Influence of Dietary Zinc and Vitamin C Supplementation on Some Blood Biochemical Parameters and Egg Production in Free-Range Laying Hens Влияние на цинка и витамин С върху биохимични показатели на кръвта и носливостта при свободно отглеждани кокошки

The study aimed to follow out the effect of antistress dietary supplements Zinteral 35 and vitamin C on the levels of some blood biochemical parameters (corticosterone, total cholesterol, glucose, total protein and creatinine) and egg production in laying hens during cold (7 o C), thermoneutral (19 o C) and hot (31 o C) periods. The fowls were divided in three groups (26 females and 3 males in each group). They were reared in a free-range management system with elements of organic production. The experimental treatments were as followed: first (control) group without dietary supplement, second group with 100 mg Zinteral 35 per kg diet containing 35 mg/kg zinc oxide, third group with the same amount of Zinteral 35 together with 250 mg vitamin C per kg diet. During the three periods with different ambient temperature, the hens supplemented either with zinc alone (second group) or co-administered zinc + vitamin C (third group) had significantly lower levels of plasma corticosterone (P<0.001), serum cholesterol (P<0.05) and glucose (P<0.05) than those from the first (control) group. The differences between the third and the first groups were bigger versus those between the second and the first groups. For the entire period (March 1 and June 21), egg production was higher by 2.22 % and 4.60 % in the second and third groups respectively in comparison to the first group. The combination of 100 mg Zinteral 35 and 250 mg vitamin C per 1 kg diet exhibited a synergistic effect in reducing cold and heat stress in laying hens and increased their egg production.