What do pigs want? A pigs’ perspective on amino acid requirements beyond growth

Research on low‐protein diets in pig production has focused on improving protein retention efficiency and reducing nitrogen waste. However, these diets have been shown to increase the risk of damaging behaviours like tail biting—a major welfare and performance issue in commercial farms. Current strategies to reduce tail biting include preventive measures such as tail docking and management approaches like environmental enrichment. Since tail docking is already banned by the EU as a routine practice, new non-invasive measures need to be investigated and integrated with existing ones in a multifactorial approach.

In Chapter 2, an experiment conducted on a commercial farm examined the effects of amino acid (AA) supplementation and environmental enrichment on pigs fed an AA-deficient low-protein diet. Pigs on this diet exhibited lower growth performance, increased activity compared to pigs fed a normal protein or AA-supplemented diet, and increased tail biting. Although extra environmental enrichment initially reduced tail biting, its effectiveness diminished over time, even though it consistently reduced ear biting. In contrast, supplementation of AA to meet the requirements for maximal growth reduced tail biting compared to the AA-deficient low-protein diet. This confirms that AA deficiency contributes to damaging behaviours, and AA supplementation counteracts these effects more effectively than enrichment.

Further studies explored the ability of pigs to self-supplement AA under standard conditions or conditions known to increase AA requirements. In Chapter 3, pigs given the choice between two and four diets achieved the same feed intake and body weight gain as animals provided with an AA-adequate diet. When offered a supplemented diet containing a combination of three AA at higher concentrations, pigs proportionally decreased consumption of the supplemented diet. However, when given a choice among individually AA-supplemented diets, pigs proportionally decreased consumption of a diet with high valine concentration, showed no aversion for a diet with high threonine concentration, and exhibited a strong preference for a diet with high tryptophan concentration. The resulting AA intake differed from that required for maximal body weight gain, indicating that pigs might choose AA to prioritize other physiological processes over growth.

In Chapters 4 and 5, a second choice-feeding experiment investigated whether the self-supplementation of AA to compensate for an AA-deficient diet differed for piglets kept under different sanitary conditions. Low sanitary conditions negatively affected piglets’ growth, immune status, and neurotransmitter levels. Contrary to expectations, piglets increased their preference for an AA-supplemented diet over time under high sanitary conditions but not under low sanitary conditions. Furthermore, low sanitary conditions led to reductions in plasma and brain tryptophan concentrations, platelet serotonin, neurotransmitter concentrations in the hippocampus and prefrontal cortex, and play behaviour compared to piglets under high sanitary conditions. Although offering dietary choices slightly reduced aggressive behaviours, sanitary conditions had a stronger influence on piglet welfare than diet alone.

Overall, these findings support a necessary shift in the conception of the ideal AA profile, from a static model determined by the human objective of maximising growth, to a more dynamic pig-centred approach that accommodates variable requirements for diverse functions and priorities.