Environmental enrichment reduces displacements and reorganises circadian patterns: a putative indicator of positive welfare in dairy calves.

INTRODUCTION: Measuring positive welfare in farm animals at scale remains one of the greatest unsolved challenges in animal welfare. Here we demonstrate for the first time that environmental enrichment alters the diurnal activity patterns and aggressive behaviour of pre-weaned dairy calves, suggesting that both could function as automated indicators of an improved welfare state. METHODS: In this quasi-experimental study, ultra-wideband location collar-mounted sensors and automatic milk feeders were used to monitor the behaviour of 300 dairy calves across 21 cohorts, five of which received stationary brushes as environmental enrichment. The diurnality index, defined as the proportion of total activity occurring during daylight hours, was derived from location data, and aggressive interactions were quantified as feeder displacements using a validated algorithm based on visit timing to the feeder. RESULTS: Enriched calves showed higher diurnality (β = 0.26 95% credible interval [0.01, 0.49]) and performed fewer feeder displacements (long duration β = -0.25 [-0.49, -0.003]; short duration β = -0.26 [-0.51, -0.01]), indicating reduced feeder-directed competition and potentially improved access to milk replacer. Crucially, within the same analytical framework, diurnality decreased during sickness and increased with enrichment. DISCUSSION: This work provides the first evidence that biological rhythms function as a candidate bidirectional indicator of welfare state, contracting under negative experiences and expanding under positive ones. Alongside this, the reduction in feeder displacements demonstrates that social aggression could also be responsive to positive welfare interventions, together establishing that precision livestock technologies can detect not only welfare deterioration but also its improvement, opening a new frontier in automated welfare monitoring for farm animals.