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Environmental Heat and Partial Water Restriction Effects on Body Fluid Spaces, Water Loss, Body Temperature, and Metabolism of Holstein Cows¹

Department of Dairy Husbandry, University of Missouri, Columbia 65201

ABSTRACT

The effects of partial water restriction on metabolism, body fluid compartments, and water balance were measured in seven mature nonlactating Holstein cows at two constant environmental temperatures, 18 and 32 C. For 7 days at each temperature, water was restricted to 50% of ad libitum consumption.

Partial water restriction at 18 C caused a 50% reduction in water loss with a reduction in total body fluid volume mainly from the extravascular fluid compartment. This was followed by a reduction in feed consumption and metabolism. At 32 C partial water restriction caused a great reduction in water loss through urine and feces, allowing more of the water to be used for heat loss by vaporization.

Partial water restriction, unlike total water withdrawal, did not change the volumes of vascular compartments at either normal or high temperatures. However, animals exposed to normal and high temperatures respond differently to water restriction. At 18 C similar percentage reductions (approximately 50%) occur in urine, fecal water, and total vaporization to reduce water loss during water restriction. But at 32 C water loss is reduced mainly by reduction of urine and fecal water, rather than vaporization.

Comparison between ad libitum animals at the two temperatures showed that heat caused a 15.9% reduction in metabolism. Water restricted cows, compared to ad libitum cows at 32 C, showed a 17.0% reduction in metabolism. Results suggest the reduced metabolism is a compensatory mechanism to reduce internal heat production in an environment of high external heat and low water availability. Also, insignificant reduction in blood and plasma volumes produced available fluids for heat loss through vaporization when water was restricted at a high temperature.

¹ Contribution from the Missouri Agricultural Experiment Station. Journal Series 6396. Approved by the Director.