Water balance of the hen during egg formation

Laying hens consume more water on days when an egg is laid, than on non-laying days; the daily water intake is more than double--from 115 g. to 255 g. and the number of drinks taken increases proportionately. The weight of water in the egg produced is only 32 g. The requirement for this excess of water implies a considerable metabolic stress associated with egg formation. Studies have been carried out to determine why this water is consumed, and its metabolic fate. Water ingestion started to increase about 12 h. before oviposition; it rose steadily until about 2 h. before lay, then fell sharply. Urine minute volume showed similar fluctuations. At the time when the urine output fell just before lay, plasma osmolarity was higher than usual; this might be predicted from the drop in water consumption. Alterations in the water content of the oviduct were not sufficient to explain the increased fluid intake, and total body weight remained constant in spite of the consumption of the additional 140 g. water. These findings suggest that the water has a metabolic role, as it is not retained as a net fluid surplus.