Research note: Gentle versus severe feather pecking: effects of synbiotic feed supplementation under chronic stress in laying hens.

Feather pecking in laying hens is a complex behavior influenced by stress and the gut microbiota, with severe feather pecking (SFP) posing a major welfare concern and gentle feather pecking (GFP) representing a non-injurious, socially mediated behavior. This study investigated the effects of a commercial synbiotic supplement on SFP, GFP, and feather damage (FD) in hens exposed to chronic social and physical stressors. A total of 394 White Leghorn hens from high, low, and unselected feather pecking lines were housed in twelve floor pens. Pens received either synbiotic-supplemented or control feed, and within each feed treatment half of the pens were exposed to stressors including social mixing, perch removal, and litter reduction over a four-week period. Behavioral observations and plumage assessments were conducted before and after stress exposure. Synbiotic supplementation did not significantly affect SFP or FD scores. However, stressors alone doubled SFP rates and tripled the likelihood of severe FD. GFP was significantly influenced by an interaction between feed and stress treatments; however post hoc comparisons indicated a trend toward higher GFP in stressed synbiotic-fed hens compared with their non-stressed counterparts, with no differences observed in control birds. These findings indicate that synbiotic supplementation via feed did not mitigate stress-induced SFP or FD, while subtle differences in GFP expression highlight the sensitivity of non-injurious pecking behaviors to behavioral context under stress. Overall, the results emphasize the importance of considering social and environmental context when evaluating microbiota-targeted interventions for laying hen behavior.