Effects of age and strain on yolk sac utilization and leptin levels in newly hatched broilers.

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 strains increased with age during 3 d posthatching. Compared with those of AA broilers, leptin levels in yolk sacs of BY broilers were greater (P <or= 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 <or= 0.0306) than that on d 0 and then decreased with age. Compared with AA broilers, BY broilers showed lower (P <or= 0.0254) levels of serum leptin on d 1 and 3. Hypothalamic leptin levels of both strains decreased with age except AA broilers on d 0. Hypothalamic neuropeptide Y (NPY) levels of BY and AA broilers increased with age until d 7 and then decreased. There were no differences in hypothalamic leptin and NPY levels between both strains during 11 d after hatch. Correlation analysis showed that average daily feed intake had a negative correlation with serum and hypothalamic leptin and positive correlation with hypothalamic NPY. Our results indicated that the dynamics of yolk sac utilization were similar between BY and AA broilers and decreased exponentially with age. The developmental changes of leptin and NPY in serum and hypothalamus with age varied in parameter and strain, and both signal molecules might be involved in the early programming of feed intake in chickens, but the mechanisms need further studies.