Proceedings of the New Zealand Society of Animal Production, 1996,
56: 133-137
AB: Six rumen-cannulated ewes, averaging 48±3.7 kg liveweight, were
used to study the effects of sodium (Na) or water loading on magnesium
(Mg) metabolism in sheep fed on a high potassium (K) diet (40 g K/ kgDM).
Animals were randomly allotted to receive one of three treatments; 1) high
Na diet (15 g Na/d) (HNa), 2) low Na diet (1.3 g Na/d) + infusion of water
(approx. 3 l/d) equivalent to the water intake of the HNa treatment
(LNaHW) and 3) low Na diet (LNa) alone. A nutritional balance and renal
clearance studies were used in two 3x3 Latin square change-over designs.
Apparent absorption of Mg was not different (p>0.05) between treatments,
averaging 0.27±0.03 g/d, inspite of an increase in the rumen Na:K ratio from
0.7:1 to 1:1 with increasing Na intake. Urinary Mg excretion was
significantly higher (p<0.01) on high Na intake (0.38±0.02 g/d), while water
loading had no effect (0.30±0.02 g/d) when compared with the low Na diet
(0.27±0.02 g/d).
Increasing Na intake, but not water loading, resulted in an increase in
glomerular filtration rate (GFR) from 58.2±2.41 to 64.5±1.27 ml/min and, as
a result total excretion (TE) and fractional excretion (FE) of Mg were
increased from 12.9±1.10 to 17.4±1.83 mg/h and 21.3±2.26 to 26.4±2.04%,
respectively.
Increasing Na intake to reduce the K:Na ratio in the rumen has been
advocated to improve Mg absorption on tetany prone pasture. These data
suggest that Na supplementation may, in fact, exacerbate an existing
hypomagnesaemia.
Keywords: NZSAPAB;
Sodium; potassium; magnesium; absorption; excretion; sheep.
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Last Updated 18-03-1997
