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Osteoporosis in laying hens has been a prominent welfare concern since the 1980s because of the relationship between osteoporosis and high fracture incidence during the laying period. In attempts to reduce the widespread occurrence of osteoporosis in laying hens, several research avenues have yielded positive results. Dietary changes to calcium-phosphorous ratios, supplemental large particle calcium strategies and genetic selection for high bone quality lines have all been moderately successful for reducing the severity of osteoporosis. Providing exercise during the rearing phase of pullets may help to build a stronger skeleton with a greater capacity for structural bone growth and medullary bone calcium reserves, thereby reducing skeletal depletion later in life. The objective of this study was to assess the effect of differing opportunities for exercise during the pullet rearing phase on the long-term bone quality characteristics of laying hens.
The effects of rearing system, standard cage (Conv) or rearing aviary (Avi), and adult housing system, conventional cage (CC), 30-bird furnished cage (FC-S), or 60-bird furnished cage (FC-L), were tested using a 2 × 3 factorial arrangement with rearing flock replicated in 4 blocks over time. Four consecutive flocks of Lohmann Selected Leghorn Lite (LSL-Lite) were obtained from a commercial hatchery at d one. For each consecutive flock, 960 pullets were conveniently selected for rearing in standard, conventional cages, and half were conveniently selected for rearing in a Portal Pullet rearing system. At 16 wk of age, 294 pullets from each rearing system (Avi and Conv) from each flock (1 to 4) were weighed and transferred to 2 adult housing rooms, each holding 12 Farmer Automatic Enrichable (Furnished) Cages, and one adult housing room holding 90 standard, conventional cages, of which 12 standard, conventional cages were included in the study. All flocks were fed identical, standard commercial diets. At 73 wk of age, 10% of the hens from each furnished cage (FC-L: n = 6; FC-S: n = 3; Total n = 54/ flock) from all 4 flocks were conveniently selected from various regions within the cage, weighed, euthanized and frozen for later bone 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).
There was an effect of rearing system on rearing BW, with the Avi pullets having a lower mean BW (1,213.2 g ± 14.8 SE) at placement into the adult housing systems than Conv pullets (1,240.7 g ± 14.7 SE; P = 0.042); however, placement of pullets was balanced for BW with no difference between adult housing treatments. Rearing system also had an effect on the BW of the adult hens at 73 wk of age. Hens that were reared in the pullet aviary had a higher mean adult BW (1,838.1 g ± 27.3 SE) compared to hens that were reared in conventional cages (1,770.1 g ± 26.0 SE; P = 0.049). The total density of the radius at 73 wk of age was greater in the Conv hens compared to the Avi hens (P < 0.001). The cortical density at 73 wk of age of the radius (P < 0.001), humerus (P < 0.001), and tibia (P < 0.001) were greater in the Conv hens than the Avi hens. Rearing system had the opposite effect on bone cross-sectional area. The radius of the Avi hens had a greater total bone mineral content (P < 0.001) and greater trabecular bone mineral content (P < 0.001) than the Conv hens at 73 wk of age. Rearing system did not affect any of the bone mineral content measures for the humerus of adult hens. There were no significant interactions between rearing and adult housing systems for any of the QCT measures. Adult housing system had an effect on several QCT bone characteristics for the radius and tibia, but not the humerus. Total bone mineral density was greatest in the FC-L compared to both the FC-S and CC for both the radius (P = 0.013) and tibia (P < 0.001). Trabecular bone mineral density was also greatest in the FC-L with no difference between the FC-S and CC for the radius (P = 0.027). Rearing system had an effect only on the breaking strength of the humerus (P < 0.001), with Conv hens exhibiting a greater BBS (9.2 kg ± 0.45 SE) compared to Avi hens (6.3 kg± 0.44 SE) at 73 wk of age. Adult housing system had an effect only on the BBS of the tibia (P < 0.006) with a higher breaking strength of hens in FC-L and FC-S (14.8 kg ± 0.76 and 14.4 kg ± 0.78 SE) than hens in CC (13.5 kg ± 0.77 SE) at 73 wk of age.
Overall, the greater total and cortical bone cross-sectional area observed in the Avi pullets at 16 wk of age, and the fact that the majority of effects of aviary-rearing were maintained through to the end-of-lay 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 aviary-reared pullet skeleton potentially affords greater space for bone mineralization and medullary bone deposition in 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.