Keel bone fractures (KBF) are considered the most important welfare problem for layer hens, with potential economic losses to the industry. The reported prevalences are alarmingly high in both caged and non-caged production systems, with up to 97% in non-caged systems. Identification of risk factors is necessary to understand KBF. Some efforts to identify risk factors have been made, however there has been some uncertanties in the definition of KBF, thus risk factor identification has shown to be not as conclusive as intended. The objective of the presented work was to identify risk factors of importance on the occurrence of KBF.
The current study presents the results from a prevalence study in Denmark including all production systems, including enriched cages, indoor non-caged flocks, outdoor non-caged flocks, and parent stock (60 weeks old). Forty flocks (120 birds/flock) were investigated at end-of-lay (>75 weeks old). Following euthanasia, the birds underwent inspection, palpation and necropsy. All observations and production data were analyzed (SAS statistical software).
The overall prevalence of KBF was 86%. In non-caged flocks, the KBF prevalence were 53-100%, whereas flocks from enriched cages ranged 50-98%. Furthermore, multiple KBF (≥4) in individual birds were often observed. Around 50% of the birds from non-caged flocks had ≥4 KBF, whereas 96% of KBF involved the distal end of the keel bone). Macroscopically KBF ranged from almost absence of callus, most frequent in caged birds, to large callus formations in and around the fracture lines, a frequent and typical finding in non-caged birds and birds with ≥4 KBF.
Parent birds had fewer KBF per bird, which is in contrast to the other birds from cage-free systems (P=0.01). Heavy birds from outdoor non-caged range flocks or parents stock had significantly lower KBF prevalence (P=0.009). Heavy birds had also fewer fractures than lightweight birds, regardless of productions system. The odds ratio of age at onset of lay (+1 week) is 0.87; for every week onset of lay is increased, the risk of having fractures at the end-of-lay is reduced by 12%. Similarly, a 100 g increase in body weight at slaughter reduces the prevalence of KBF in end-of-lay hens by 3%. Other identified factors like housing, hen hybrid, and egg production are discussed.