Animals utilise a wide range of regulatory systems to control the conditions within their bodies or homeostasis. These regulatory systems control for example, body temperature, nutritional state, water balance, social interactions and fear and these systems react to environmental and endogenous stimuli so as to correct or prevent displacements from the optimal range. The behavioural and physiological responses utilised by the animal are usually proportional to the challenge to homeostasis. The stress response commences with the central nervous system perceiving a potential challenge to homeostasis. Once the central nervous system perceives a threat, it develops a biological response or defence that consists of some combination of the four general biological defence responses: behavioural responses, responses of the autonomic nervous system, responses of the neuroendocrine system and responses of the immune system. Although biological regulation is occurring constantly, adaptation is not always possible and when homeostasis fails, there are biological costs for the animal, which may include growth and reproductive failure, injury, disease as a consequence of immunosuppression or even death. While animal comfort is not strictly defined in the scientific literature, an appropriate definition of comfort that is utilised in this review is a dictionary definition, “at ease in body and mind”.
With the likely trend to greater intensification of grassland production, there are a number of factors that will affect animal comfort. Both group size and space allowance are key features of the social environment that may affect animal comfort, while the nature of human contact is an important factor, which through fear, may also affect animal comfort. Thermal stressors can have significant impact on animal comfort with cold around parturition being a serious problem for the offspring and heat having adverse consequences for adult animal production and welfare. The concerns for animal comfort over husbandry procedures include practices ranging from simple interventions such as shearing that involve challenges including restraint, close human contact and isolation to more complex surgical interventions such as tail docking and castration that may include additional challenges such as acute and chronic pain, as well as short-term production depressions. Others issues include risks associated with animal biotechnologies, including simple manipulations such as twinning to more complex transgenic manipulations
