The main findings of the study were severe multiple organ damage in rats fed the GM maize and low levels of Roundup, both separately and in combination. These effects did not become evident until after the end of the 90-day trial length favoured by industry and regulators.

1. Toxicological findings: The main findings were multiple organ damage in rats fed the GM maize, whether or not the crop had been sprayed with Roundup, and independently, in rats fed low levels of Roundup in drinking water.

Biochemical analyses of blood and urine was conducted on the samples taken at 15 months, the latest timepoint when at least 90% of the rats were still alive in each treatment. Statistically significant damage was found to mammary tissues, liver, kidneys, and pituitary glands of the rats fed the GM maize grown with and without Roundup, and in the rats given Roundup in drinking water.

Thus, rat health was impacted – both separately and jointly – by GM maize and Roundup. Roundup impacts were recorded at exposure levels permitted in animal feeds in the US and in drinking water permitted in both the US and EU. 

2. Mortality and tumours: Unexpectedly, both the timing and rate of mortality and tumour growth were affected by the treatments in this study.

Mortality reflected both spontaneous deaths and euthanasia due to tumours that impeded functions such as breathing, nutrition, and digestion. Spontaneous deaths accounted for most male mortality, while euthanasia accounted for most female mortality during the study.

Male and female rats responded differently to the GM maize and Roundup treatments. Whereas 30% of control males and 20% of control females died before the mean survival time, up to 50% of males and 70% of females died prematurely in some groups containing GM maize. However, the rate of mortality did not increase proportionately with the treatment dose, reaching a threshold at the lowest dose (11%) or, for some groups, the mid dose (22%) of GM maize, both with and without Roundup spraying during treatment.

In males, the maximum difference between treatment groups and controls was 5 times more deaths occurring during the 17th month in the group consuming 11% GM maize, and in females 6 times greater mortality during the 21st month on the 22% GM maize diet with and without Roundup. In the female treatment groups, there were 2–3 times more deaths compared with controls by the end of the experiment, and these occurred earlier. Females were more sensitive to the presence of Roundup in drinking water than males, as evidenced by a shorter lifespan. The most common causes of death were linked to large mammary tumours in females, and liver and kidney damage in males.

Three types of tumours were reported: non-regressive palpable tumours (NRPT), small internal tumours, and metastatic (spreading to other parts of the body) tumours. As with mortality, tumour incidence appeared to vary between male and female rats. Small internal tumours accounted for most tumours in males, and roughly half of those in females, although the proportion varied among treatments. NRPT in female rats were largely mammary tumours. Metastatic tumours were rare.

None of the treatments affected incidence of small internal or metastatic tumours, but all treatments increased NRPT as compared to the control. Furthermore, NRPT began to occur earlier in treated than in control rats. For male and female rats, respectively, the first NRPT occurred at about 700 and 400 days in controls, compared with 100 and 200-300 days in GM maize treatments, and at around 530–600 and 200–400 days in Roundup-dosed water treatments.

As explained in subsequent sections, this was not a carcinogenicity study but a chronic toxicity study, but tumour occurrence is relevant for two reasons. First, researchers are required to report tumours even in toxicity studies, according to the chronic toxicity protocol set by the Organisation for Economic Cooperation and Development (OECD).[1] Second, specific types of tumours may also be an indicator of specific metabolic dysfunctions to be explored in further studies.

Nonetheless, of pivotal importance to GMO safety testing is the timing of the tumour onset. The first NRPTs were detected at 4 and 7 months for male and female rats respectively, with most tumours occurring after 18 months. This illustrates the futility of relying on 90-day feeding trials to detect potential risks from chronic exposure to GMOs and their associated pesticides.


[1] Organisation for Economic Cooperation and Development (OECD) (2009). OECD guideline no. 452 for the testing of chemicals: Chronic toxicity studies: Adopted 7 September 2009. Note: The OECD uses the word “lesions”, which would include tumours.

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