How to manage Stress and Behavioral Problems in Poultry.

Dr Stuart Wilkinson

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      1. Stress has a direct impact on animal biological functions. This includes the following:
        • Abnormal behavior including aggressiveness.
        • Weakened immune system.
        • Upset of the digestive system including depressed appetite.
        • Less energy for production
        • Lower reproductive performance

        With layers in free range or barn settings there is a considerable challenge around behavior that not only impacts performance but is a significant welfare challenge.  Issue such as feather pecking, aggressive behavior, flightiness all leads to lower feed intake, poor production, and compromised livability.


        Let us look at what options we must overcome to address this problem.


        To measure the efficacy of anti-stress products we need 2 types of analysis:


        1. Behavior: Evaluate the capacity of the animal to adapt to stressful situation (=anti-stress serotonin effect) by behavior tests
          • Preening behavior, immobility test and social motivation are 3 tests well documented to evaluate the stress level of the birds.


        1. Physiological: Measure serotoninergic neurons activity
          • Measure the impact on 5-HT (serotonin) turnover in the hippocampus.


        Serotonin is well-known for its role in regulation of behavior and adaptation capacity however total levels of serotonin does not tell the full story. It is the release of this neurotransmitter and its turnover which is important.


        A popular option has been to use the amino acid tryptophan given it is a precursor of serotonin. Typically, tryptophan is added to feed at around 2kg/MT. Several studies have been conducted to measure the effect of high-level tryptophan on aggressive bird behavior. Authors studies different physiological & behavioral indicators with variable effects (table1):


        Table 1:

  • Physiological results: Vanhierden et al., (2004) studies observed positive impact on 5-HT (serotonin) turnover in the hippocampus and TRP/LNAA ratio.


  • Behavior results: Vanhierden et al., (2004) studies shown less gentle feather picking behavior (interpreted as social interaction) on high feather picking birds. Feeding is increased but preening behavior (which is welfare indicator) and resting time had decreased. Savory et , (1995) studies shown better body score for the highest tryptophan diet (22.6g/kg of diet) but not for the lower tryptophan diet (12.6g/kg of diet).


  • Concentration of tryptophan: studies (table 1) added 10 to 20 kg of L-trp/ton of feed compared to basic needs. This represents 5 to 10 times the nutrition level of tryptophan (even more).


The last study conducted by Yildririm et al., (2017) also evaluated tryptophan in behavioral tests: immobility test and social motivation. In this study they used immobility as a marker to evaluate the animal stress level. Unfortunately, there was no difference between treatments. In addition, the anxiety measures through social motivation test were not improved.


Adding high level of tryptophan in birds™ diet provide inconsistent results on behavior. The studies (table 1) have shown an increase of serotonin (5-HT) without observing a release of the neurotransmitter (explaining why results are inconsistent). These high levels of 10-20kg/MT of Tryptophan are impractical from a cost point of view ($100-200/MT).


Another way to modify behavior has come to light.


VeO is a dietary citrus extract that stimulates the olfactory system to send sensory messages to the brain which moderates the animals stress response. Consequentially VeO moderates the behavioral and physiological effects of stress resulting in improved productivity.


Remember we need two types of measurements when evaluating products for reduction in stress:


1. Effect on behavior


A study was made in the University of La Molina Peru (2017). The aim of the study was to test the impact of VeO under 2 rearing densities. Results have been published in Poultry Science, 2019.


  • Immobility test



This study showed the impact of VeO under psychosocial stress (density at 12 birds/sqm). Birds with VeO are less stressed in high density compared to control group. Same effect have been observed on growth performance.


A second study was made in INRA (France). The aim of this study was to measure quail social motivation (= anxiety level):


  • Social motivation test

Figure 2

VeO reduces quail™s anxiety.


2. Effect on physiological parameters


Guiard et al., (2020) measured serotoninergic neurons activities through 3 treatments:

  • NS: non stressed animals
  • S-Control: stressed animals under placebo
  • S-VeO: stressed animals under VeO



Figure 3: Serotoninergic neurons activities under 3 treatments

Under chronic stress serotoninergic activity is reduced. VeO restores activity (figure 3) allowing animals to adapt longer to a stressful situation.


Summary: Comparing VeO with Tryptophan



  • To get an effective evaluation of products to alleviate stress you need to consider both behavior and physiological responses.
  • Tryptophan, even at high dose rates, does not modify behavior and while physiologically serotonin is increased the serotoninergic neurons activity does not seem to change.
  • VeO has a positive effect on both behavior and physiological parameters in poultry.
  • The cost of VeO is significantly lower than Tryptophan.


If you wish further information, please no not hesitate to contact me:

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