Effect of different zinc levels on performance, egg quality traits and immune response of laying hens reared under high and recommended stock density

Introduction: Zinc (Zn) is required for many enzymatic and metabolic functions in the animal’s body (Prasad and Kucuk 2002). Also, it affects the antioxidant defence system, where deficiency of Zn increases oxidative damage to cell membranes (Prasad and Kucuk 2002). Zn was assumed to improve the antioxidant capacity (Liu et al., 2011) since Zn is necessary for the structure and function of superoxide dismutase, which protecting the brain, lungs, and other tissues from oxidation (Noor et al., 2002). However, it has been suggested that Zn increases the synthesis of metallothionein, a cystine-rich protein that acts as a free radical scavenger (Oteiza et al. 1996). Nys et al. (1999) reported that the deficiency of Zn decreased egg production and eggshell quality linked to its role as a cofactor in the enzyme carbonic anhydrase, which is essential for shell deposition. The NRC (1994) recommends a level of 40–75 mg/kg of Zn in various poultry diets. Due to the high amount of phytate, Zn can be less bioavailable, despite the high level of Zn available in pulses and cereals. Therefore, this micronutrient can be supplemented to diets of livestock and poultry (Sahin et al. 2009). ZnO and ZnSO4 are the two most common inorganic Zn supplements used for poultry diets (Batal et al. 2001). The present study has examined the effects of Zn supplementation of laying hens on performance, egg quality traits and immune response under high stock density.Material and methods: This experiment was conducted to evaluate the effects of dietary inclusion of different levels of zinc on performance, egg quality traits and immune response of laying hens under high stock density with 160 Hy-Line W-36 leghorn hens for 10 weeks. Treatments consisted of different levels of zinc (40, 80, 120 and 160 ppm) and 2 cage densities [3 hens/cage (recommended) or 5 hens/cage (high density) (38 × 38 × 40 cm)], that was performed in completely randomized design with 2×4 factorial arrangement with 5 replicates. The basal diet was formulated to meet all of the nutrient specifications according to the Hy- Line W-36 recommendations (Hy-Line International, 2007). The first 2-wk (61–62 wk of age) was considered as the adaptation period. The main trial period commenced from 63 wk of age and lasted for 8 weeks. Lighting program was set on 16 light: 8 dark using an artificial light in a windowless house. The hens had free access to feed and water at all times.The temperature was maintained at 20±2 °C throughout the study. Egg production and egg weight were recorded daily and feed intake (FI), feed conversion ratio (FCR) and egg quality were recorded weekly. Egg quality traits were evaluated every35-d period. All eggs produced during thelast2-d of each period were collected and egg quality indices including Haugh unit (HU), yolk color, egg shell thickness, andshell breaking strength were measured as followings: Each egg was weighed and the eggshell breaking strength (kg/cm2) was measured by a quasi-static compression device. The broken eggs were put onto a glass surface. The height of the albumen, midway between the yolk and the agde of the thick albumen, was measured with micrometer. Haugh units were calculated using the formula: HU=100log (H+7.57– 1.7×W0.37), where H is the mean height (mm) of the albumen and W is the weight (g) of egg (Haugh, 1937).Yolk color was visually scored by the Roche yolk colorfan. Eggshell thickness was determined on 3 points (air cell, equator, and sharpend) by using a micrometer screw gauge. In order to study immune responses, suspension of SRBC were injected into the breast muscle of two birds of each replicate at the first of 6 and 7th weeks of experiment and blood parameters were analyzed 7 days after each injection. Also reproductive parameters, blood lipid parameters were analyzed at the end of the experiment. Results and discussion: The results of this experiment showed that dietary supplementation of zinc in different stock density could not affect performance of layers (P > 0.05). Although, dietary increasing levels of zinc caused significant decrease in egg production percentage and egg weight during the whole period of experiment. Sorosh et al (2019) showed hens receiving 130 mg Zn/kg of diet, laid more eggs than the birds which receiving 40 and 70 mg Zn/kg of diet. Also, they reported hens receiving 130 mg Zn/kg of diet had lower feed consumption compared with the other treatments. FCR was influenced by supplemental Zn in which dietary inclusion of 70, 100, and 130 mg Zn/kg of diet improved FCR compared with that of the 40-mg inclusion level. Mirfendereski and Jahanian (2015) showed that hens in cages with higher stocking density had lower hen-day egg production, egg mass, and feed intake compared with those in normal density cages. Also they reported plasma concentrations of triglycerides and high-density lipoproteins were not influenced by dietary treatments. Sarica et al. (2008) observed that hen-housed egg production, egg mass, viability, and live weights were decreased by the higher stocking densities. In addition, the same authors reported that hens housed at the lower stocking densities reached sexual maturity significantly earlier than those in higher stocking densities. In the current experiment haugh unit was significantly lower in birds with high stock density compared with those in normal density cages. Jahanian and Mirfendereski (2015) showed that eggshell thickness was greater in hens under high stocking density challenge during the second 35-dperiod. Ovary weight was lower in hens under high stocking density challenge (P <0.05). Dietary supplementation of zinc (160 ppm) increased plasma LDL level and high stocking density increased plasma glucose level (P <0.05). The present findings indicated that dietary zinc supplementation in high stock density could not affected performance or egg quality of birds, although, levels of 120 and 160 mg Zn/kg of diet increased eggshell strength.