In recent decades there has been a growing concern about animal stress on intensive pig farms due to the undesirable consequences that stress produces in the normal physiology of pigs and its effects on their welfare and general productive performance. This review analyses the most important types of stress (social, environmental, metabolic, immunological and due to human handling), and their biological consequences for pigs. The physio-pathological changes associated with stress are described, as well as the negative effects of stress on pig production. In addition an update of the different biomarkers used for the evaluation of stress is provided. These biomarkers can be classified into four groups according to the physiological system or axis evaluated: sympathetic nervous system, hypothalamic-pituitary-adrenal axis, hypothalamic-pituitary-gonadal axis and immune system. Stress it is a process with multifactorial causes and produces an organic response that generates negative effects on animal health and production. Ideally, a panel of various biomarkers should be used to assess and evaluate the stress resulting from diverse causes and the different physiological systems involved in the stress response. We hope that this review will increase the understanding of the stress process, contribute to a better control and reduction of potential stressful stimuli in pigs and, finally, encourage future studies and developments to better monitor, detect and manage stress on pig farms.

Traditionally, research on farm animal welfare has mainly been focused on welfare problems thought to be common in intensive systems, whereas the welfare of animals kept in extensive systems has attracted much less attention. This may be due to the generally held belief that extensive systems are advantageous in terms of animal welfare. Although it is undeniable that extensive systems have many benefits in terms of animal welfare, they are by no means free of welfare problems. This review highlights the animal welfare problems that are most likely to be found in extensive systems following the four animal welfare domains of “nutrition,” “environment,” “health,” and “behavior.” Extensive environments are highly variable and heterogeneous in terms of climate conditions, food quality, and access to high-quality water, and this can raise serious welfare concerns related to chronic hunger and thirst, and thermal stress. These problems will vary depending on the location and time of year. Some diseases are more likely in extensive systems than in intensive ones and this can be compounded by supervision of animals being more difficult in extensive systems. Several painful husbandry procedures as well as neonatal mortality and predation are other potential welfare issues for animals raised in extensive systems. Finally, infrequent handling and / or potentially aversive handling can impair human-animal relationship and have a negative effect on the welfare of extensive livestock. Detection and monitoring of welfare problems in extensive systems are essential for implementing practical solutions adapted to local challenges. Selecting animals that are adapted to local conditions reduces some of the welfare problems encountered in extensive systems. Practice-led innovations should be undertaken in extensive systems and should support knowledge-exchange strategies with producers.