The aim of this study was to compare the effect of changing a range of biological traits on farm profit and greenhouse gas emissions (GHG; expressed as carbon dioxide equivalent, CO2-eq.) for dairy cows in Northern Ireland, and also in the whole of the UK. An average cow was modelled for each population of animals, using average values from milk recording records. Previous work developed a dynamic model, to include nutrient partitioning to allow investigation of GHG abatement options over an animal’s lifetime. A Markov chain approach was used to describe the steady-state herd structure, as well as estimate the CO2-eq. emissions per cow and per kilogram of milk solids (MS). The effects of a single phenotypic and genetic standard deviation change in a range of production and fitness traits were assessed. For each dairy cow population, the study will identify traits that will improve production efficiency by bringing about a desirable increase in profit, and reduce average CO2-eq. emissions per cow and per kg MS of herds. Selective breeding and appropriate management can both potentially improve health, fertility and feed utilisation of dairy systems and reduce its environmental impact.