Introduction
Zinc (Zn) is an important trace mineral which has various roles in poultry nutrition. It serves as an essential nutrient and a metal cofactor for several enzymes. Sufficient zinc intake and absorption are required for various functions such as immunity against pathogens, growth, and quality of the egg. The importance of zinc in nutritional biology was first reported by Raulin (1869) when he observed that zinc was required for growth of Aspergillus niger. The enzyme carbonic anhydrase was identified and purified in 1940. It contains a metalloenzyme zinc at 0.33%, which catalyses the breakdown of carbonic acid into CO2 and H2O. Being a component of carbonic anhydrase, zinc facilitates transport of CO2 from tissues to lungs. Zinc is vital for the proliferation of cells and their differentiation. It is a structural constituent of many enzymes such as glutamic dehydrogenase, alkaline phosphatase and RNA polymerase. These enzymes are responsible for the metabolism of carbohydrates, proteins, and lipids. Zn catalyzes enzyme reactions and controls many cellular processes including DNA synthesis, normal growth, brain development, behavioral response, reproduction, fetal development, membrane stability, bone formation, and wound healing. It is also required for vitamin-A utilization and conversion of pyruvic acid to lactic acid. Zinc prevents free radical formation and protects biological structures from damage. It is an important element of thymus gland functioning. Zinc deficiency increases lipid peroxidation in mitochondria and microsomal membranes. It was observed that zinc deficiency is responsible for poor growth and abnormal bone development in chicks receiving purified diets. The dietary requirement of zinc for poultry is 40 mg/kg of feed. But regular poultry diets contain less amount of zinc concentration in feed. Recently, specific focus has been given on zinc and its importance in poultry. Consumers, producers, feed manufacturers and others related with poultry industry has developed a good attraction towards the role of zinc in poultry nutrition. Some important areas of research on zinc such as growth performance, egg production, egg quality, and immune functions are discussed as follows.
Zinc for growth performance
Zinc is essential for growth, maintenance and other functions such as bone development, feather development, enzyme structure and function, and appetite control in poultry. At the level of 0.012–0.018% on the total weight basis, Zn is commonly added as a supplement to all formulated poultry diets. It is typically added to the diet in the form of inorganic feed grade zinc oxide (ZnO) and zinc sulfate (ZnSO4) or in organic form in which zinc is bound to a ligand such as a propionate or methionine. ZnO is less bioavailable for poultry than reagent-grade or feed-grade Zn sulfate. However, sulfate (acid salt) is highly water soluble, allowing reactive metal ions to promote free-radical formation, which can facilitate reactions that lead to the breakdown of vitamins and ultimately to the degradation of fats and oils, decreasing the nutrient value of the diet. The oxide is less reactive, but also less bioavailable. Organic zinc sources are stable, electrically neutral and protect zinc from chemical reactions during digestion that would render the mineral unavailable to the bird. Organic zinc is more biologically available in the animal digestive system than inorganic zinc and that perhaps results in less mineral excretion and pollution of the environment. Researchers have observed that supplementation of zinc in inorganic form didn’t improve feed efficiency in broilers. But they reported that supplementation of zinc in broilers from three different organic sources improved feed efficiency. It is suggestive that supplementation of zinc from organic minerals instead of inorganic sources improves feed efficiency in broilers.
With respect to growth, zinc deficiency has shown poor growth rate because of improper nucleic acid synthesis. Chicks fed with basal diet and not supplemented with zinc grew poorly, did not feather properly, developed footpad dermatitis and could not walk normally. Chicks that were fed the same basal diet with added zinc did not develop any deficiency symptoms. This shows the importance of zinc in growth, feather development, skin and foot health, and feed efficiency in poultry. Zinc supplementation has increased weight gain in broilers, whereas some reports didn’t show any positive effect on weight gain. Some studies with inorganic zinc have shown that growth performance, leg abnormalities, and meat yields were not improved with excess zinc supplementation than NRC (1994) recommendation of 40 mg/kg. But in contrast, many researchers have observed an improvement in broiler growth performance when their diet was added with zinc in organic form. In addition, the presence of compounds such as phytate and high calcium in diet forms an insoluble complex with zinc and hampers zinc absorption and affects growth. Deficiency in some enzyme activity is responsible for the loss of appetite and taste.
Zinc in poultry nutrition
Zinc has shown many effects on the immune system. The nutritional immunity phenomenon of zinc deficiency and immune response has been widely studied. Zinc is a component of hormone thymosin (produced by the thymus cell) which is required for immunity purpose. It helps in the production of T-lymphocytes in the thymus. Zn deficiency has shown to decrease cellular immunity, thymus and spleen development. Zn is an important element of thymus gland functioning and supports in T cell maturation and differentiation by Zn-dependent thymic hormone called thymulin. Zinc impacts immunity in poultry and zinc supplementation in breeder diets has shown to enhance immunity of their progeny. Symptoms of zinc deficiency in young chicks included retarded growth, shortening, and thickening of leg bones, poor feathering, poor feed efficiency, loss of appetite, mortality in severe cases and reduced bone ash. Zinc supplementation in poultry has observed marked changes in immune system responses and disease resistance. Particularly zinc propionate and cellular immunity has been an area of study nowadays.
Dietary organic zinc supplementation at 80 mg/kg in old broilers and 40 mg /kg in young broilers improves immunity. Dietary organic zinc supplementation at the level of 40 or 80 mg/kg in the layer diet improves survival of E. coli challenged old laying hens. Similarly, immunity in broilers can be improved by the level of organic zinc in the diet. Layer birds supplemented with organic zinc had increased thymus weight and improved livability of progeny. Above studies indicates that zinc has a role in generating immune response and disease resistance activity in poultry birds.
Zinc in egg quality
Quality of eggshell is an important parameter in the egg industry as its value is totally dependent on it. If eggshell quality is not good, it directly affects the hatchability. For preventing some of the harmful organism like Salmonella which is a vertically transmitting bacteria, eggshell thickness, and its strength is essential. In the case of old layers, eggshell weakness is a major concern where broken egg percentage could be even more than 20% at the end of the laying period. Macro-minerals like calcium and phosphorous, and vitamin D3 are major nutrients which are responsible for eggshell quality in laying birds. Additionally, for the mineralization process, some enzymes are linked with macro-minerals. One of the enzymes is carbonic anhydrase, a zinc-dependent enzyme essential for supplying carbonate ions to convert calcium into calcium carbonate needed for eggshell formation in laying hens. Zinc concentration in birds is directly related to the activity of this carbonic anhydrase enzyme. Zinc and manganese are cofactors of metalloenzymes which are responsible for carbonate and mucopolysaccharides synthesis and are important in eggshell formation. Moreover, zinc, calcium, and manganese can affect structural properties of eggshell by affecting on calcite crystal formation and modification of the crystallographic structure of eggshell.
Zinc supplementation in diet can have a visible effect on egg quality and hatchability. A study reported that supplementing broiler breeder birds with a combination of an organic and inorganic source of zinc reduces the incidence of cracked eggs and improved eggshell quality. It was also observed that birds supplemented with a combination of an organic and inorganic source of zinc in diet showed the benefit of highest hen-day egg production as well as highest hen-housed chick production. Supplementation of organic sources of minerals like Mn, Cu, and Zn also resulted in greater eggshell thickness. Usage of an organic source of Zn helps in increasing the strength of eggshell in older hens.
Conclusions
Zinc plays multiple roles in poultry nutrition and helps in increasing body weight, feed intake and feed efficiency in broilers. Zinc also provides better eggshell quality in layer birds. Its role in immunity and disease resistance is remarkable. Even though zinc as a nutrient is known over many decades, still there is a need for study in aspects of both nutrition and production performance. According to NRC (1994), usually zinc in poultry ration is not sufficient but often diets are formulated with zinc at the level of 120 mg/kg of diet. So, wide safety margins are often used to avoid trace mineral deficiencies and allow birds to achieve their genetic potential. This higher level of addition possibly reduces deficiency under commercial condition.
by Rishikesh Jagzap, Kemin Industries South Asia Pvt Ltd
