Zootecnica International - World Poultry Journal

D.R. Korver

Department of Agricultural, Food and Nutritional Science,
University of Alberta,
Edmonton, Alberta,
Canada

Modern poultry production methods have allowed for increased levels efficiency of production for both meat- and egg-type poultry. Although advances in genetics, nutrition and management have largely allowed this progress, care must be taken by the poultry industry to ensure a balance between the desire for greater and more efficient production and the health of the birds. As has been observed in the past with broilers and turkeys, and is currently the case with laying hens, the pursuit of increased performance without concurrent attention paid to the metabolic and physiological needs of the animal leads to decreased health, performance and welfare.
The broiler and turkey breeders have addressed many of the skeletal problems of the past through intensive selection for skeletal health; the breeders of egg-type hens have begun to address these problems as well.
New techniques to assess bone development and integrity in vivo will allow researchers to more closely monitor individual birds throughout production, and identify factors that may be useful in allowing future increases in production to take place with minimal impact on skeletal health.

Introduction

Genetic selection and improvements in nutrition and management have led to dramatic increases in potential for growth in meat-type birds and egg production in laying hens. Table-egg layers are typically small-framed birds bred for high levels of egg production. The huge demand for calcium for eggshell formation, the typically small appetite of many strains of layers, and the effects of bird age can all have a negative impact on bird skeletal health. In the case of meat-type birds, rapid growth can lead to abnormal skeletal development; intense efforts by primary breeders in recent years have minimized these problems, although continued effort is required to keep ahead of the problem. Broiler breeder hens are typically feed-restricted, which can interfere with the normal mechanisms used by egg-laying birds to maintain calcium balance and support both skeletal integrity and eggshell formation.

Laying hen skeletal abnormalities

Laying hens are typically the most sensitive type of poultry to perturbations in Ca metabolism, and subsequently, skeletal disease. The requirement of the hen for Ca for eggshell formation places a huge demand on the relatively small frame of the hen. A table-egg-type hen deposits approximately 2.3 g of Ca in an eggshell, which represents about 10% of the total Ca present in the skeleton of the hen (Etches, 1987). By the end of the production cycle, the hens are susceptible to osteoporosis (Whitehead and Fleming, 2000), with up to 30% of hens having broken or healed broken bones at depopulation (Gregory and Wilkins, 1989).
The hens have a natural storage compartment of labile Ca called medullary bone. Medullary bone is typically formed approximately 2 weeks prior to the onset of lay (Hurwitz, 1964), in response to an increase in circulating estrogen. This type of bone is deposited in the shafts of long bones, and is readily mobilized when Ca demand for eggshell formation is greater than Ca supply from the diet. Medullary bone is rapidly deposited from dietary Ca during periods of low Ca demand (Van de Velde et a.l, 1985).
During periods of high demand (such as near peak egg production), and as the hen ages (when efficiency of Ca absorption and deposition decreases), the hen may not be able to mobilize sufficient Ca from the medullary stores, and cortical (structural) bone may be mobilized. In addition, endosteal surfaces not covered by a layer of medullary bone result in cortical bone degradation by osteoclasts, further weakening the bones (Whitehead, 2003). Although total bone density may actually increase by the end of a production cycle, almost all of the new bone is medullary bone (Whitehead, 2003), which has little inherent strength (Fleming et al., 1998).

Broiler and turkey skeletal abnormalities

Meat-type birds (broilers and turkeys) have been genetically selected for many generations for rapid growth rate, mainly as skeletal muscle, and efficiency. At times over the years, the capacity of the skeletal system of these birds has lagged behind the ability of the birds to deposit skeletal muscle. Skeletal problems are often related to the rate of growth; many of the compensatory growth (eg. light restriction) programs developed in the 1980s and early 1990s decreased the incidence of skeletal abnormalities as a consequence of slowed early growth (Classen et al., 1991; Riddell and Classen, 1992). Nutrition plays both direct and indirect roles in the development of skeletal disease in meat-type poultry, but genetics are also very important (Riddell, 1992). In recent years, poultry primary breeding companies have substantially decreased the incidence of metabolic and developmental skeletal disease through intensive selection of stock with rapid growth rate, good feed efficiency and a resistance to development of skeletal disorders.

Broiler breeder hen skeletal abnormalities

Broiler breeder hens not only face many of the same issues of Ca supply and demand as table-egg layers, but additional challenges brought on by standard breeder management techniques. Breeder hens are typically feed-restricted to control body weight; the breeders have similar potential for growth as broilers, and must be nutrient-restricted in order to prevent obesity and associated reproductive difficulties. In most management situations, breeder hens are fed a limited amount of feed, early in the morning. This presents a challenge to the hens, as most eggshell formation takes place at night (Etches, 1987). As the Ca from the diet is available for only a brief period during shell formation, Ca must be sequestered in the medullary bone stores to make up for the lack of dietary Ca during much of the shell-forming period within a day.
Maintaining flock uniformity is extremely important in breeder hen management, especially in terms of the age at sexual maturity. A non-uniform flock is likely to have an extended period in which individual hens lay their first egg. This will result in some hens being capable of depositing medullary bone, and also having increased Ca requirements for eggshell formation many weeks before the last hens to lay. Hens are not capable of developing medullary bone reserves until approximately two weeks prior to sexual maturity, and a change from a low Ca grower diet to a high Ca breeder diet will necessarily not meet the requirements of the entire population if the flock is not uniform.
Research at the University of Alberta has shown that increasing dietary Ca to breeder pullets to soon (before they are capable of forming medullary bone), as well as too late (six weeks after photostimulation) can both lead to decreased egg production, shell quality and bone quality at 31 weeks of age (Petruk and Korver, unpublished observations). Over an extended period of time, the hens with impaired Ca reserves may be susceptible to the same perturbations in Ca metabolism as laying hens. Thus, increased flock uniformity and appropriate timing of increased dietary Ca will decrease the number of hens that are switched to a high Ca diet either too soon or too late to support maximum production and bird health.

New methods for measurement of bone quality in poultry

In the past, most assessments of skeletal health in poultry involved removing a sample of birds from the study population at various times during production, killing the birds and conducting destructive tests such as breaking strength, dried bone weight, ash and specific mineral measurements. Patterns of bone accretion and remodelling were followed on a population basis, with the assumption that the sampled birds reflected the state of the entire population. Obviously, this approach limited the ability of researchers to correlate individual bird traits to entire production cycles, especially if the birds were sampled before the end of the cycle. Although much of our current knowledge of avian bone biology has been obtained using these techniques, newer, more sophisticated methods have been adapted for use in poultry.
In a symposium on avian osteoporosis, held in Madison, some of the newer techniques discussed include digitised fluoroscopy (DF), amplitude-dependent speed of sound (Ad-SoS) ultrasonography, dual-emission x-ray absorptiometry (DEXA) and peripheral quantitative computed tomography (pQCT). DF uses video digitisation of radiographic image intensity to create an image suitable for computer analysis of bone density (Fleming et al., 2003). Ad-SoS ultrasonography measures the speed of sound through a bone across a precisely measured distance to quantify bone density; the greater the density, the higher the velocity of sound (Fleming et al., 2003). DEXA analysis employs a system in which x-rays at two distinct energies are passed through the bone of interest, the attenuations are recorded and used to calculate a two dimensional radiographic density (Hester et al., 2003). Analysis of bone by pQCT involved passing an x-ray through the bone at numerous angles in a single plane; the attenuation of the x-ray at each angle is used to calculate a three dimensional, volumetric density (Korver et al., 2003). Each of these techniques has advantages and limitations, but all can be used on live birds. By keeping the birds alive, an individual bird can be followed through an entire production cycle, and factors such as feed intake, egg production, body weight and bone density can be correlated at multiple ages, giving a much more powerful data set than can be generated by removing birds for production as is required with the traditional methods.
These newer techniques will not replace the traditional methods by which so much of our current knowledge has been gained. However, some of these techniques allow researchers to follow individual birds throughout the entire production cycle and identify factors related to skeletal health or disease. Ultimately, fewer birds will be sacrificed prematurely, and the rate of advancement of avian bone biology will increase, and hopefully keep pace with advances in productivity.

Conclusions

The skeleton is an essential support element of poultry production; providing support for the body, allowing movement and in the case of egg-laying birds, providing an essential store of Ca for eggshell formation during times of low dietary Ca supply. Many of the production expectations placed on modern poultry put the bird at risk for diseases of the skeletal system.
Advances in genetics, nutrition and management have and will continue to play a role in maximizing productivity and minimising skeletal abnormalities. The use of traditional analytical methods for bone quality will be used in conjunction with the newer techniques to rapidly advance the knowledge of avian bone biology.

References

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