Magnesium metabolism in the rumens of lactating dairy cows fed on spring grass

Magnesium homeostasis in cattle: absorption and excretion

Magnesium (Mg2+) is an essential mineral without known specific regulatory mechanisms. In ruminants, plasma Mg2+ concentration depends primarily on the balance between Mg2+ absorption and Mg2+ excretion. The primary site of Mg2+ absorption is the rumen, where Mg2+ is apically absorbed by both potential-dependent and potential-independent uptake mechanisms, reflecting involvement of ion channels and electroneutral transporters, respectively. Transport is energised in a secondary active manner by a basolateral Na+/Mg2+ exchanger. Ruminal transport of Mg2+ is significantly influenced by a variety of factors such as high K+ concentration, sudden increases of ammonia, pH, and the concentration of SCFA. Impaired Mg2+ absorption in the rumen is not compensated for by increased transport in the small or large intestine. While renal excretion can be adjusted to compensate precisely for any surplus in Mg2+ uptake, a shortage in dietary Mg2+ cannot be compensated for either via skeletal mobilisation of Mg2+ or via up-regulation of ruminal absorption. In such situations, hypomagnesaemia will lead to decrease of a Mg2+ in the cerebrospinal fluid and clinical manifestations of tetany. Improved knowledge concerning the factors governing Mg2+ homeostasis will allow reliable recommendations for an adequate Mg2+ intake and for the avoidance of possible disturbances. Future research should clarify the molecular identity of the suggested Mg2+ transport proteins and the regulatory mechanisms controlling renal Mg excretion as parameters influencing Mg2+ homeostasis.

Magnesium absorption by wethers fed potassium bicarbonate in combination with different dietary magnesium concentrations

We hypothesized that the decrease in the absolute amount of Mg absorbed in the total digestive tract, as induced by K, would remain constant if Mg intake by ruminants was increased. This hypothesis was based on earlier studies that used temporarily isolated rumens of sheep and the fact that the rumen is the major site of Mg absorption in ruminants. To test the hypothesis, six rumen-fistulated wethers were fed diets at two concentrations of K and three concentrations of Mg in a 6 x 6 Latin square design. Diets contained either 10 or 36 g of K/kg of dry matter and 1.3, 2.5, or 3.7 g of Mg/kg of dry matter. Extra K was added in the form of KHCO3, and Mg was added in the form of MgO. For wethers fed the low K diets, absolute Mg absorption rose by 0.32 g/d for each 1 g/d of Mg intake that was in excess of requirements. The high K diets reduced absolute Mg absorption by a mean of 0.36 g/d; this reduction was independent of Mg intake. Magnesium intake and Mg concentrations in rumen liquid were positively related. Extra KHCO3 in the diet increased K concentrations in rumen liquid, but the concentrations of Mg remained unchanged. Rumen pH was elevated by a mean of 0.45 units when the high K diets were fed. This study indicated that, in practical ruminant feeding, the supplementation of Mg to either low or high K diets increased absolute Mg absorption to the same extent.

A model of magnesium metabolism in young sheep. Magnesium absorption and excretion

A model of Mg metabolism in sheep is proposed. It is based on standard Michaelis-Menten enzyme kinetics to describe the transport of Mg across the rumen wall and passive diffusion to describe the absorption of Mg in the hindgut. Factors known to have an effect on Mg metabolism in farm animals, namely the concentrations of K and Mg in the diet, and the physico-chemical conditions within the rumen as determined by the type of diet, are incorporated into the model. Consideration of the rumen as the only site of Mg absorption provided an inadequate mechanistic description of Mg metabolism in sheep. To ensure compatibility between predicted Mg absorption and recent independent data sets for Mg balances, it was necessary to include in the model aspects of Mg absorption that operate in the hindgut. The results from this model suggest that there is a need for a series of experiments to determine the important aspects of Mg transport in the hindgut of sheep. Mechanisms of homeostasis are discussed.

Influence of pH and rapidly fermentable carbohydrate on mineral release in and flow from the rumen

Objectives for this study were to determine the effect of energy supplementation on mineral release in and outflow from the rumen, independent of pH. Nonlactating Holstein cattle with ruminal cannulas were limit-fed chopped alfalfa hay with or without energy supplement. Treatments were 1) control, alfalfa plus ruminal infusion of 3 L/d of distilled water; 2) acid, alfalfa plus infusion of 3 L/d of 1N HCl; 3) energy supplement, alfalfa and ground corn plus infusion of 3 L/d of distilled water; and 4) buffered energy supplement, alfalfa, and ground corn plus infusion of 3 L/d of distilled water containing 90 g of NaHCO3. Macromineral intake was standardized across treatments by adjustment of infusion solutions with inorganic mineral sources. Dry matter consumed was 5 kg/d on treatments 1 and 2 and 8.4 kg/d on treatments 3 and 4. Mean ruminal pH was 6.8, 6.1, 5.6, and 6.2 for treatments 1 through 4, respectively. Ruminal pH was more significant than energy supplementation in dispersion of Ca, Mg, and P in ruminal digesta. Only 1, 17, and 27% of dietary K, Mg, and P flowed with solids from the rumen, but 52% of Ca and 41% of Cu remained with the solid phase.

Renal regulation of electrolyte and acid-base balance in ruminants

The kidney maintains volume, electrolyte, and acid-base homeostasis. These functions are examined in the ruminant in response to differing dietary intakes and disease states. The consequences of renal disease for these homeostatic processes and the interpretation of urinary excretion data are reviewed.

Effect of change of diet on the mineral composition of rumen fluid, on magnesium metabolism and on water balance in sheep

1. The effects of four diets on water intake, rumen fluid outflow-rate, rumen pH and mineral metabolism were studied in wether sheep. The diets were barley and hay, flaked maize and hay, dried grass and frozen grass. 2. Experimental periods were of 12 d duration, and plasma magnesium concentrations were lower at the end of treatment periods when the grass diets were given and were significantly different (P less than 0.05) at 11.00 and 20.45 hours. Also, the concentration was significantly lower with the dried-grass diet than with the frozen-grass diet (P less than 0.05). 3. The concentration of Mg in rumen fluid centrifuged at 30,000 g (ultracentrifuged) varied with the diet. Maximum concentrations (tmax) were reached 4 h later on the grass diets than on the hay and concentrate diets. In the latter case tmax coincided with that for calcium, potassium, chloride and ammonia. At this time sodium and phosphate were at a minimum. The concentration of Mg in ultracentrifuged rumen fluid was negatively correlated (r -0.89) with pH, which was significantly higher (P less than 0.01) at all times on the grass diets. This relation was also reflected in the apparent availability of Mg. 4. Total water intake on the frozen grass was about twice that on the barley and hay diet. The outflow rate of liquid from the rumen was higher on the frozen grass than on the other three diets. 5. The proportion of absorbed Mg excreted in urine was significantly influenced by diet.