Understanding Mortality Rates of Laying Hens

by Sara Shields

The issue of hen mortality in cage-free egg production is a recurring theme in this blog and in the wider debate over the use of battery cage confinement systems for laying hens. High mortality is an obvious indicator of poor welfare. It is important to note, however, that mortality can vary substantially between hen flocks, and that some cage-free systems have very healthy flocks that do not suffer substantial death losses. These systems can serve as models for the rest of the industry, since mortality is not inherent to any particular system, cage or cage-free, but rather to how well the system is managed.

It is also important to put the problem into historical perspective. Prior to about 1930, flocks were small. Many diseases, such as coccidiosis, respiratory diseases, and salmonellosis[1,2] became more problematic with the commercialization of poultry production, when flock sizes increased and there was a growing effort to raise more birds on less land.[3,4] When the industrialization of poultry production began, the average mortality rate of chickens jumped from 5-6% to 20%.[5] Although high mortality rates were a significant problem, economic incentives led the industry to seek strategies for mitigating these losses.

For the egg industry, one remedy was to move laying hens into cages, where wire floors separated the chickens from their manure. While this may have helped reduce intestinal disease and parasites, intensive confinement restricted the hens to small, barren cages that offer no opportunity to display important natural behavior, impinging on physical health due to lack of exercise and resulting in a very poor quality of life (for more information, click here).

It is also critical to note that the improved flock health status seen today is not due solely to cage confinement, but also to vaccine development, better hygiene practices such as “all in, all out” policies,[6] disease eradication programs,[7] and genetic selection for disease resistance.[8] These factors will continue to be important as egg production systems evolve to meet societal animal welfare concerns.

Role of Hen Genetic Strain

Another key concept in understanding hen mortality rates is the interaction between the genetic background of the hen and her environment. Hens must be genetically adapted to their surroundings in order to thrive, and studies and practical experience are beginning to show that a large portion of mortality in cage-free production can be explained by differences in the genetic strain of hen used in the system.

Laying hens are bred almost entirely by a select few international companies. Until recently, since most egg production took place in battery cages, breeding goals were aimed at producing hens who could produce many eggs in cages. Since cage-free egg production is becoming an international trend (in part a result of legal reforms in the EU and California), however, breeding companies are beginning to select hens on the basis of their performance and survival in cage-free environments. [9]

To illustrate the importance of hen genetic background, we can look to the experience in Denmark, where hens were not moved into cages until 1980. When cages were first introduced to the country, one of the common breeds used was the Danish Skalborg hen. The Skalborg hen had been genetically adapted to the floor systems in use at the time, and this was reflected in the mortality rate when the housing environment changed. In breed comparison tests initiated during this time, it was discovered that the mortality of the Skalborg hen was five times higher in cages compared to floor pens. [10]

In 2005, the World’s Poultry Science Journal published a study in which the authors reviewed every English, French, and German study that reported hen mortality rates in aviary and cage housing systems since 1980. This study eliminated much of the previous bias in the scientific literature by including only studies that used the same hen strain, age, and beak trim status in both cages and aviaries. Only 14 studies met these criteria, a telling result in itself. When these factors were accounted for in the statistical analysis, the mortality rate did not differ between cages and aviaries (multi-tiered cage-free systems). This means that, in previous studies showing a higher mortality rate in aviary systems, the apparent difference was due to factors other than the housing system. [11] One very important factor was the choice of hen strain and, in some studies that report differences in mortality, the type of production system is confounded with the strain of hen used in that system.

In many studies, brown egg-laying strains tended to experience higher mortality rates, primarily as a consequence of injurious pecking. [12,13,14] Because these birds have been popular in cage-free production, however, these studies can give the impression that the system itself is the cause of the increased mortality when, in fact, the hen strain or an interaction between the hen strain and her environment, is the major factor of importance.

In some segments of organic production, where brown hybrid hens have historically dominated, white hybrid hens are now becoming more popular, mainly due to their more agreeable temperament. A Swedish survey found that, where farmers have experienced severe outbreaks of cannibalism, they are changing the hens in their systems from brown to white hybrids. [15] To meet potential consumer demand for cage-free brown eggs, however, it is also possible to breed brown strain hens specifically for cage-free production (see the Stonegate example below). [16]

Role of Management

An important prerequisite to good management is the attitude of the producer. Scientists have noted that, “Attitudes of those in charge of management and husbandry are likely to be a major determinant of animal welfare.” [17] Alternative systems undoubtedly require more skills and experience, [18] and are thus more sensitive to poor management. [19] Currently, differences in management can contribute to inconsistency among cage-free farms, with some performing well and others experiencing difficulties. Thus, while mortality can be high, it is also highly variable between farms, [20,21,22] with some being highly successful. Where mortality is excessive, steps should be taken to correct the problem. Poultry producers have shown the ability to be innovative in the past, overcoming obstacles with the use of enhanced biosecurity measures and aided by advances in veterinary science and genetics, as previously mentioned. It seems likely that as demand for cage-free eggs increases, producers will once again need to show innovation.

In organic production, it has been demonstrated that, as farmers gain experience, feather pecking damage is reduced. One such study found that farmers who understood the behavioral biology of their chickens, including their origin in a forested environment, have adapted their management, provided enhanced outdoor areas and paid greater attention to the early rearing experience of their laying hens. Successful control of feather pecking in this study was dependent on the motivation and devotion of the farmer. In other words, the attitude of the producer matters. [23]

There are many important management steps that producers can take to control mortality rates. For example, some steps to reduce injurious pecking—in addition to choosing the right hen strain—include dimming the lights, [24] delaying the age at which hens lay their first egg, [25] providing early access to perches, [26,27] and providing attractive foraging substrates. [28,29,30] At stocking densities that provide more than 1000cm²/hen, the provision of additional space to facilitate the use of perches, nest boxes, foraging materials and access to feed and water may also reduce the likelihood of cannibalism. [31] The primary methods of controlling coccidiosis are with vaccine and coccidiostats added to feed. In barn systems, the use of raised slatted floors (as is common in breeding flocks, which are almost always housed on the floor), can prevent infection with parasites. [32] Small, free-range producers can use mobile housing to prevent the build-up of parasites around the hen house, [33] pasture rotation, and lowered stocking density. [34] A complete discussion of management factors is beyond the scope of this blog post, but interested readers can find additional information on keeping free-range flocks healthy and safe on the web sites of the Soil Association (www.soilassociation.org) or the National Sustainable Agriculture Information Service (http://attra.ncat.org).

Low-Mortality Cage-Free Farms

Low-mortality cage-free farms can be found in instances where corporations and producers have worked together to find solutions. For example, Stonegate supplies eggs to Waitrose, a leading supermarket chain in the United Kingdom. Stonegate has developed its own line of hens, Columbian Blacktails, who are birds with hardy characteristics that make them more suitable to free-range production. Beak trimming is not permitted, yet the genetic background of these birds makes them “almost totally free of feather pecking and cannibalism.” [35] Stonegate is the United Kingdom’s second largest egg producer and packer, [36] demonstrating that welfare improvements can take place on a large scale. The Columbian Blacktails fit the environment—they are genetically adapted to free-range production.

There are many other examples of low-mortality cage-free egg production operations. For example, in Switzerland, the FiBL research institute uses a subdivided barn system with rotational access to four outdoor paddocks. One paddock is in use at a time, giving the other three a chance to rest, which reduces the parasite load. Overhead netting protects free-range hens from birds of prey. The flock, a mix of brown and white strains, is subdivided into groups of 500 birds. The environment is enriched with perches, dustbaths, nest boxes and plenty of space. The birds are not beak trimmed, yet they have a mortality rate of less than 2%.[37]

Himmelsfarm (Heaven's Farm) in The Netherlands has three barns with around 32,000 H&N Brown Nick birds. They use a Vencomatic aviary system. The mortality rate after 51 weeks was less than 2%.[38]

The Meyers farm in Lincolnshire, England is a free-range farm using Lohmann Tradition birds. After making adjustments to their refurbished barn, the mortality rate at 48 weeks was just 1.5% in their flock of over 4,000 birds. [39]

There are also examples of low-mortality cage-free production in the scientific literature spanning several decades. For example, in a 1986 study conducted by researchers at Scottish Farm Buildings Investigation Unit and the North of Scotland College of Agriculture, ISA Brown birds were used in a comparison between battery cages and a perchery system. The cumulative mortality in the perchery was 1.36% from 20-44 weeks while it was 2.47% in the comparison group housed in battery cages. [40]

A 2009 University of British Columbia study reported mortality rates of beak-trimmed Lohmann White, Lohmann Brown, and H&N White birds. Half of the birds of each strain were confined in cages and half were kept in floor pens with a perch and a nest box. Birds reared in floor pens were given access to perches beginning at two weeks of age and were vaccinated against coccidiosis. Mortality during the laying period was 3.33% for the Lohmann White birds in the floor pens and 10.8% in cages. Similarly, for the Lohmann Brown birds it was 1.67% in the floor pens and 15.8% in cages, and for the H&N White birds mortality figures were 5.71% on the floor and 13.3% in cages, although mortality was higher during the rearing period for Lohman Brown and H&N White strains on the floor. [41]

These examples show that mortality is not inherently high in cage-free egg production. Experienced producers who are truly committed to improving the welfare of their birds have found viable methods for controlling mortality.

The egg industry is not static. It continues to evolve and reinvent itself in response to consumer demand. Changing the face of egg production is not only a matter of consumer concern, however, as meeting the welfare needs of the animals is an ethical imperative. The cooperative efforts of producers working together with retailers, scientists, consumers, and advocacy groups could bring about needed improvements quickly. While it may take some time for North American producers to make cage-free systems perform optimally, the longer we wait to make the transition, the longer hens in battery cages suffer.

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