How much is enough? The amount of straw necessary to satisfy pigs' need to perform exploratory behaviour

Since 10 years, EU-legislation states that 'pigs must have permanent access to sufficient quantity of material to enable proper investigation and manipulation activities'. While much research has focused on which materials 'enable proper investigation and manipulation activities', little has been done to determine what constitutes 'sufficient quantity' and 'permanent access'. Based on the hypothesis that a reduced level of oral manipulation of pen mates reflects an increased level of fulfilment of pigs' behavioural need to explore, we chose oral manipulation of pen mates as target behaviour. In three batches, we investigated the relation between oral manipulation of pen mates and amount of straw provided to the pigs in order to identify the amount of straw, where additional provision of straw did not reduce the occurrence of oral manipulation of pen mates any further. From 30 to 80 kg body weight, the pigs were housed in groups of 18 animals in pens (5.48 m x 2.48 m) with concrete floor (1/3 solid, 1/3 drained and 1/3 slatted). Pens were cleaned manually twice a week and fresh uncut straw was provided daily onto the solid part of the floor. In the first batch, 48 pens were assigned to either 10, 500 or 1000 g straw per pig and day (N = 16 pens per straw allocation). A reduction in oral manipulation of pen mates was found when pigs were given 500 compared to 10 g (P = 0.03), but no further reduction when increasing the straw amount to 1000 g was detected. In the second and third batch, a total of 96 pens were assigned to 8 treatments (10, 80, 150, 220, 290, 360, 430 or 500 g straw per pig and day) (N = 12 pens per straw allocation). There was a linear relation between straw amount and oral manipulation of pen mates, the latter being reduced from 8.4% to 6.7% of active time, when pigs were provided 500 compared to log (P = 0.01). Based on the concept of bioequivalence, 387 +/- 10 g straw per pig and day was identified as the amount of straw where a further increase in straw provision did not reduce the oral manipulation of pen mates. Thus, the straw amount identified to meet pigs' need to explore was close to 400 g straw per pig and day. A criterion of permanent access (defined by a minimum of 1 I(approx. 60 g) unsoiled straw in a pen 24h after allocation) was achieved at lower levels of straw provision, especially during the initial weeks of the growing period. (C) 2014 Elsevier B.V. All rights reserved.