Net exchange of certain inorganic ions and water in the alimentary tract of the milk-fed calf

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Abstract
Calves of different ages were suitably fistulated so that samples could be obtained from different points in the alimentary tract and were fed on milk containing polyethylene glycol as a marker. Preliminary experiments showed this substance to be recovered almost quantitatively from the distal end of the small intestine although a mean loss of about 10% apparently occurred in the large intestine. In different calves of different ages the net effect of the exchanges of sodium, chloride and water up to the exit from the abomasum was equivalent to the progressive addition of a fluid containing these constituents in approximately constant ratios to each other except that the relative amount of chloride increased somewhat after feeding. Calcium, magnesium and inorganic phosphorus were temporarily held up in the abomasum presumably in association with the milk clot. In young calves (age 2-5 weeks) the mean net absorptions of magnesium up to the distal end of the small intestine and in the large intestine respectively were about 25 and 35% of the intake. The former figure did not change markedly with age or vitamin D status; the latter decreased greatly with age. It appeared that the true percentage absorption of magnesium up to the end of the small intestine decreased with increasing intake. Mean net absorption of calcium up to the end of the small intestine was about 86% of the intake in the young calves and apparently decreased with age. Vitamin D markedly improved the absorption in older calves (age about 3 months) but wide variations in magnesium intake had no apparent effect. Net exchange of calcium in the large intestine was negligible at all times. Water, potassium and chloride were, as a net effect, efficiently absorbed up to the end of the small intestine (mean values 80-95% of intake) but further net absorption of all occurred in the large intestine. Mean net absorption of sodium was only about 40% of the intake up to the end of the small intestine but almost complete in the large intestine. Net exchanges of these constituents did not vary markedly with age. The pH of the tract contents varied from 4.2 to 2.3 at the exit from the abomasum and was about 7.6 at the distal end of the small intestine.