The effects of stocking density on behavior and biological functioning of penned sheep under continuous heat load conditions

• Published in: Journal of animal science Vol. 101
• Main Authors: Mayes, Bonnie T, Taylor, Peta S, Cowley, Frances C, Gaughan, John B, Morton, John M, Doyle, Brendan P, Tait, L Amy
• Format: Journal Article
• Language: English
• Published: US Oxford University Press 03-01-2023
• Subjects: Adrenal glands; Animals; Behavior, Animal; Blood; Climatic conditions; Density; Diurnal variations; Feeding Behavior; Glucocorticoids; Hot Temperature; Housing and Management; Humidity; Intervals; Livestock housing; Male; Metabolites; Necropsy; Respiration; Sheep; Sheep, Domestic; Stocking; heat stress; allometry; live export; sheep welfare
• ISSN: 0021-8812; 1525-3163
• DOI: 10.1093/jas/skad223

Abstract
Stocking density may impact sheep welfare during live export voyages that occur under hot and humid conditions. The aim of this study was to assess the welfare implications for sheep housed at three allometric stocking densities (k = 0.030, 0.033, 0.042), while exposed to hot and humid climatic conditions. For 21 d, Merino wethers (n = 216) were housed in 12 pens of 18 wethers, in two climate-controlled rooms where wet-bulb temperature (TWB) mimicked the conditions of a live export voyage with high heat and humidity, and limited diurnal variation. Scan sampling of standing and lying behaviors was conducted on days 2, 5, 8, 11, 15, 18, and 20, at hourly intervals. Agonistic interactions were scored continuously on the same days between 1750 and 1800 h. Liveweights were recorded at the start and end of the study. For a subset of focal wethers (3 per pen), whole blood variables were assessed at the start and end of the experiment, along with fecal glucocorticoid metabolites (FGCM), which were also assessed on days 7 and 14. Rumen temperatures (TRUM) of focal wethers were recorded at 10-min intervals, and their respiration rates (RR) were measured every 2 h on days 1, 3, and from days 7 to 21. Focal wethers were slaughtered for necropsy after the study, and both adrenal glands were excised and weighed. The expression of some lying positions was impaired at high stocking densities, and lying with outstretched legs increased at high TWB. For respiration rates, there was an interaction between stocking density and TWB, such that RR was reduced by the provision of additional space at high TWB. TRUM was relatively unaffected by stocking density but increased at higher TWB, and any effects of stocking density on FGCM concentrations, liveweights (LW), adrenal gland weights or blood variables were minimal. Necropsy examination showed no indication that the wethers had experienced ongoing respiratory distress. These results suggest that the wethers were able to cope with these increases in stocking density under the conditions imposed. However, based on this evidence, the provision of additional space under hot conditions may be beneficial to facilitating the expression of some lying positions. Whilst the experiment was designed to emulate certain conditions relevant during live export voyages, other factors that may induce stress during this mode of transport were not present, and so the conclusions must be interpreted in the context of the experimental conditions.
For sheep housed intensively under hot and humid climatic conditions, stocking density impacted lying behaviors, but had limited important effects on heat stress and physiology in the highly controlled experimental environment.
Lay Summary
There is a high demand for Australian sheep to be exported to the Middle East, and for live export voyages which depart during an Australian winter, heat, and humidity increase rapidly as ships cross the equator and approach destination countries. Concern about sheep becoming heat stressed during these voyages has increased, and industry attention has focused on the potential role of stocking density in determining heat stress risk in this context. High stocking densities limit the body surface area available for heat loss and can increase heat exchange between individual sheep. This study aimed to assess the welfare implications of three stocking densities, for sheep exposed to climatic conditions similar to those experienced during a live export voyage to the Middle East. Higher stocking densities restricted the ability of sheep to lie in some positions, but stocking density had limited effects on heat stress indicators or the physiology of the sheep. These results suggested that the sheep were able to cope with these increases in stocking density under the conditions imposed, but the conclusions must be interpreted in the context of the controlled experimental conditions.