Influence of the protein status of piglets on their ability to select and prefer protein sources. 
Pigs may have retained the capacity to choose feeds based on their nutritional requirements, even after decades in which they are not allowed to select their diet composition due to the common feeding systems of the intensive pig industry.  We used 480 early-weaned piglets in two experiments to assess their ability to select and prefer protein-related sources, depending on their protein status.  Piglets were fed after weaning with two isoenergetic diets formulated to contain an optimal or sub-optimal crude-protein (CP) content, a high-protein (HP, 204g CP/kg as-fed) or a low-protein diet (LP, 142g CP/kg), respectively.  In Experiment 1, the preference of piglets was assessed by using a choice test between protein (porcine digestible peptides [PDP] 40g/L) and carbohydrate (sucrose 40g/L) water-based solutions for a period of 3min.  Piglets showed higher intake and preference for the sucrose 40g/L than for the PDP 40g/L solution, independently of the dietary CP content (9.8mL/kg body weight [BW] vs. 3.7mL/kg BW and 10.4mL/kg BW vs. 4.3mL/kg BW in HP and LP pigs, respectively).  In Experiment 2, piglets were given eight training sessions in which two equally preferred flavors were mixed with protein (porcine animal plasma 60g/L, CSp) or carbohydrate (maltodextrin 60g/L, CSc) solutions.  In the subsequent choice test, piglets fed the HP diet showed a tendency to a higher intake of CSc than of CSp (6.5mL/kg BW vs. 5.4mL/kg BW).  On the other hand, piglets fed the LP diet showed a higher intake and preference for CSp than for CSc (15.5mL/kg BW vs. 10.2mL/kg BW), differences being higher for medium and low BW piglets than for heavy ones.  The results show that piglets are unable to express a specific appetite for protein to correct previous underfeeding with it; however, they may show an appropriate dietary selection pattern in order to overcome protein deficiency through associative learning.