Nitrogen balance and yolk corticosterone levels of laying hens fed low-protein diets from 33 to 64 weeks of age

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OBJECTIVE: The study aimed to investigate the effects of varying dietary crude protein levels on laying performance, nitrogen balance, odor emission, and yolk corticosterone in laying hens from 33 to 64 weeks. METHODS: Two hundred and forty laying hens (Hy-Line Brown) were randomly assigned to one of four varying protein diets from 18.5% to 12.5% at 33-45 weeks, from 18.0% to 12.0% at 46 to 55 weeks, and from 17.0% to 11.0% at 56 to 64 weeks. RESULTS: Laying performance (i.e., egg weight, egg production, and egg mass, p<0.05) was linearly declined with decreasing dietary crude protein levels. Eggshell thickness decreased (quadratic effect, p = 0.033) at 44 weeks of age, while increased (linear effect, p<0.001) at 52 weeks of age as dietary protein levels lowered. Decreasing dietary protein levels linearly increased (p<0.05) Haugh unit at 52 and 64 weeks. Apparent digestibility of dry matter and crude protein increased at 45 and 55 weeks of age as dietary protein levels decreased. Lowering dietary protein levels increased the concentration of high-density lipoprotein cholesterol (linear effect, p = 0.012) but decreased (linear effect, p = 0.021) uric acid levels in serum samples at 64 weeks of age. Nitrogen excretion linearly declined as dietary protein levels decreased at all ages. Among the odors analyzed, fecal volatile fatty acids increased at 55 weeks of age with decreasing dietary protein levels. Finally, yolk corticosterone was not altered by dietary protein levels during the laying cycle. CONCLUSION: It is concluded that decreasing dietary protein levels fortified with limiting amino acids can be applied to mitigate nitrogen excretion without affecting nutritionmediated stress in laying hens.

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