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Ruminal Mg Transport and Assessment of Mg Intake in Dairy Cows: Two Sides of one Coin. H. Martens and F. Stumpff ? Freie Universitaet Berlin, Dept. of Veterinary Physiology, Germany
The objective of this compilation of published data was to combine knowledge about Mg transport mechanisms in the rumen with a recent meta-analysis about Mg absorption for a better understanding of Mg absorption in vivo and prediction of Mg intake in lactating cows for the daily requirement. Ruminal Mg absorption is mediated by 2 apical uptake mechanisms: A potential-dependent or K-sensitive (K-s) and a potential-independent or K-insensitive (K-ins) mechanism. The K-s mechanism is suggested to work at low and the K-ins one at high Mg concentrations. The negative effect of K on Mg absorption should be high at low Mg (uptake via K-s) and low at high Mg intake (uptake via K-ins). Indeed, in sheep fed a low Mg diet (1.64 g/d), elevating dietary K from 1% to 3.6% severely reduced Mg digestibility from 35% to 16%. Conversely, when Mg intake was high (4.66 g/d), digestibility only dropped from 33 % to 24 % (1 % K versus 3.6 %; Ram et al., J. Dairy Sci. 1997 : 2485-2492). In a recent meta-analysis of Mg absorption in dairy cows Schonewille et al. (J. Dairy Sci. 2008 : 271-278) demonstrated that the true Mg absorption depends on Mg intake and K concentration: True Mg absorption (g/d) = 3.6(g/d) + 0.2xMg intake(g/d) ? 0.08(kg/d) x K(g/kg DM). An endogenous Mg secretion of 2.8 g/d has to be subtracted to yield the apparent Mg absorption (g/d) = 3.6(g/d) - 2.8(g/d) + 0.2xMg intake (g/d) ? 0.08(kg/d) x K (g/kg DM). This equation clearly shows that the effect of K on Mg digestibility is high at low Mg intake and vice versa in accordance with the ruminal Mg transport model.
In lactating cows, apparent Mg absorption has to cover both the secretion in milk (M l/d x 0.12 g/l) and endogenous secretion (2.8 g/d). Entry of these values into equation  yields
Mg intake (g/d) = M (l/d) x 0.6 (g/l) + 0.4(kg/d) x K (g/kg DM) + 10 (g/d).
Equation  permits the quantitative calculation of Mg intake according requirement and dietary K concentration.
Key words: Mg, rumen, K, dairy cow