A role for plasma aromatic amino acids in injurious pecking behavior in laying hens

Neither FP nor feather damage was observed in the present study, but aggressive pecking directed at the head/neck area occurred in several groups with an onset of this aberrant behavior between weeks 22 and 29. Eight HP pens and seven LM pens were affected by severe head/comb injuries inflicted via aggressive pecking. Therefore, our exploratory data analysis focused upon the possible interplay between the variability of our outcome measures (absolute levels of AAA in plasma as well as the ratios PHE/TYR and TRP/(PHE + TYR)) and the aggressive head/comb pecking as an expression of social stress within the pens. We found significantly lower TRP availability relative to PHE and TYR (TRP/(PHE + TYR) ratio) and higher TYR concentrations at week 24 in pens with an early onset of injurious aggressive behavior at weeks 22-23. This was most pronounced in the LM line, but at week 29, TRP availability normalized in both lines. It was furthermore evident that in LM birds, higher aggressive pecking activity per pen was associated with higher TYR levels (n = 78 birds, r = 0.643, p < 0.001) and lower TRP/(PHE + TYR) ratios at week 24 (r = − 0.541, p < 0.001). In the HP birds, these associations were of lower strength and were negatively correlated (TYR: n = 73, r = − 0.308, p = 0.005; TRP/(PHE/TYR) ratio: r = 0.314, p = 0.004). Our findings indicate that in LM birds, lower TRP availability at week 24 may be attributable to higher stress levels in pens where injurious aggressive pecking developed early on. These findings may lay the important groundwork for the analysis of AAA plasma levels as a useful avenue of research to investigate underlying physiological mechanisms of behavioral problems in laying hens.