Sternum injuries in laying hens – a multifactorial problem
By the end of the laying period, sternum damage occurs in more than 40% of laying hens in a flock on average. Studies have shown that affected birds experience pain and reduced mobility, suggesting that sternum damage impairs the animals’ welfare. But how does sternum damage occur, what can be done about it, and what are its effects? Dr Stefanie Petow (Friedrich Löffler Institute, Celle) and Dr Lisa Jung (University of Kassel) have been addressing these questions for several years. In a webinar organised by the Network Focus Animal Welfare project, they shared their findings with participants.
Firstly, it is important to bear in mind the implications of sternum injuries for the animal, but also for the farm: in laying hens, the pectoral muscles are attached to the sternum. Consequently, with every wing movement, mechanical forces exerted by the contracting muscle act upon the potentially fractured sternum. If the sternum is injured, the laying hens try to minimise their movements to avoid pain. This restriction in mobility often leads to reduced feed and water intake, particularly in multi-tiered aviaries, and in some cases to misplaced eggs or reduced laying performance. Thus, sternum injuries have an impact not only on animal welfare but also on the farm’s economics.
In the case of sternum injuries, a distinction is made between fractures and deformities.
Deformities refer to deviations from the sternum’s naturally straight central axis. These changes in the sternum are observed in both hens and cocks. The pressure exerted on the sternum when perching plays a role in the development of deformities. Furthermore, day length in conjunction with vitamin D may also play a role.
In the case of sternum fractures, a distinction can be made between traumatic and non-traumatic fractures.
Traumatic fractures can affect the entire sternum. The healing process for these fractures often takes 4–6 weeks. They occur, amongst other things, when laying hens collide with a perch whilst landing.
Non-traumatic fractures include, for example, stress fractures. These mainly occur at the rear end (tip) of the sternum. Healing is significantly delayed or not observed in these fractures. One possible cause discussed for this type of fracture is that laying begins in laying hens as early as between the 18th and 22nd week of life, which increases calcium requirements. Consequently, there may be a lack of calcium for the ossification process, which does not occur at the posterior end of the sternum until the 33rd week of life.
One question being investigated by the two scientists is which factors underlie the development of sternum damage. Based on current knowledge, it is assumed that laying performance, genetics, age in weeks, feeding and the housing system all play a role.
Impact of laying performance
Based on the existing body of research, Dr Petow identifies high laying performance as a primary cause of fractures. For instance, approximately 2.5 g of calcium is required to form an egg. However, the animal cannot meet this requirement through feed alone, according to the expert. To form the eggshell, the birds additionally mobilise calcium from the interior of the bone, the so-called medullary bone, a bone structure found only in laying hens. To form medullary bone, other components, such as structural bone, are remodelled. Medullary bone serves as a calcium store for the animals. The more and the earlier the animal lays eggs, the more bone substance is lost, as it has no way of replenishing this store, for example through a longer break from laying. This has a negative effect on bone stability.
Influence of genetics
An experimental study of different breeds has shown that white genetics (WLA hens – a parent line of the Lohmann Selected Leghorn) exhibited fewer bone fractures but more deformities, compared with brown genetics (BLA hens – a parent line of the Lohmann Brown), which showed more fractures but fewer deformities. Furthermore, the incidence of bone fractures varied from breed to breed.
Influence of diet
Various feed additives have already been investigated here, but without promising results. An increased calcium intake, for example, had no effect on sternum damage, as the capacity for absorption in the small intestine is limited.
Vitamin D plays a crucial role in the mineralisation of bone with calcium. However, studies on increased vitamin D supplementation showed that the vitamin mainly accumulated in the egg but did not lead to improved bone stability. Although further studies on the administration of vitamin D in relation to the development of sternum deformities are currently being conducted, it must be noted that the uptake of vitamin D as a feed additive is limited.
Furthermore, the addition of omega-3 fatty acids was investigated. Whilst this approach led to positive effects in terms of bone elasticity, it had a detrimental effect on egg mass, which decreased.
One factor identified as a risk factor in a practical study is underweight hens. However, it remains unclear whether the birds consumed less feed due to the pain associated with the fracture, leading to underweight, or whether the birds’ underweight condition prior to the fracture contributed to its development because of an insufficient supply of nutrients.
Nevertheless, the experts recommend carrying out feed analyses in cases of increased incidence of sternum damage to ensure that the feed contains sufficient nutrients. In addition, supplementary calcium should be provided.
Influence of the housing system
An important factor in the development of traumatic sternum injuries is the housing system. As mentioned above, traumatic fractures occur, amongst other things, when hens fall from perches or collide with them. This is because, if the approach angle in aviary housing is incorrect, the hens sometimes fly into the wall or drop themselves in front of it. Solutions to reduce the incidence of traumatic fractures include installing ramps, non-slip perches, an adjusted twilight phase and wider walkways to improve approach and landing angles (more on this in the M-Tool). One tip from the researchers is to provide perches to the birds as early as possible in the rearing process, so that they become accustomed to them and learn to move around the aviary. Furthermore, the birds should be socialised with humans at an early stage so that they do not panic when someone enters the house.
Assessing sternums – how does it work?
To get an idea of the condition of the sternums yourself in the coop, it is necessary to carry out a self-check. This is best done at dusk or in the dark, when the birds have already ruffled their feathers, advises Dr Lisa Jung. It is important here that animals are assessed from all angles. When assessing the sternum, the expert recommends evaluating the sternum independently of its tip (tail end). This is because the tip of the sternum is particularly prone to abnormalities and is difficult to assess. It should therefore be recorded as a separate point.
The scientist recommends a three-tiered scheme for assessment.
- Grade 0: intact sternum with no abnormalities
- Grade 1: Sternum with thickening of the bone or separation of the connective tissue or deformation of the midline ≤1cm
- Grade 2: Sternum with bone thickening or separation of the connective tissue or midline deformation of ≥1 cm
For the examinations, the animals are placed on the forearm and a finger is placed between their thighs. This prevents the joints from rubbing against each other. To assess the sternum, the animals are then turned onto their backs. The index finger and thumb are used to palpate the sternum crest from top to bottom. Care should be taken to apply appropriate pressure so that the bone can be felt. In total, at least 30, but preferably 50, animals from a flock should be assessed.
All in all, it should be noted that sternum damage is a very prevalent problem, as on average between 50% and 100% of the hens in a flock may be affected. Due to the pain associated with sternum damage and the resulting restricted mobility, the welfare of affected birds is severely compromised. By recording sternum damage in their flock, farmers gain an overview of the situation in their house.
Although the causes of sternum damage are varied, various approaches were presented at the event that can help improve the situation in the barn.
Author: Regine Revermann, Lower Saxony Chamber of Agriculture