Effect of reduced dietary protein level on finishing pigs’ harmful social behaviour before and after an abrupt dietary change

Tail biting in pigs is a harmful behavioural disorder that negatively affects their welfare. Nutrition has played a crucial role in improving farm efficiency; however, an imbalance in key nutrients is an accepted risk factor for tail biting. The exact contributing factors and the level of risk associated with inadequate nutrition remain unclear. We aimed to assess the effect of dietary protein level on harmful social behaviours in finishing pigs before and after an abrupt dietary change. A total of 80 pigs (Duroc x (Landrace x Large White)) were assigned to this trial over two batches. Pigs were housed in mixed sex groups of 10 and assigned to one of two treatments at 12 weeks of age. These consisted of a control treatment (CON) containing 15.5% Crude Protein (CP) and a low CP treatment (LowCP) containing 13.5% CP. The abrupt dietary change occurred at 16 weeks of age (71.45 kg) in batch 1 and at 15 weeks of age (67.04 kg) in batch 2. This change in diet led to a 1.2% drop in CP in the CON treatment and a 1.4% drop in CP in the LowCP treatment. Behavioural observations were performed before and after the diet change and largely focused on the pigs' social behaviour. Using a "hurdle" model analysis, we investigated whether diet, diet change or sex influenced: 1) the probability that a behaviour was not performed, and 2) the frequency and/or duration when the behaviour was performed. The LowCP treatment led to an increased duration of limb-directed behaviour (P = 0.03) when performed at all, and also an increased duration of ‘tail-in-mouth’ behaviour (P = 0.02) in males but not females. However, LowCP did not affect the level of ear-directed behaviours in this way (P > 0.05). An increase in standing frequency occurred after the abrupt diet change (P = 0.04). Our study indicates that a small reduction in dietary protein and sex affect behaviour in the finishing phase, with some impacts on important harmful social behaviours. Furthermore, an effect of abrupt diet change on behaviour is suggested, which will require further confirmation.