Increasing magnesium intakes in relation to magnesium absorption in dry cows Surasak Jittakhot, J Thomas Schonewille*, Hugo Wouterse, Anton WJ Uijttewaal, Chalermpon Yuangklang and Anton C Beynen Department of Nutrition, Faculty of Veterinary Medicine, Utrecht University, P.O. Box 80.152, 3508 TD Utrecht, The Netherlands Earlier studies with temporarily isolated rumen of heifers show saturation kinetics of Mg efflux across the rumen wall. Therefore, we hypothesized that high Mg intakes would not further increase the rate of Mg absorption in cows. To test our hypothesis, six ruminally fistulated non- pregnant dry cows were given diets with different Mg concentrations in a 6r6 Latin square design. Desired concentrations of Mg were attained by adding MgO to the basal diet and the Mg concentrations in the total rations were 3 . 8, 6 . 4, 9 . 1, 11 . 8, 14 . 1 and 17 . 3 g Mg/kg dry matter, which provided Mg intakes of 27 . 1, 44 . 6, 64 . 6, 83 . 5, 100 . 4 and 124 . 3 g/d, respectively. Increasing Mg intakes were associated with increased (P <0 . 001) faecal Mg excretion. However, apparent Mg absorption expressed as g/d was not significantly different for Mg intakes from 100 . 4 to 124 . 3 g/d while Mg absorption expressed as a proportion of intake was not significantly different for Mg intakes ranging from 64 . 6 to 124 . 3 g/d. Mg concentrations in rumen fluid after feeding increased (P <0 . 001) with increasing Mg intakes. Apparent absorption of Mg appeared to become saturated at a ruminal Mg concentration of 17 . 5mM (Mg intake of 83 . 5 g/d). Group- mean post-feeding concentrations of Mg and Na in rumen fluid were significantly correlated (Pearson’s r =–0 . 96; P =0 . 003, n = 6). This study showed that under conditions of practical dairy cow feeding, Mg absorption was maximal at Mg intakes o84 g/d. Keywords : Magnesium intake, absorption, cows. In the course of our studies, much attention has been given to the inhibitory action of K on Mg absorption under practical feeding conditions (Schonewille et al. 1997 ; Ram et al. 1998; Schonewille et al. 1999a; Schonewille et al. 1999b). Tomas & Potter (1976b) showed that the inhibitory action of K is restricted to the reticulorumen, which is the predominant site of Mg absorption in ruminants (Rogers & Van ’t Klooster, 1969; Tomas & Potter, 1976a; Greene et al. 1983; Khorasani et al. 1997). Obviously, Mg absorp- tion is also determined by the Mg concentration in rumen fluid. Indeed, Martens (1983) showed that Mg transport across the temporarily isolated rumen of heifers responds in a dose-dependent fashion up to Mg concentrations in buffer solutions as high as about 10 mM. At higher con- centrations, Mg transport increased non-linearly and it was suggested that Mg transport across the rumen wall be- comes saturated at a luminal Mg concentration of 12 . 5mM (Martens, 1983). These results imply that Mg absorption, when expressed as a proportion of intake, will decrease at high Mg intakes. However, in a study with sheep (Ram et al. 1998), a 2 . 9-fold increase in Mg intake was followed by a similar increase in absolute Mg absorption (g/d), in- dicating that the efficiency of Mg absorption (% of intake) remained constant. Corresponding measurements in cows are scarce but Field & Suttle (1979) reported that an in- crease in Mg intake from 5 to 7 . 8 g/d decreased Mg ab- sorption from 25 . 3 to 20 . 4% of intake in Ayrshire cows weighing 334–376 kg. Corresponding dietary Mg con- centrations were 1 . 1 and 1 . 8 g/kg of dry matter (DM) re- spectively, but rumen Mg concentrations were not reported. Thus, the results of Field & Suttle (1979) appear to conflict with those of Ram et al. (1998). Therefore, we studied Mg absorption in relation to increasing Mg intakes in non-pregnant, dry cows. It was expected that the results would provide information on the capacity of cows to absorb Mg under practical feeding conditions. *For correspondence ; e-mail : j.schonewille@vet.uu.nl