Publikationsdatenbank

The influence of egg production, genetic background, age, and housing system on keel bone damage in laying hens (2020)

Art

Hochschulschrift

Autor

Eusemann, Beryl Katharina (WE 11)

Quelle

Berlin: Mensch und Buch Verlag Berlin, 2020 — VII, 88 Seiten

Sprache

Englisch

Verweise

URL (Volltext): https://refubium.fu-berlin.de/handle/fub188/28838

Kontakt

Institut für Tierschutz, Tierverhalten und Versuchstierkunde

Königsweg 67
14163 Berlin
+49 30 838 61146
tierschutz@vetmed.fu-berlin.de

Abstract / Zusammenfassung

The keel bone is the prominent ventral part of the sternum in birds where the flight muscles attach. It is fractured or deviated, i.e., deformed, in up to 97 % or 83 % of laying hens within one flock, respectively. Both symptoms are often summarized to the term “keel bone damage” (KBD). Keel bone fractures and possibly also deviations are likely to cause pain and impair the mobility of affected hens. For these reasons, KBD is considered to be one of the most severe animal welfare problems in the egg production industry. The etiology of KBD is not yet fully understood but it is widely defined as a multifactorial disorder. There are external factors such as housing system and nutrition as well as internal factors such as genetic background and age influencing the prevalence of KBD. However, there is no agreement about the direction of these effects and knowledge about the extent to which each of these factors contributes to the etiology of fractures and deviations is lacking. Another internal factor that may influence the keel bone is egg production. There is a high demand of calcium for the eggshell. To meet this demand, female birds possess a special kind of woven bone which is located in the medullary cavity of some bones. It is suggested that once the hen comes into lay, osteoblasts change their function from forming structural bone to forming medullary bone which leads to a decrease in the amount of structural bone and, thus, in bone strength. It is further suggested that these mechanisms are mediated by estrogens. However, the role of egg production and estrogens in KBD has never been investigated into detail. The aim of the present work was to get a better insight into the etiology of KBD. A special focus was put on comparing the external factor housing system with the internal factors genetic background, age, egg production, and estradiol-17β. To that aim, three studies were carried out. In the first study, a method to assess keel bone fractures and deviations in living hens was established using radiography. Furthermore, hens of five layer strains differing in phylogenetic background (brown versus white layer lines) as well as laying performance (high versus low performing) were kept in two different housing systems (single cages versus floor housing) and repeatedly radiographed. Brown layer lines showed more keel bone fractures while the severity of keel bone deviations tended to be higher in white layer lines. Within the brown layers, the high performing layer line showed more keel bone fractures and deviations compared to the low performing layer line. More fractures were found in the floor housing system whereas keel bone deviations were more severe in cages within some of the layer lines. Fracture prevalence increased with age. The presented radiographic examination of the keel bone allowed to clearly differentiate between fractures and deviations and to assess the severity of the latter. It was further shown to be a suitable and quick method for longitudinal studies on KBD. The aim of the second study was to establish an animal model with non-egg laying hens which could further be used to assess the influence of egg production on different traits in laying hens. 40 hens were kept in a floor housing system. Ten hens received a sustained release implant containing the gonadotropin releasing hormone (GnRH) agonist deslorelin acetate before and ten hens after the onset of lay. The remaining 20 hens were kept as control hens. The implant inhibited egg laying activity in all hens that were treated after the onset of lay and protracted the onset of lay in all hens that were treated before. However, duration of effectiveness was relatively short and showed that a new implant should be administered after approximately twelve weeks in order to constantly inhibit egg laying activity. Furthermore, estradiol-17β plasma concentration was decreased in treated hens. All hens of this study were also radiographed twice. Egg laying control hens showed significantly more keel bone fractures and more severe keel bone deviations compared to non-egg laying hens within the group that was treated after the onset of lay. Furthermore, severity of keel bone deviations increased with age in this group. The results of the first two studies were used for the third study whose aim was to assess the influence of egg production and estradiol-17β on KBD. A total of 200 laying hens of two strains differing in laying performance were kept in a floor housing system. Half of each layer line was administered a deslorelin acetate implant every 90 days and, thus, did not lay eggs. Part of these hens as well as of the egg laying hens was further given an implant with estradiol-17β. All hens were repeatedly radiographed and fracture prevalence as well as prevalence and severity of deviations were compared between the four treatment groups and both layer lines. Furthermore, radiographic density of the keel bone was assessed. The risk of keel bone fracture was much lower in non-egg laying compared to egg laying hens while no effect of egg laying activity on keel bone deviations was found. Radiographic density of the keel bone was higher in aged non-egg laying hens compared to aged egg laying hens. Treatment with exogenous estradiol only showed a relatively small effect on keel bone fracture risk within egg laying hens and no effect on deviations or radiographic density. The high performing layer line showed a higher risk of keel bone fracture than the low performing layer line but layer lines did not differ in terms of keel bone deviations. Taken together, a method to assess keel bone fractures, deviations and radiographic density in a longitudinal study as well as a model with non-egg laying hens have been established and can be used in further studies. Different risk factors have been found for keel bone fractures and deviations indicating that these are two different and independent phenomena and that it is very important to clearly differentiate between them. Both external and internal factors have been found to contribute to the etiology of keel bone fractures and deviations. Part of the keel bone fractures seem to be caused by collisions with housing equipment. However, the very large difference in risk of keel bone fracture between egg laying and non-egg laying hens clearly indicates that there is a fundamental weakness of the keel bone in laying hens caused by egg production which makes it very susceptible to fractures. Findings about the higher prevalence of keel bone fractures in high compared to low performing layer lines support this assumption. It is, thus, necessary to figure out which are the differences in bone structure and composition between egg laying and non-egg laying hens in order to find solutions against this huge animal welfare problem.