Effect of housing system, group size and perch position on H/L-ratio in laying hens
The objective of the present study was to assess the level of stress imposed on Lohmann Silver laying hens kept in a small group housing system with elevated perches (Eurovent (EV) 625a-EU, group sizes 40, 60 hens, four perches with two different heights) compared to furnished cages (Aviplus, group sizes 10, 20, 30 hens) and an aviary housing system with a winter garden (Natura, two pens, 1,250 hens) under identical management and feeding conditions. Each two perches within compartments of EV were either incorporated in a stepped position (ST, front and back perches heightened) or with only the back perches being elevated (BE). In the 3rd, 6th, 9th and 12th laying month, approximately 36 hens were randomly chosen from each housing system (430 hens in total) and heterophil to lymphocyte (H/L) ratio was determined. Laying hens kept in EV had significantly lower H/L-ratio compared to hens housed in Aviplus (P < 0.05), whereas no difference was detected between EV and aviary system. Hens kept in groups of 40 layers in compartments with perches in the BE position showed significantly lower H/L-ratios compared to hens kept in groups of 10, 20, 30 layers (Aviplus) and groups of 40 and 60 hens with perches incorporated in the ST position. Hens kept in furnished cages reflected the greatest stress exposure. Group sizes of 40 hens together with elevated back perches were associated with lowest levels of H/L-ratios and these ratios were even lower than in hens kept in the aviary system. Differences between group size 40 (BE) and the aviary system nearly achieved the significance level (P = 0.054). Keeping hens in groups of 40 layers together with perches incorporated in the BE position indicated to be most favourable in terms of imposing the least environmental stress on layers.