Introduction

Abstract

Avian osteoporosis has been recognized as a problem in the laying hen industry for at least 45 years. The onset of this disease in individual hens decreases bird welfare, eggshell quality, and egg production by the flock but also increases the incidence of broken bones and lameness. Webster presents an overview of the welfare implications of osteoporosis in laying hens. This author discusses the perception of pain by birds, and relates the lesions associatedwith osteoporosis to the degree of pain perceived by the bird. Attention is also given to humane handing and disposition of spent hens.The large demand for Ca for eggshell formation compounds the inability of actively laying hens to deposit new structural bone. The symposium paper presented by Whitehead reviews basic avian bone biology, with special reference to the factors involved in development and prevention of excessive structural bone loss in laying hens.Medullary bone is a unique form of labile bone found in egg-laying birds; potential methods to optimize medullary bone reserves and utilization by the hen are presented.Estrogen plays an important role in the regulation of Ca metabolism in laying hens. Beck and Hansen discuss the metabolic functions of estrogen in the laying hen, and address the topic of changes in estrogen synthesis and receptor numbers as the hen ages. As in humans, simple nutritional interventions are of limited use in preventing avian osteoporosis. Therefore, a deeper understanding of the factors that regulate metabolism in laying hens is required.Because of the serious implications for laying hen productivity and welfare, much research over the years has been conducted into the causes and prevention of osteoporosis. Measurements of production such as egg numbers and eggshell quality have been used to indirectly assess the effects of dietary, pharmaceutical, or management treatments on Ca metabolism of laying hens. Direct measurement of the skeletal health of laying henswas almost always conducted postmortem. Traditionally, measurements of avian bone quality involved sacrificing a portion of the population at various times throughout or at the end of a trial and conducting destructive tests on the bones. These tests included measurement of specific bonemass and volumetric density, breaking strength, and bone ash andmineral measurements. Although many important fundamental discoveriesweremade using these techniques, the availability of modern analytical techniques and equipment has spurred an increase in the use of noninvasive testing of bone quality of laying hens. Fleming and co-workers present results of their studies using quantitative ultrasound, in which the speed of sound through a bone is dependent upon the density of the specific location on the bone in question (amplitude-dependent speed-of-sound ultrasound). Hester et al. offer details on their use of dual-energy x-ray absorptiometry to assess bone density of entire bones. X-rays of 2 distinct energies are passed through the bone and recorded, and the attenuation of each of the energies by the bone is used to calculate a two-dimensional radiographic density. Korver et al. communicate their work with quantitative computed tomography. This technique involves passing an x-ray at numerous angles through a fixed point on a bone. The resultant cross-sectional image allows calculation of a true volumetric mineral density as well as spatial distribution of bone mineral at that specific crosssection of bone.The previous 3 papers compare each of the newer noninvasive