4Steenfeldt S,Kjaer J B,Engberg R M.Effect of feeding silages or carrots as supplements to laying hens on production performance,nutrient digestibility,gut structure,gut microflora and feather pecking behaviour[J].British Poultry Science,2007,48(4):454-468.
5Kucuk O,Salin N,Salin K.Supplemental zinc and vitamin A can alleviate negative effects of heat stress in broiler chickens[J].Bio logical Trace Element Research,2003,94(3):225-235.
6Lin H,Wang C F,Song J L,et al.Effects of dietary levels of vita min A on the egg yield and immune responses of laying hens[J].Poult Sci,1998,77(4):542-546.
