What does "stacks" mean here?

Context:

On 19 September 2012, Séralini et al. published online in the scientific journal Food and Chemical Toxicology a publication describing a 2-year feeding study in rats investigating the health effects of genetically modified maize NK603 with and without Roundup WeatherMAX^® and Roundup^® GT Plus alone (both are glyphosate-containing plant protection products). As requested by the European Commission, EFSA reviewed this publication taking into consideration assessments conducted by Member States and any clarification given by the authors. The assessments of Member States and EFSA revealed an overall agreement. The study as reported by Séralini et al. was found to be inadequately designed, analysed and reported. The authors of Séralini et al. provided a limited amount of relevant additional information in their answer to critics published in the journal Food and Chemical Toxicology. Taking into consideration Member States’ assessments and the authors’ answer to critics, EFSA reaches similar conclusions as in its first Statement (EFSA 2012). The study as described by Séralini et al. does not allow giving weight to their results and conclusions as published. Conclusions cannot be drawn on the difference in tumour incidence between treatment groups on the basis of the design, the analysis and the results as reported. Taking into consideration Member States’ assessments and the authors’ answer to critics, EFSA finds that the study as reported by Séralini et al. is of insufficient scientific quality for safety assessments. EFSA concludes that the currently available evidence does not impact on the ongoing re-evaluation of glyphosate and does not call for the reopening of the safety evaluations of maize NK603 and its related stacks. EFSA’s evaluation of the Séralini et al. article is in keeping with its role to review relevant scientific literature for risk assessment on an ongoing basis to ensure that the advice it provides is up-to-date.
© European Food Safety Authority, 2012