The dynamics of yolk sac utilization and changes of leptin levels in serum, hypothalamus, and yolk sac with age were investigated in Beijing-You (BY) and Arbor Acres (AA) male broilers during 11 d after hatch. The growth rate and feed intake of BY broilers were lower (P < 0.0001) than those of AA broilers, but the dynamics of the weights and total energy contents of yolk sacs were similar between both strains and decreased exponentially with age. Leptin levels in yolk sacs of both broiler

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... s increased with age during 3 d posthatching. Compared with those of AA broilers, leptin levels in yolk sacs of BY broilers were greater (P ≤ 0.0413) on d 0 and 3. There was no change in serum leptin levels in BY broilers, whereas in AA broilers, serum leptin levels on d 1 and 3 were greater (P ≤ 0.0306) than that on d 0 and then decreased with age. Compared with AA broilers, BY broilers showed lower (P ≤ 0.0254) levels of serum leptin on d 1 and 3. Hy-