Comment: Soil scientist Dr Jacqueline Rowarth looks into the controversy around glyphosate, the active ingredient in Roundup.

Glyphosate, the active ingredient in Roundup, is the most widely used herbicide in the world. It affects a specific enzyme pathway in plants and so stops growth.

Concerns about human health have been raised in the media, and various local government bodies are choosing other methods to destroy weeds. The evidence, however, is that when used as directed, glyphosate remains one of the safest ways to destroy weeds. This article discusses the evidence.

The Facts


• No issues with human health where studies had been conducted according to scientific principles were found by the United States Environmental Protection Agency in a review of open literature in 2017.

• Fifteen agencies around the world have indicated that there is no reason to ban glyphosate from use, including the New Zealand Environmental Protection Authority (EPA).

• All agencies advise that instructions for use should be followed whatever substance is being used.

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The Concern

Concern about the use of glyphosate has escalated because the International Association for Research on Cancer (IARC) classified it as a 'probable human carcinogen' in 2015.

The same year, the European Chemicals Agency (ECHA) Committee for Risk Assessment stated that "on the available scientific evidence, there were no grounds to classify the controversial herbicide, glyphosate, as a carcinogen, as a mutagen or as toxic for reproduction".

ECHA examined all the information that was available, including human evidence and "the weight of the evidence of all the animal studies reviewed".

The difference in the outcome of the two reports reflects the fact that IARC classify chemicals according to hazard whereas ECHA considers "risk" in its evaluation.


The difference between hazard and risk is that risk considers likelihood.

For example, the sea is a hazard. People minimise risk by wearing lifejackets, staying between the flags, and checking the weather forecast. People are less likely to encounter hazardous conditions if they take the precautions to minimise risk.

IARC have classified tobacco, UV radiation and ethanol in alcoholic beverages as Category 1 "known human carcinogen".

Risk is minimised by not smoking, wearing sunblock and limiting intake of alcoholic drinks, respectively.

Listen to Jamie Mackay's interview with Dr Jacqueline Rowarth on The Country:

Any risk in the use of glyphosate is minimised by diluting the chemical as instructed and using personal protection gear as advised.

The guidelines for domestic use include wearing rubber gloves, a face mask and choosing a calm day for spraying.


The same applies to using any other chemical for weed control, including vinegar. Note that household vinegar is generally too weak to be of use.

Trials have shown that the 5 per cent concentration of acetic acid known as vinegar has little effect on weed growth.

A 20 per cent concentration can burn off up to three quarters of weedy material but can also burn skin and eyes.

Furthermore, there is no long-term effect of acetic acid on plants at any concentration… more frequent spraying with the acetic acid will be required than if glyphosate was used.

In 2016, Bristol City Council reported that using vinegar was 3.6 times more expensive and far less effective (weeds reappeared within a month instead of five to six months) than using glyphosate.

Contractors were also unwilling to use vinegars they were "afraid of complaints about the smell".

Dr Jacqueline Rowarth. Photo / Supplied
Dr Jacqueline Rowarth. Photo / Supplied

The Court Case

Confusion about the use of glyphosate persists partly because of the high-profile court case in California in 2018.

The judgement hinged on the fact that Bayer/Monsanto had to prove that glyphosate hadn't caused the groundsman's cancer (Non-Hodgkin's lymphoma); the groundsman did not have to prove that glyphosate had caused the cancer.

Dr Andrew Kniss, Professor of Weed Science at University of Wyoming, has calculated that 97 per cent of people with non-Hodgkin's lymphoma have had no exposure to glyphosate.

A large research project in the US involving almost 55,000 people, 83 per cent of whom had used glyphosate, reported in 2017 that "glyphosate was not statistically significantly associated with cancer at any site".

The authors did note an increased, but statistically non-significant, risk of acute myeloid leukaemia (AML) in the study in the highest exposure quartile compared with "never users". AML can arise during non-Hodgkin's lymphoma treatment.



Chemical companies spend time and money evaluating new products to ensure that they work on the target while having no impact on human health and the environment.

Phillips McDougall reported in 2016 that only one in 160,000 chemicals achieved registration between 2010 and 2014.

The cost was up to NZ$300 million to research, develop and register a new crop protection product and the average time for the process was estimated at over 11 years.

In addition to tests in the country of development, new products for New Zealand must comply with laws here.

These are the Hazardous Substances and New Organisms Act (EPA) and the Agricultural Compounds and Veterinary Medicines Act (Ministry for Primary Industries (MPI)).


Three years of testing and trials must be completed locally over multiple seasons and in different regions of the country. Costs were estimated by Agcarm in 2015 to be at least $250,000 but often around $750,000.

These trials must prove the efficacy of the product, and measure residues as well as generating health and safety data. The data from the trials are submitted to the EPA and the MPI to support an application. Approval is required from regulators before the product can be marketed.

The US research investigating the health of almost 800,000 agricultural workers and their families found that main illnesses were hypertension, diabetes mellitus and mental health.

Cancer was not found.

This research indicates that the current approval process is sufficient to ensure human safety.

- Dr Jacqueline Rowarth CNZM CRSNZ HFNZIAHS has a PhD in Soil Science and has held various academic and government positions across the environment, agriculture and business spectrum. She is currently a farmer-elected director on the boards of DairyNZ and Ravensdown.