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

Sixteen 2-year old female sheep were fitted with rumen and duodenal cannulae at the Johnstone Memorial Laboratory, Lincoln University during 1989-1990. They were offered, at 2 hourly intervals, a pelleted concentrate diet (900 g/day) and chaffed 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, 2.3 or 3.1 g Mg/kg food DM/day) within a latin square design and with the liquid and solid phase markers ⁵¹Chromium EDTA and ¹⁴¹Cerium 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 0.08 g Mg/day at intakes of 1.3 g Mg/day and 16 g K/kg DM/day to 0.46 g Mg/day at intakes of 3.1 g Mg/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