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Despite the importance of hedonic reactions in pig's intake, feed palatability has been typically inferred from preference or acceptance measures. However, these measures are influenced by factors beyond palatability, such as energy density and hunger. The aim of this study was to evaluate palatability responses in pigs to sweet and umami taste at different inclusions levels. Pigs (24 per experiment) were video recorded while exposed in pairs to different sucrose (Exp. 1) or monosodium glutamate (MSG, Exp. 2) solutions over seven consecutive 10 min tests (one concentration per day). In both experiments, palatability was estimated through consumption patterns (consumption time per approaches, CT/A), facial expressions (snout openings and tongue protrusions), and consumption during a brief 2 min period. Data were analyzed by sucrose or MSG concentration. Sucrose concentration affected total intake, producing an inverted-U function and a quadratic relationship with sucrose concentration (P = 0.012). In contrast, CT/A and snout openings showed a dose effect (P < 0.005) with a direct correlation between sucrose concentration and CT/A (R = 0.23, P = 0.033) but not for openings (R = 0.18, P = 0.105) where a quadratic relationship appears (P < 0.001). Tongue protrusions and brief consumption time were not affected by sucrose concentration (P = 0.144 and 0.205, respectively). MSG concentration affected consumption, CT/A, snout openings, and brief consumption time (P < 0.001), with significant (P < 0.001) positive correlations (R = 0.59, 0.56, 0.56, and 0.68), respectively. As with rats, CT/A appears to provide a novel and interesting measure reflecting the palatability of preferred ingredients in pigs. However, brief consumption time and orofacial reactions show less similarity between pigs and rodents. Thus further studies are necessary both to better understand the measurement methods themselves and relationship between hedonic reactions and simple consumption in pigs.

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The fact that consumption of normally palatable foods is affected by stress in both humans and rats suggests a means to assess hedonic reaction in non-verbal animals. However, little is known about anhedonia and stress in productive animals such as pigs. Thus we examined the separate effects of social stress and restraint stress in 42-day old pigs on the preference for dilute sucrose solutions over water. Pigs in the social stress group (SS) were mixed with unfamiliar animals from separate pens for two 20 minute periods (Experiment 1). Pigs in the restraint stress group (RS) were immobilized three times a day, for 3-min periods, on 3 consecutive days (Experiment 2). Consumption of dilute sucrose solutions and water was examined after these stress manipulations and in the unstressed control groups (CG). Pigs were tested in pairs (12 control and 12 experimental) with a choice between water and sucrose solutions (at either 0.5% or 1%) during 30 min sessions. In both experiments CG pigs showed higher intakes of 0.5% and 1% sucrose solutions over water. Neither SS nor RS pigs consumed more 0.5% sucrose than water, but both groups did consume more 1% sucrose than water. Both social stress and restraint stress reduced sucrose preference at low concentrations but not at higher concentrations suggesting that stress may limit food consumption in pigs unless a palatable feed is present. In addition, the results suggest that stress reduces the hedonic impact of dilute sucrose. Therefore, sucrose preference may be a useful test for the presence of stress and anhedonia in pigs.