of Glyphosate‑Resistant and Conventional Soybeans
Barney Gordon (2007)
A controlled field study in Kansas revealing glyphosate-resistant soybeans had less than half the leaf manganese of conventional isolines and suffered yield losses of about 7 bushels/acre. Manganese supplementation reversed the yield gap .
URL: https://fluidfertilizer.org/wp-content/uploads/2016/05/Barney-Gordon-1.pdf
Manganese Management in Glyphosate-Resistant Soybean Cropping Systems
Purdue & Indiana Soybean Studies (2007–08)
Confirmatory research showing glyphosate disrupts manganese uptake and soil microbe activity—leading to early-season Mn deficiency in Roundup-resistant soybeans .
URL: https://www.agry.purdue.edu/cca/2008/proceedings/vyn.pdf
Economic & Environmental Impacts of First‑Generation GMOs
IISD (2008)
A review noting that early Roundup Ready soybean varieties showed yield drag, though newer varieties have reduced it, but still lag behind top conventional cultivars
URL: Economic and Environmental Impacts of First Generation Generation Genetically Modified Crops
Maize yield stability under organic and conventional farming systems in sub-humid agro-ecozones of Central Kenya
2025, European Journal of Agronomy
A long-term study (2007–2022) comparing organic and conventional maize systems in Kenya’s sub-humid agro-ecozones. While conventional systems initially outperformed, organic yields gradually caught up—and in lower-fertility sites, organic systems showed greater yield stability. The findings underscore the resilience potential of organic practices in degraded soils.
URL: https://www.sciencedirect.com/science/article/pii/S1161030125002424
Rodale Institute – Farming Systems Trial (ongoing since 1981)
A long-term, side-by-side comparison of organic versus conventional cereal cropping systems. After a five-year transition, organic yields match or exceed conventional, particularly during droughts (up to 31% higher). It also shows significant advantages in soil health, water retention, energy use, and greenhouse gas emissions—all without glyphosate or synthetic herbicides.
URL: https://rodaleinstitute.org/science/farming-systems-trial/
Further reading:
The following books are linked to Amazon.com for your convenience. If you decide to purchase through these links, we may earn a small commission — at no extra cost to you.
Glyphosate and the Swirl: An Agroindustrial Chemical on the Move
By Vincanne Adams (2023, Duke University Press)
Medical anthropologist Adams explores glyphosate’s global journey—its corporate origins, shifting scientific consensus, and contested safety narratives. A timely look at how glyphosate’s reputation is shaped by competing stakeholders in the food system.
Glyphosate and the Swirl [amazon.com]
Whitewash: The Story of a Weed Killer, Cancer, and the Corruption of Science
By Carey Gillam (2017, Island Press)
An award‑winning exposé that uncovers Monsanto’s campaign to suppress harmful glyphosate research, revealing corporate influence on regulators and attacks on scientists. A must‑read for context on institutional cover‑ups.
Whitewash [Our review]
Toxic Legacy: How the Weedkiller Glyphosate Is Destroying Our Health and the Environment
By Stephanie Seneff (2021, Chelsea Green)
MIT researcher Seneff compiles evidence linking glyphosate to gut dysbiosis, chronic disease, and environmental harm, with clear explanations and practical recommendations. Rises to the level of a modern Silent Spring.
Toxic Legacy [Our review]
Something to Think About…
If science is meant to evolve with new evidence, then clinging to outdated claims about glyphosate and GMOs serves no one — except maybe those profiting from the status quo. The studies below raise questions mainstream discourse has largely ignored. You don’t have to take our word for it. Read them. Question them. Decide for yourself.
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