Pasture-based milk is increasingly preferred by consumers owing to its perceived socio-economic, animal welfare and environmental benefits. However, nitrogen excretion from pasture-based dairy farming is also a large source of nitrogen leaching and emission of the potent greenhouse gas nitrous oxide. Ryegrass bred to express elevated concentrations of water-soluble carbohydrates ('high-sugar' grass; HSG) has been shown to decrease dietary nitrogen excretion in urine of cattle, and may increase milk yields per cow, but it is unclear how this translates to environmental footprints at the farm- and product levels. This study evaluates, for the first time, the environmental footprint of HSG dairy systems with life cycle analysis, measured as land occupation in addition to global warming, eutrophication, acidification and resource depletion potentials (energy-based and economic allocation methods). Data from meta analysis and simulation were combined to model a pasture-based dairy farm under a conventional perennial ryegrass-based scenario (Sc-CTR) and an HSG-based scenario (Sc-HSG). In addition, grass type interactions with six manure management permutations were considered, leading to 12 scenarios in total. It was found that eutrophication and acidification potentials per unit of energy-corrected milk could be reduced by 4-6% and 7-11% respectively when switching from Sc-CTR to Sc-HSG, and that these reductions could reach 22% and 40% respectively with more efficient manure management. It is concluded that a simple change in choice of grazing sward may deliver substantial environmental gains, especially when combined with improved farm technology. However, the high costs for improving manure management could drive expansion of HSG pastures as a more attractive short-term measure for farmers, while regulation and access to capital could drive investment in improved manure storage infrastructure and spreading equipment. (C) 2018 The Authors. Published by Elsevier Ltd.