Grazing environment plays a key role in the welfare of domestic herbivores. In the UK, donkeys are typically kept on species-poor, agriculturally improved grasslands that offer limited dietary and behavioural opportunities. Obesity, driven by low exercise levels and unsuitable diet, remains a primary welfare concern in this species. This study examined whether field habitat heterogeneity, measured as structural variation, botanical diversity, and sward height, affects donkey welfare. Over a six-month grazing season, three donkey herds rotationally grazed 10 fields that differed in their level of habitat heterogeneity. Animal herd welfare was assessed using Qualitative Behaviour Assessment (QBA) of 194 videos, each scored independently by three trained enumerators. Habitat heterogeneity had a highly significant influence on donkey herd emotional states. Fields with greater structural variation were associated with more energetic behavioural expressions, while taller swards were linked to calmer, more relaxed states. Taller grass may have reduced competition for resources, leading to a more relaxed herd, while structural variation created environmental differences that encouraged more energetic expressions. These findings demonstrate that increasing field habitat heterogeneity can promote positive welfare states in donkeys, highlighting the importance of integrating environmental or semi-natural habitat features into grazing management practices.