RESUMEN
Pigs are mixed into new social groups at various stages of production, and this study investigated the response of gilts to mixing with older unfamiliar sows. Four groups of 6 gilts and 1 group of 5 gilts were housed in stable groups until the most recently bred female reached d 39 of gestation. Then, stable groups were split such that 15 gilts remained in their home pens (CON) and 14 gilts were mixed (MIX) with 3 multiparous sows for 1 wk (M1), returned to the home pen for 1 wk, and then mixed with 3 different sows for 1 wk (M2). Continuous behavioral observations of gilt location in the pen, posture, aggressive, and other social interactions were made from the start of M1 and M2 for 5 h. Five-minute scans of location, posture, and behavior as well as salivary cortisol measurements were taken before, during, and after each mix week, whereas skin lesions were counted before and after each mix. During the mixed period, MIX gilts spent more (P ≤ 0.009) time in the individual feeding stalls, ventral lying, sitting, involved in aggression, and idle and less (P < 0.001) time in the straw-bedded area, lateral lying, and performing exploratory behavior (chewing, nosing, and rooting) than CON gilts. Additionally, MIX gilts had greater (P < 0.001) salivary cortisol concentrations and accumulated more (P < 0.001) skin lesions during mixing sessions compared to CON gilts. Mixed gilts in M2 spent more (P ≤ 0.013) time in the feeding stalls and ventral lying and less (P ≤ 0.029) time standing and in the dunging passageway compared to M1. Compared to M1, gilts had lower (P < 0.001) salivary cortisol concentrations and fewer (P ≤ 0.027) lesions in front and middle regions of the body during M2. During M2, salivary cortisol concentration was positively correlated with time spent in the dunging passageway (Spearman's rank correlation [rs] = 0.650, P = 0.022) and negatively correlated with time spent in the feeding stalls (rs = -0.762, P = 0.004). Results indicated that gilts adapted their behavior during M2 by using the feeding stalls more, and they became less active, especially when interacting with the sows. Despite this adaptation in M2, lesions and salivary cortisol in M IX gilts were much greater than in CON gilts, highlighting the severity of mixing gilts with multiparous sows.
