Effect of light intensity on daily and hourly activity patterns of growing-finishing pigs

Domestic pigs are diurnal animals, and light exposure affects their physiological processes and consequently behaviour. Light intensity, as one aspect of light, has been shown to affect activity levels, including behaviours such as locomotion and lying. However, previous studies have tested a relatively narrow range of light intensities and primarily focused on overall daily activity or inactivity rather than examining distance moved or hourly activity patterns over a full day. Therefore, the objective of this study was to assess the effects of different light intensities on daily and hourly activity patterns of pigs. An experiment was conducted on a commercial farm during a 12-week growing-finishing phase with a total of 448 pigs (approximately 10 weeks old). These pigs were divided into four treatments: low intensity (45 lux, considered the baseline slightly exceeding European Union (EU) requirements), medium intensity (198 lux), high intensity (968 lux), and a spatial gradient (from 71 to 330 lux within a pen). Active behaviours and pig postures were manually observed from video recordings every 2 weeks using scan sampling at 20-minute and 1-h intervals, respectively, both during the light and dark periods. In addition, postures and distance moved were assessed per second using a You Only Looked Once (YOLO) v8x-based detection model. Daily levels of active behaviours (defined as moving or interacting with pen mates or environment regardless of the posture, observed manually), proportions of active postures (standing and sitting, both manually observed and automatically detected) and distance moved (automatically detected) were analysed using generalised linear mixed models. Hourly patterns of distance moved and active postures were compared between light intensities using generalised additive mixed models. Light intensity influenced daily activity levels (significant light by week interaction), but the few specific differences observed between light intensity treatments varied inconsistently across the experimental weeks. Pigs in high light intensity had a different hourly activity pattern, showing reduced afternoon activity peaks compared to pigs in other treatments. Pigs in the medium intensity also showed a different hourly pattern of active postures, with clearer morning and afternoon activity peaks compared to pigs in other treatments. Pig activity in low light intensity and spatial gradient treatments did not differ from other treatments. To conclude, light intensity effects on active behaviour varied over weeks and affected activity patterns but not daily activity levels; therefore, light intensity effects on activity remain ambiguous.