EHects of dietary sodium bicarbonate on egg production and egg quality of laying hens during high summer temperature

Eggshell quality is one of the most common problems in layer stocks. Results from the field have shown that the incidence of cracked eggs in layer stocks ranges from 1-2% to 30-35% (ALTAN and ALTAN, 1992). This problem is more specific in older flocks when the birds were raised under high environmental temperatures. The eggshell consists of 98% calcium carbonate. The necessary Ca for shell formation is derived from the feed and the medullary bone. The source of carbonate ions is dissolved CO2 in the blood and in the uterine fluid. During the shell calcification, pH of uterine fluid and of blood decreases due to areduction in bicarbernia resulting from the use of bicarbonates by the uterus (MONGIN, 1968). Metabolic acidosis, caused by excess hydrogen ions that are produced during shell formation , adversely affects eggshell quality. Acidosis is partly compensated by renal and respiratory systems. Various feed additives have been used to prevent or to decrease the acidosis in laying hens. One of them, NaHC03, prevents the metabolic acidosis by reducing the cation deficit or by increasing the cation/chloride ratio (COHEN and HURWITZ, 1974). Blood pH, bicarbonate and total carbon dioxide are increased by NaHC03 supplementation to the diet (JUNQUEIRA et al., 1984). On the other hand the acid/base balance of the birds is affected by heat stress. Under high temperature conditions, birds regulate heat loss by evaporation of water from their lungs. As a result of hyperventilation, levels of plasma CO2 and HC03 are reduced and respiratory alkolosis occurs. Several researchers have reported reduced laying performance and eggshell quality due to high temperature (ETCHES et al 1995; PRAJAPATI and GHODASARA, 1992). ERNST et al (1975) reported that hens receiving NaHC03 produced significantly fewer rough-shelled eggs at high temperatures. It wa reported that the addition of NaHC03 to drinking water improves the eggshell quality (FRANK and BURGER, 1965; ODOM et al. , 1985) and increases egg production (FRANK and BURGER, 1965). However, reports have conflicting results about dietary NaHC03 supplementation; while some authors indicate benefits (FRANK and BURGER, 1965; HOWES, 1966; COHEN and HURWITZ, 1974;