Using testicular cells of rats for tests, they injected concentrations of one part million to ten parts per million of glysphosate, the active component in Roundup. This range of concentration was used to similate the exposures that might encountered in the normal activity of rats.
Even at the one part per million concentration level, the glysphosate was responsible for causing endocrine disruption that reduced testosterone levels by thirty-five percent. The one part per million exposure level is considered to be extreme low. At higher exposure levels, death occurred in as little one hour and as long as forty-eight hours.
Contrary to claims made the Monsanto, there is no safe level of application for Roundup. It has been show to destroy cells and cause reproductive harm.
The results of related research using similar procedures are discussed.
The link to this paper is found here.