Effects of stocking density during a livestock export voyage on sheep behaviour, liveweight, indicators of stress, and responses to environmental heat load

Context Stocking density can impact animal welfare during livestock export voyages. Aims This study assessed selected welfare and heat stress measures for sheep housed at two allometric stocking densities (k = 0.033 and 0.042) during a voyage from Western Australia to Kuwait. Methods Merino wethers (n = 479) were loaded onto the vessel and housed in eight pens in groups of 57–66, for a 14-day voyage. Scan sampling of the number of wethers standing was conducted at 3-hourly intervals on Days 1–12 using video footage. For a subset (five per pen; ‘focal’ wethers), liveweights were recorded on Days 1 and 13, and faecal glucocorticoid metabolite concentrations were assessed on Days 0 and 13. Focal wethers had a rumen bolus inserted on Day 0 to record rumen temperature. Respiration rates (RR) and open-mouthed breathing were recorded for focal wethers three times daily. Air ammonia was measured three times daily, and faecal pad assessments were conducted daily. Key results Additional pen space decreased the number of sheep standing over the first 4 days, but stocking density had limited effect on lying synchrony. The effects of stocking density on RR were unclear, and estimates for the effects on open-mouthed breathing and rumen temperature were consistent with small effects at most. Higher wet-bulb temperature led to increased RR and an increased likelihood of breathing with an open mouth. For both rumen temperature and RR, some wethers had lower (and others higher) average values, and some displayed smaller (or greater) increases for every 1°C increase in wet-bulb temperature. Understanding how sheep differ in their rumen temperature and RR, and in their responses to hot conditions may be useful for facilitating sheep selection and management before and during voyages. Conclusions The biological cost of attempting to cope with restricted pen space was relatively low and achieved with minor behavioural changes only, but attempting to cope with the warm conditions seemed more challenging for sheep in this study. Implications This work indicates usefulness in identifying and selecting sheep that are more able to cope with hot and humid conditions for export.