Estimates of magnesium (Mg) balances and plasma Mg concentrations of 51 dairy cows were obtained at 2-weekly intervals from calving during the first 3 months of lactation (late August to early December). During that period estimated 24h Mg excretion increased from 1.1 to 3.2g Mg/d (P<0.001) and varied with age. Plasma magnesium concentrations in 10% of samples were below the threshold (0.6 mM) at which productivity is considered to be impaired. Within cow repeatability for plasma Mg was 0.70 ± 0.048. Individual cows showed unique patterns in the relationship between plasma Mg and estimated urinary Mg output. Mean milk Mg concentration was 104 ± 0.54 mg/l, almost 20% lower than published values used in estimates of net requirements for Mg. Individual cow repeatability for milk Mg concentration was 0.67 ± 0.082 which, together with the plasma data, suggest an important genetic component in Mg homeostasis. This will need to be accounted for in models of Mg metabolism.
Proceedings of the New Zealand Society of Animal Production, Volume 61, Christchurch, 152-155, 2001
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