## 4 Discussion
### Laying Period
#### 4.2.4 Influence of Enrichment and Litter  
According to a meta-analysis of the relevant literature, flocks provided with environmental enrichment on the layer farms have a reduced incidence of feather pecking, irrespective of the presence or absence of enrichment materials during rearing [52]. Several studies have demonstrated the positive influence of providing environmental enrichment during the laying period, on plumage damage and SFP [49,51]. [PERSON] et al. [49] suggested that husbandry procedures during the rearing and the laying periods can be used to prevent feather pecking, and thus to improve laying hen welfare. Our results point in the same direction. The variable 'enrichment' was significant for LV1 of the first rearing period in the univariate analysis and in the model. However, the enrichment materials used were very different, in regard to quality and quantity between the farms, which may explain why this factor was not significant for LV2 and LV3. Future research should focus on specific recommendations for enrichment that would be suitable for practical use on commercial farms.  
The provision of litter and the litter quality are useful for preventing SFP [41]. Our univariate analysis showed a significant effect of the litter quality on plumage damage, whereas the litter depth had no effect. This result was confirmed in the model: the risk factor 'litter quality' was significant for all three visits (LV1: \(p\) = 0.027; LV2: \(p\) = 0.021; LV3: \(p\) = 0.015). Remarkably, another study found evident genotype-environment interactions between strains and groups [66]. In groups with environmental enrichment, plumage damage decreased in LB and LSL, but increased in BB. In view of the diverging effects between strains, the authors suggested that future practical recommendations for laying-hen husbandry should be strain-specific [51].  
Feather pecking is a behavioral disorder with multifactorial reasons. Various aspects of pullet and laying-hen husbandry--such as housing conditions, management, feeding, and genetics--must be considered, to prevent negative outcomes such as severe plumage damage, skin injuries, and high mortality rates due to SFP [1,61,75]. All data for the present study were gathered on voluntarily participating commercial farms, which differed largely in terms of housing conditions and management. The lack of standardized conditions needs to be considered when interpreting the statistical analysis. The univariate multifactorial analysis of variance showed an overdetermination of the model used, because the adjusted partial eta-squared of the model was smaller than the partial eta-squared. This was most likely due to the influence of other risk factors, such as feed composition [54] and genetics [19,76], which were not analyzed in this study. The results need to be interpreted carefully, because there are many possible influencing factors for feather pecking and subsequent plumage damage. Future research should verify the herein-proposed risk factors with a larger number of flocks. Because beak trimming is not practiced anymore in several European countries, it is highly important for farmers to gain knowledge on the prevention of feather pecking, by implementing optimized husbandry and management conditions.