Australian rainfed cotton is grown in regions with highly variable rainfall, with in-crop rainfall ranging from 100 mm to 800 mm. The CSIRO cotton breeding program conducts rainfed germplasm evaluations at its core research site, as well as a number of regional locations. As a direct result of our variable rainfall environment, yields 550 kg ha?1 were not achieved in 27 seasons (18%). These 27 seasons underwent further simulations to determine the most suitable soil water content and crop growth stage where yield was increased with a single irrigation. It was determined that an irrigation should be applied to a ‘rainfed’ experiment if plant available water (PAW) reached approximately 40% by peak flowering (100-110 DAS). Paired rainfed and MSE experiments with 21 genotypes were established in 2013/14, 2014/15 and 2015/ 16 to provide field validation of the developed protocol. Genotype performance was assessed in terms of lint yield and fibre quality. Results show that in dry seasons (2013/14) irrigating the ‘rainfed’ treatment was necessary to reduce within experiment variability and increase yields above 550 kg ha?1 . However, once rainfed yield levels increase due to greater in-crop rainfall (2014/15 and 2015/16), irrigation was no longer necessary. This was further supported by the result that genotype yield ranking differed between rainfed and MSE treatments. Genotype changes in fibre quality between treatments were small. It was concluded that a MSE, designed to produce experimental data better matched to our breeding target environment as well as reducing the risk of experimental failure, would be a worthwhile addition to rainfed evaluations conducted in variable rainfall environments.