4. Production and pathology
4.1. Production and welfare
The relationship between production level and welfare is far from simple and unidirectional. As the resource allocation theory puts it, all the energy used for adaptation purposes is no longer available for production that should therefore decrease (Beilharz et al., 1993). Indeed, corticosteroid hormones and catecholamines are catabolic and the activation of these neuroendocrine systems is therefore antagonistic to anabolic processes involved in production. Glucocorticoid hormones reduce the activity of sex neuroendocrine systems and therefore reduce the efficiency of reproduction (Wan et al., 1994). Behavioural adjustments are also energy consuming (Schütz et al., 2002).
There is no doubt that health problems, recurrent pain due to skin lesions or wounds, adverse environmental conditions like excessive density, bad air quality, usually reduce weight gain and overall productivity. However, the reverse is not true that a maximal production rate necessarily reflects optimum welfare. Practices to improve production via genetic selection or the use of growth promoters have been questioned since they may have a negative impact on welfare or mask the negative effect or bad welfare on production performances. On the other hand, resources used for adaptation purposes, and therefore unavailable for production, may eventually be used in processes improving welfare. Although most studies aimed at removing negative inputs, less work has been done on positive emotions that may activate adaptation systems and consume energy as well. For instance, I have cited earlier experimental data showing that the HPA axis may be activated in an enriched environment (de Groot et al., 2000; De Jong et al., 1998, 2000). However, experiments comparing husbandry methods for growing-finishing pigs show that enriching the environment can also increase production. Pigs raised on bedding and eventually with free access to an outdoor area are more active, eat more and grow faster than controls raised in a conventional system with a totally slatted floor, the differences in meat quality being at most modest, although a wide range of variation can be found in the literature (Van der Wal et al., 1993; Lyons et al., 1995; Nicks et al., 1996; Sather et al., 1997; Geverink et al., 1999; Beattie et al., 2000; Klont et al., 2001; Lebret et al., 2002, 2004, 2006).
Finally, it should be kept in mind that production is usually monitored at the level of the group, when welfare is an individual experience, so that a good overall performance of a production unit should not preclude attention given to individual animals.
4.2. Pathology and welfare
Good health is the first prerequisite for a sound production system ensuring animal welfare together with efficient production. The question here is whether pathology can be used as an index of the impact of the environment on the animal, and therefore as a measure of welfare.
The immune system is a major player in the defense of the organism against pathogens, and is also sensitive to stress factors. Indeed cortisol is a powerful anti-inflammatory hormone and can influence a number of immune mechanisms. This led to the general concept of stress-induced immunosuppression that is an over-simplification of the complex bi-directional relationships between stress responses and the immune system (Dantzer and Mormède, 1995; McEwen et al., 1997; Dantzer, 2004). Just to cite two examples among thousands, we showed in rats that the consequences of repeated defeat in social encounters on immune functions were different according to the social status of the animals (Raab et al., 1986). In another experiment, chronic social stress induced by daily reallocation of animals in different social groups, and that provokes an intense activation of the HPA axis and/or the ANS, did not change the indices we used to monitor immune system activity (Klein et al., 1992). Although various parameters related to immune functions are being monitored in welfare-related studies, their interpretation in terms of stress and welfare is far from obvious (see for instance Hicks et al., 1998; de Groot et al., 2000; Tuchscherer et al., 2002).
Besides major epizootic diseases determined by specific pathogens and highly contagious, enzootic diseases, like respiratory, digestive and reproduction disorders, are characterized by a low mortality, high morbidity with variable expression of the disease, frequently associated with bad zootechnical and economical performances, and by a complex, multifactorial determinism in which pathogens do not play the main role. Ecopathological approaches have been used to disentangle the various factors involved in the etiology of these ‘production diseases’. They consider numerous factors related to stockmanship, genetics and general health of the animals, herd management, characteristics of the environment, feed composition, distribution and intake, technical performance of the farm and relate these factors to the occurrence of disease with multi-dimensional statistical analyses (Madec and Tillon, 1988). For instance in a study of digestive disorders at weaning, factors like the number of piglets per pen, the number of litters of origin per pen, the feeder length available per pig, the stocking density were shown to be risk factors to the occurrence of the disease (Madec et al., 1998). These various characteristics of the environment are obviously relevant to animal welfare as well, so that these production diseases may be an important integrative indicator of the quality of the environment. Furthermore, this approach has the advantage of being multidimensional, eventually exhaustive, and without any prevailing hypothesis about the causal factor(s) and the intermediate mechanisms leading to disease. It would be worthwhile extending this methodology to other parameters characterizing animal welfare, like neuroendocrine profile or behavioural traits.