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Osteoporosis is described as the development of fragile, brittle bones from loss of bone mass and deterioration of the microarchitecture of the bone leading to a greater susceptibility to fractures. In laying hens, osteoporosis poses an animal welfare concern because of its high prevalence and association with fractures during and at the end of the laying period. In the 1950s, initial investigations into the causes of severe bone loss in laying hens suggested the importance of exercise, noting that the problem was highly prevalent in hens housed in confined, conventional (i.e., battery) cages, yet was rarely seen in hens housed in open, barn-floor systems. The overall objective of this study was to assess the effects of different opportunities for exercise during the pullet rearing period. In this paper, we present results on keel bone and musculoskeletal characteristics in a sample of pullets obtained from 2 consecutive flocks at 16 wk.
Two consecutive flocks of Lohmann Selected Leghorn Lite (LSL-Lite) chicks were obtained at 1 d from a commercial hatchery. From each flock, 960 of the chicks were placed in standard conventional cages (Conv) with 16 pullets/cage during wk 0–6 and a space allowance of 145 cm2/pullet, followed by 8 pullets/cage during wk 6–16 with a space allowance of 290 cm2/ pullet. The remaining 756 chicks from each flock were placed in a single Farmer Automatic Portal Pullet rearing system (Avi) with system space allowance of 285 cm2/pullet during wk 0–6, and system + outer platforms + litter space allowance of 754 cm2/pullet during wk 6–16. The aviary system was selected to allow maximum opportunities for exercise. Both the Conv and Avi pullets for both flocks were fed identical. Both rearing treatments also followed identical vaccination and lighting programs. A subsample of 100 pullets/flock from each rearing system were weighed biweekly. At 16 wk of age, a sample of 20 Avi and 20 Conv pullets from each flock were euthanized and frozen at −20°C for later collection of muscle and bone tissues. Following muscle collection, the bones from the right side were used for Quantitative Computed Tomography (QCT) analysis and the bones from the left side were used to test bone breaking strength (BBS). Measurements of the keel were taken immediately following dissection and weighing of muscle tissues.
There was no difference in mean BW (P = 0.875) between Avi (1,204.6 g ± 18.1 SE) and Conv (1,202.1 g ± 18.2 SE) pullets at 16 wk. Rearing system affected all muscle and keel characteristics, except for keel height. The muscle weight of the bicep brachii, pectoralis major, and pectoralis minor was greater in the Avi pullets compared to Conv pullets (P < 0.001); however, the weight of the leg muscle group was greater in the Conv pullets compared to Avi pullets. The length of the metasternum was greater in the Avi pullets compared to the Conv pullets (P = 0.003). The length of the cartilage on the keel bone (P < 0.001) and percentage of the keel comprised of cartilage (P < 0.001) were greater in the Avi pullets compared to the Conv. Keel area was greater in the Avi pullets compared to the Conv pullets (P = 0.026). The distance between the keel and pubic bone (P < 0.001) was greater in the Conv pullets compared to the Avi pullets; however, there was no difference in the distance between the pubic bones (P = 0.717) for the Avi and Conv pullets. The total bone mineral content and cortical bone mineral content was greater in the Avi pullets compared to the Conv pullets for the radius, humerus, and tibia (P < 0.001). The BBS of the humerus, radius, and tibia were greater in the Avi pullets compared to the Conv pullets (P = 0.014 or less).
Avi pullets developed stronger bones as evident by greater BBS values for all 3 bones compared to the Conv pullets. Overall, the greater total and cortical bone area and breaking strength observed in the Avi pullets at 16 wk of age indicates that opportunities for diverse, loading exercise during the rearing phase substantially alters the geometry of growing pullet bones. This increase in size of the Avi pullet skeleton potentially affords greater space for bone mineralization and medullary bone deposition during the laying phase of adult hens. Perhaps other avenues of research targeting methods to increase the rate of calcium absorption or medullary bone deposition can be used in conjunction with this increased skeletal growth to capitalize on the newly available skeletal framework.