Effect of intraruminal infusion of potassium on the site of magnesium absorption within the digestive tract in sheep

SUMMARY Sixteen 2-year old female sheep were fitted with ruminal and duodenal cannulae at Johnstone Memorial Laboratory, Lincoln University during 1989-90. They were oered, at 2 hourly intervals, a pelleted concentrate diet (900 g}day) and chaed lucerne hay (100 g}day). In a split-plot experiment they were infused, intraruminally and at four rates, with potassium (providing 16, 26, 36 or 46 g K}kg food DM}day) and magnesium (providing 1-3, 1-8, 2- 3o r 3 - 1gM g }kg food DM}day) within a Latin square design and with the liquid and solid phase markers &"chromium EDTA and "%"cerium chloride. Net absorption of Mg before and after the duodenum was estimated from dietary intake, duodenal flow and urinary and faecal excretion of Mg. Increasing K intake resulted in a decline in net absorption of Mg from the entire digestive tract, supporting data in the literature. Increasing K intake from 16 to 46 g}kg DM decreased urinary Mg excretion by between 0-14 and 0-30 g}day, the extent of which was independent of the level of Mg intake. At high K intake Mg absorption from the rumen was reduced, the amount absorbed ranging from 0-07 g Mg}day at intakes of 1- 3gM g }day and 46 g K}kg DM}day to 0- 66 gM g }day at intakes of 3- 1gM g }day and 16 g K}kg DM}day. However, at high K intake, and when Mg absorption from the rumen was reduced, net Mg absorption from sites distal to the rumen was increased to an extent which suggested compensatory absorption. Increase in K intake was associated with a consistent reduction in plasma Mg concentration which was independent of Mg intake. Increases in Mg intake resulted in increases in Mg absorption and plasma Mg concentration at all rates of K intake in direct proportion to rate of intake. The reduction in Mg absorption from the rumen at high K intake was associated with an increase (0-3 units) in pH of rumen digesta.