Low levels of organic compound trace elements improve the eggshell quality, antioxidant capacity, immune function, and mineral deposition of aged laying hens oleh K.K. Zhang, M.M. Han, Y.Y. Dong, Z.Q. Miao, J.Z. Zhang, X.Y. Song, Y. Feng, H.F. Li, L.H. Zhang, Q.Y. Wei, J.P. Xu, D.C. Gu, J.H. Li