Platelet serotonin uptake, N/L ratio and (abnormal) behaviour of pigs with a low or high social breeding value in barren or enriched housing

Sammanfattning: Pigs that are kept intensively and in groups may show excessive aggressive and/or manipulative behaviour. These behaviours may cause both welfare problems for the pigs and economic problems for the farmer. These problems may be solved by adjusting the environment the animals are kept in, and by (genetic) selection of animals. A social breeding value (SBV) has been designed that is used to estimate the heritable effect of an animal on the traits of its pen mates. Selecting for high social breeding values for the growth of pen mates might be an indirect way to reduce behavioural problems related to social interactions, but so far the effects of selecting for SBV for growth on behaviour and physiology of pigs are unknown. The main aim of the current study was to examine the relation between SBV, serotonin, Neutrophil to Lymphocyte (N/L) ratio, (abnormal) behaviour and aggression in animals with a high or low SBV, housed in an enriched or barren environment. The experiment was set up as a 2x2 arrangement, with SBV (high vs. low) and housing (barren or enriched) as factors. In total, 32 groups of pigs (192 animals) divided over two batches were used, the pigs were housed barren (partially slatted) or enriched (with sawdust and straw) in groups of six (1:1 sex ratio). Animals diverged in SBV, and half of the pigs were high SBV animals, with an average estimated genetic effect of +2.72 on average daily gain (ADG) of pen mates, and the other half were low SBV animals, with on average a negative effect of -1.5 g ADG on pen mates. To study both SBV and housing, behavioural observations in the home pen were conducted, a regrouping test (i.e. mixing of individuals from the same SBV) was performed, lesion and tail bite scores were recorded, and at three times some blood was taken from the pigs to determine N/L ratios and serotonin uptake velocity by platelets. Housing was found to have a greater impact on explorative, active and manipulative behaviour during this experiment than SBV, and no interactions were found between SBV and housing. Pigs in enriched pens were more active compared to barren housed animals, showed more explorative behaviour and less oral manipulation of pen mates. Furthermore, tail damage scores were lower in enriched housing. Body lesion scores after regrouping were not affected by housing, but under stable social conditions, pigs in enriched pens showed more lesions than barren housed pigs. SBV or its interaction with housing did not affect behaviour, body lesion scores or tail damage of pigs. SBV did affect platelet serotonin uptake velocity, with a higher uptake for the low SBV than for the high SBV pigs. Also N/L ratio was affected by SBV, with a higher N/L ratio in low SBV animals than in high SBV animals. Housing did not significantly influence serotonin (5-HT) uptake or N/L ratio. In conclusion, SBV did not affect behaviour of pigs, but effects on physiological variables were found. It is possible that selection for a high SBV may genetically make the animals less prone to show damaging behaviours, but in the experimental environment or method of this study, this was not observed. Alternatively, as this study was part of a larger experiment, the sample size may have been too small to reveal potential effects of diverging SBVs on behaviour of pigs. Further research is needed to unravel which traits of pigs are changing when selecting for SBV for growth of pen mates.