In Norwegian dairy farming, the usage of automatic milking systems (AMS) has increased during the last decades. AMS is primarily a productivity increasing and labor reducing technology, but previous research shows that AMS can have other (secondary) effects that could impact the environmental performance of farms. Effects of AMS-adoption found, include increased total production, increased farm size, changes in grazing patterns and feed mix and changes in energy consumption. This paper aims to study the hypothesis that AMS-adoption, through these secondary effects, affect farm-level GHG-emissions. Using a difference-in-difference approach, we provide evidence of the presence of secondary effects and shows that AMS-adoption negatively affects farms’ eco-efficiency, particularly by increasing enteric fermentation. The causal effect is identified by considering adopting farms and non-adopting farms observed at two periods in time. Apart from providing this empirical result, the paper also presents a general procedure of how to go about evaluating farm-level effects of technology adoption.