Effect of different levels of zinc oxide in diets containing sodium selenite on growth performance and some immune indices of suckling Holstein calves

Introduction: Microelements in organisms have several structural, catalytic, and regulatory functions and play an important role in the functioning of the immune system. The trace mineral content of colostrum and milk is not optimal and in suckling calves, the inclusion of trace mineral supplements in the diet is necessary. Selenium (Se) is a basic mineral for humans and animals and has now been identified as an integral component of more than 35 selenoproteins which are involved in enzymatic, structural, or as yet unidentified functions. In livestock, Se is important for fertility and the prevention of diseases. Sufficient intake of Se increases fertility and enhances antioxidant protection systems and immunological potential. Se deficiency in cattle is associated with several problems, including delayed conception, muscular degenerative disease in calves, myocardial necrosis, heart failure, impaired immune function, increased risk of mastitis, abortion, perinatal mortality, and growth retardation in young calves. Based on the recommendation of the National Research Council, the daily requirements for Se and vitamin E in growing calves are 0.30 mg and 40 IU, respectively. To avoid Se deficiency and to promote animal health, inorganic sources of Se (sodium selenite and selenium) are routinely used as a supplement to the diet of farm animals. Zinc is involved in over 300 enzymes either as a component or as an activator. Zinc (Zn) is involved in immunity, metabolism, growth, and reproduction. It plays a role in the structure and function of antioxidant enzymes. Due to mineral interactions, the inorganic forms are less bioavailable in the gastrointestinal tract. The aim of this study was therefore to assess the effects of supplementation with sodium selenite and Zn oxide on growth performance, immune response, blood parameters, and antioxidant status in Holstein calves. Materials and methods: The calves were kept in well-ventilated sheds with concrete floors and with individual feeding and watering facilities. The basal diet was prepared from locally available feed ingredients to meet the nutritional requirements of calves growing except for Zn and Se. Calves are fed milk at 10% of body weight in the morning and evening. The experiment started at four days of age and the calves were kept in individual pens for up to 42 days. The study used 36 Holstein calves with an average weight of 34.5±2 kg over 42 days in a completely randomized design. The basal diet (containing 40 mg Zn and 0.15 mg Se) was without Zn and Se supplementation (diet 1). Diet 2 contained 0.3 mg sodium selenite, diet 3 contained 40 mg Zn oxide, diet 4 contained 55 mg Zn oxide, diet 5 contained 0.3 mg sodium selenite + 40 mg Zn oxide, and diet 6 contained 0.3 mg sodium selenite + 55 mg Zn oxide. Feed intake was calculated daily, and calves were weighed weekly to calculate the feed conversion ratio and average daily gain. Blood samples were taken on days 20 and 42 to determine trace mineral levels in plasma and biochemical, enzymatic, hormonal, antioxidant, and hematological parameters. Data on growth and blood parameters were analyzed using the GLM procedure of the SAS program. Significant difference between treatments was determined by Duncan's test and the results were considered significant if the P-value was less than 0.05. Results and discussion: The results showed that the use of sodium selenite and different levels of Zn oxide did not have a significant effect on feed intake, final weight, body weight gain, growth performance, and immune response (P>0.05). No significant differences in blood metabolite concentrations were observed between treatments (P>0.05). Supplementing milk with 0.3 mg sodium selenite and 40 and 55 mg Zn oxide significantly increased the activity of superoxide dismutase and decreased the levels of malondialdehyde and aspartate aminotransferase (P<0.05). Se plays a role in the defense against the accumulation of hydroperoxides from cellular metabolism. This biological function is mediated by selenoproteins such as glutathione peroxidase (GPx), iodothyronine deiodinases, and thioredoxin reductases, of which selenium is a structural component. Se also acts on enzymes involved in the production and regulation of thyroid hormones and is recognized as a factor in immunological function. The role of Zn as an antioxidant and its role in cell replication and proliferation are the two most directly related links between Zn and the immune system. Conclusions: Supplementing milk with Zn oxide and sodium selenite improved the antioxidant status and helped to relieve the stress experienced by suckling calves.