Life cycle changes in bone mineralization and bone size traits of commercial broilers

Life cycle changes in bone mineralization and bone size traits of the tibia and humerus were evaluated in commercial male and female broilers using dual-energy x-ray absorptiometry (DEXA). Experiment 1 evaluated weekly changes in bone traits from 2 to 7 wk of age, whereas experiment 2 compared the bone traits of 4 strains of commercial meat-type chickens from 4 to 8 wk of age. Birds were restrained without anesthesia, and the humerus and tibia were scanned in vivo. After each scan, individual BW was determined. From the DEXA scans, bone mineral density (BMD), bone mineral content (BMC), as well as bone length, width, and area were determined. Bone mineralization and size traits were analyzed as an analysis of covariance with BW as the covariate. If BW was NS as a covariate, then an ANOVA was used. The BMD reached its peak at 4 wk of age. The BMC of the humerus changed little from 2 to 8 wk of age, whereas tibial BMC increased as the birds aged, especially in males (P < 0.0001). In experiment 1, bone length, width, and area also increased with age (P < 0.0001), with the tibia growing in length at a faster rate than the humerus. In experiment 2, the BMD did not differ among the 4 strains of commercial broilers. Interactions with strain of chicken were also NS, indicating that all strains of chickens responded similarly with respect to age (4, 6, and 8 wk of age), sex, and type of bone (humerus vs. tibia). Coefficients of variation for BMD ranged from 15 to 16%, indicating a potential use of DEXA for selection to improve skeletal integrity. In conclusion, the tibia continued to grow, especially after the initiation of the growth spurt at 3 to 4 wk of age, as indicated by bone length, width, and BMC, but it did not become denser in mineral after 4 wk of age as its surface area increased.