INFLUENCE OF SALINE DRINKING-WATER ON EGGSHELL QUALITY AND FORMATION

Many studies have shown that saline drinking water supplied to mature laying hens at concentrations similar to those found in underground bo re water has an adverse effect on eggshell quality and on the incidenc e of eggshell defects. This response can be observed in the absence of any adverse effects on production parameters such as food intake, egg production and egg weight. Strain differences appear to exist, and ev en within a strain there is considerable variation in the responses of individual hens to saline drinking water. The sensitivity of the hen also increases with age with a greater occurrence of broken and shell- less eggs. The incidence of shell-less eggs also increases with higher concentrations of NaCl in the water. The use of saline drinking water with breeder flocks significantly reduces the production of day-old c hicks through reduced numbers of settable eggs and lower hatchability. The primary metabolic lesion associated with the poor eggshell qualit y which results from the intake of saline drinking water appears to be related to the supply of bicarbonate rather than calcium to the lumen of the shell gland for eggshell formation. A reduced activity of carb onic anhydrase in the shell gland mucosa is of particular importance. This limits the supply of bicarbonate and the dependent calcium to the lumen of the shell gland. Treatments which have been found to be effe ctive are preventive rather than remedial in nature. The two most prom ising treatments, apart from desalination of drinking water, appear to be the use of ascorbic acid supplements in the diet or drinking water and the use of zinc-methionine supplements in the diet. The mechanism by which ascorbic acid exerts its effect is unknown but the response is dose-dependent. The zinc component of zinc-methionine appears to be the important factor with this treatment, perhaps through its effect on carbonic anhydrase, a zinc-requiring enzyme.