The value assigned to six different rooting materials by growing pigs

According to EU legislation pigs must have access to rooting material, and the aim of the study was to quantify growing pigs' preferences among six different rooting materials. The relative attractiveness of six rooting materials was assessed in an operant conditioning set-up using concurrent schedules of reinforcement. Twelve pigs were tested with all six combinations of the reference material (peat) and one of the six test materials in a balanced design. The cost of access (fixed ratio of presses per reward (FR)) to both reference material and test material was varied (reference/test: FR8/FR40, FR16/FR32, FR24/FR24, FR32/FR16, FR40/FR8). For each combination, demand functions for both materials were estimated as a function of the cost of the reference material leading to a cross point of the two demand functions. The intercept of the demand functions for the test materials differed (1.27, 0.97, 0.91, 0.64, 0.53, 0.48 (±0.14) for maize silage with straw, compost, spruce chips, seed grass hay, sisal rope and chopped straw, respectively; P < 0.001). The slopes of the demand functions for the six test materials did not differ. Furthermore, the demand functions for the reference material were not affected by test material. The cross point of the two demand functions for each of the six combinations was calculated to assess the relative attractiveness of the six test materials using the reference material as a common scaling factor. The cross points (95% confidence interval in brackets) revealed the following ranking (the lower values are the most preferred): maize silage with straw (14.2 (9.5-18.5)), spruce chips (18.0 (13.8-21.9)), compost (18.2 (13.8-22.3)), sisal rope (25.5 (21.4-29.6)), seed grass hay (27.1 (22.7-31.8)), chopped straw (28.5 (24.5-32.8)). All the tested materials were valued as much as chopped straw, but maize silage with straw, spruce chips and compost were valued higher. The results confirm that pigs prefer more complex and compound rooting materials.