Dietary influences on the hydration and acid-base status of experimentally dehydrated dairy calves.

The incorporation of oral rehydration solutions (ORS) into 'milk meals' is potentially an effective, time-saving method of treating diarrhoeic calves. Although milk-based ORS are effective in improving the hydration and acid-base status of healthy calves, this effect remains to be confirmed in dehydrated/diarrhoeic animals. In this study, six experimentally-dehydrated calves were fed with either milk replacer (MR) or an ORS prepared in either water (WORS) or MR (MORS). In one experiment, calves were not treated and blood samples were taken before and after feeding. Parameters of hydration status were determined and blood gas analysis was performed. Plasma volumes increased significantly following the intake of a 'fluid meal' whereas they remained constant in the absence of treatment. The rate of plasma volume expansion was reduced by the feeding of MR relative to WORS or MORS. In dehydrated calves, the expansion of plasma volume was more pronounced following the intake of WORS but the increase was less and plasma osmolality increased significantly following the ingestion of MORS. The acid-base status of animals improved as a result of fluid absorption, but this effect was less obvious as the experimental protocol resulted in severe dehydration and moderate acidosis. Feeding hypertonic MORS raised the plasma osmolality in dehydrated calves, and may increase the risk of hypernatraemia in diarrhoeic calves, which should therefore have ad libitum access to water when undergoing treatment with hypertonic ORS. Further research is planned to assess whether feeding ORS reconstituted in milk or MR combined with ad libitum access to water offers a practical treatment for diarrhoeic calves.