The strength of pigs’ preferences for different rooting materials measured using concurrent schedules of reinforcement

Abstract

The aim of the present experiment was to measure pigs’ preferences for four different rooting materials using a set-up where pigs were given the opportunity to consider the relative attractiveness of two different materials simultaneously. Ten pigs had access to work for 50 min daily in an operant conditioning experiment using concurrent schedules of reinforcement. During the 50 min two different rooting materials were available after completing a fixed number of presses on two different operant panels inside a test pen. Long straw was always used as reinforcement on one panel, while one of four alternative rooting materials (chopped straw, fir branches, peat or long straw) was used as reinforcement on the other panel. The cost of access to the resources varied on both panels according to five pairs of Fixed Ratio schedules (FR8/FR40, FR16/FR32, FR24/FR24, FR32/FR16, FR40/FR8). For each material all pigs went through three test runs of the five FR pairs. Measures of preferences were based both on the generalized matching law (bias) and on the cross-points between the demand curves for the reference material and for the alternative material. Both the bias and the cross-point was significantly affected by type of alternative material in that the cross-point for peat (FR9.0) was significantly lower ( P < 0.05) and bias (−0.34) significantly higher ( P < 0.05) than for all other alternative materials (cross-point: branches FR18.3, chopped straw FR23.8, long straw FR29.2; log(bias): branches −0.27, chopped straw −0.01, long straw 0.40). The intercept of the demand curves for the alternative materials were significantly affected ( P < 0.05) by type of material in that it was higher for peat (1.73 ± 0.14)(lsmeans ± std. err.) than for all other materials. The intercept for branches (1.31 ± 0.14) was higher than for chopped straw (0.92 ± 0.14) and long straw (0.54 ± 0.17). The slopes of the alternative demand curves were unaffected by type of material as were the intercepts and slopes of the reference curves. The number of times of pressing the alternative panel in succession before shifting to the reference panel was significantly higher ( P < 0.001) across all FR's for peat (11 ± 0.7 at FR24) and branches (9.4 ± 0.7 at FR24) than for chopped straw (8.2 ± 0.6 at FR24) and long straw (7.0 ± 0.6 at FR24). The pigs achieved a new reward sooner following delivery of the previous one ( P < 0.05) when the alternative material was either peat (1.3 ± 0.13 min) or branches (1.3 ± 0.13 min) compared to long straw (1.8 ± 0.19 min) and chopped straw (1.7 ± 0.13 min). All three alternatives were able to substitute completely for long straw, in that the slope of the demand curve of the reference material was not influenced by type of alternative material. In conclusion, both measures of cross-points and bias indicated a preference for peat and branches over chopped straw and long straw.