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

Invited review: Mineral absorption mechanisms, mineral interactions that affect acid-base and antioxidant status, and diet considerations to improve mineral status

Several minerals are required for life to exist. In animals, 7 elements (Ca, P, Mg, Na, K, Cl, and S) are required to be present in the diet in fairly large amounts (grams to tens of grams each day for the dairy cow) and are termed macrominerals. Several other elements are termed microminerals or trace minerals because they are required in much smaller amounts (milligrams to micrograms each day). In most cases the mineral in the diet must be absorbed across the gastrointestinal mucosa and enter the blood if it is to be of value to the animal. The bulk of this review discusses the paracellular and transcellular mechanisms used by the gastrointestinal tract to absorb each of the various minerals needed. Unfortunately, particularly in ruminants, interactions between minerals and other substances within the diet can occur within the digestive tract that impair mineral absorption. The attributes of organic or chelated minerals that might permit diet minerals to circumvent factors that inhibit absorption of more traditional inorganic forms of these minerals are discussed. Once absorbed, minerals are used in many ways. One focus of this review is the effect macrominerals have on the acid-base status of the animal. Manipulation of dietary cation and anion content is commonly used as a tool in the dry period and during lactation to improve performance. A section on how the strong ion theory can be used to understand these effects is included. Many microminerals play a role in the body as cofactors of enzymes involved in controlling free radicals within the body and are vital to antioxidant capabilities. Those same minerals, when consumed in excess, can become pro-oxidants in the body, generating destructive free radicals. Complex interactions between minerals can compromise the effectiveness of a diet in promoting health and productivity of the cow. The objective of this review is to provide insight into some of these mechanisms.

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.

Evaluation of mineral status in high dietary sulfur exposed or sulfur-induced polioencephalomalacia affected beef cattle

Amat, S., McKinnon, J. J., Penner, G. B., Simko, E. and Hendrick, S. 2014. Evaluation of mineral status in high dietary sulfur exposed or sulfur-induced polioencephalomalacia affected beef cattle. Can. J. Anim. Sci. 94: 139–149. We examined the mineral status in beef heifers fed high S containing diets with differing forage-to-concentrate ratio (F:C), and in S-induced polioencephalomalacia (PEM) affected feedlot steers. A metabolism trial was conducted as a randomized complete block design using a 2×2 factorial treatment arrangement with main effects of dietary S and F:C using 16 ruminally cannulated heifers. The F:C was modified by altering the proportion of barley silage (4 vs. 51% dry matter), whereas, the S content was modified by using differing sources of wheat dried distillers’ grains with solubles (WDDGS) to achieve low and high S diets (LS=0.30 vs. HS=0.67%). Minerals including Cu, Co, Fe, Mg, Mn, Mo, Se and Zn were determined from rumen fluid, blood, brain tissue and urine. Urinary mineral excretion was also assed. During the course of the metabolic trial, an outbreak of S-induced PEM in a commercial feedlot was documented and brain minerals of these PEM steers (n=4) were contrasted with the experimental heifers fed HS diet. There were no interactions between dietary S concentration and F:C (P>0.05). Heifers fed HS diet had reduced (P<0.05) mineral intakes (except for Mo), ruminal Co, Fe and Mn, and serum Mg and Fe relative to those fed LS diet. Heifers fed low F:C diet had reduced (P<0.05) Cu, Fe, Mo and Se intakes, greater (P<0.05) ruminal Cu, Fe, Mn and Zn, and reduced (P<0.05) serum Cu and Se, and greater (P<0.05) serum Mg than heifers fed high F:C diet. Brain minerals were not affected (P>0.05) by F:C or dietary S. However, the PEM brains had reduced Cu (P=0.058), Fe (P=0.003) and Mo (P<0.001) relative to normal brains. Dietary S and F:C did alter the mineral status of the heifers, but no deficiencies or PEM were induced.

Effect of chloride on pH microclimate and electrogenic Na+ absorption across the rumen epithelium of goat and sheep

Active Na+ absorption across rumen epithelium comprises Na+/H+ exchange and a nonselective cation conductance (NSCC). Luminal chloride is able to stimulate Na+ absorption, which has been attributed to an interaction between Cl-/HCO3- and Na+/H+ exchangers. However, isolated rumen epithelial cells also express a Cl- conductance. We investigated whether Cl- has an additional effect on electrogenic Na+ absorption via NSCC. NSCC was estimated from short-circuit current (Isc) across epithelia of goat and sheep rumen in Ussing chambers. Epithelial surface pH (pHs) was measured with 5-N-hexadecanoyl-aminofluorescence. Membrane potentials were measured with microelelectrodes. Luminal, but not serosal, Cl- stimulated the Ca2+ and Mg2+ sensitive Isc. This effect was independent of the replacing anion (gluconate or acetate) and of the presence of bicarbonate. The mean pHs of rumen epithelium amounted to 7.47 +/- 0.03 in a low-Cl- solution. It was increased by 0.21 pH units when luminal Cl- was increased from 10 to 68 mM. Increasing mucosal pH from 7.5 to 8.0 also increased the Ca2+ and Mg2+ sensitive Isc and transepithelial conductance and reduced the fractional resistance of the apical membrane. Luminal Cl- depolarized the apical membrane of rumen epithelium. 5-Nitro-2-(3-phenylpropylamino)-benzoate reduced the divalent cation sensitive Isc, but only in low-Cl- solutions. The results show that luminal Cl- can increase the microclimate pH via apical Cl-/HCO3- or Cl-/OH- exchangers. Electrogenic Na+ absorption via NSCC increases with pH, explaining part of the Cl- effects on Na+ absorption. The data further show that the Cl- conductance of rumen epithelium must be located at the basolateral membrane.

Effects of intrinsic potassium in artificially dried grass and supplemental potassium bicarbonate on apparent magnesium absorption in dry cows

Literature data indicate that the form of K in the ration can affect its inhibitory influence on Mg absorption in ruminants. We tested whether identical amounts of K either intrinsically present in artificially dried grass or present in added KHCO3 have different effects on Mg absorption in dry cows. In a 3 x 3 Latin square design, six cows were fed rations consisting of low-K grass and concentrate with or without KHCO3 or a ration consisting of high-K grass with concentrate without added KHCO3. Each ration was given for a period of 4 wk. The ration low in intrinsic K contained 26 g of K/kg of dry matter, the ration low in intrinsic K plus KHCO3 contained 43 g of K/kg of dry matter, and the ration high in intrinsic K also contained 43 g of K/kg of dry matter. The three rations were balanced for crude protein, crude fat, crude fiber, Mg (2.2 g/kg of dry matter), Ca, P, and Na. Apparent Mg absorption was 10.8 +/- 1.54% of intake (mean +/- SE, n = 6) when the cows were fed the low-K ration, but dropped to 1.9 +/- 3.4 and 2.1 +/- 1.9% of intake, respectively, when the rations high in KHCO3 and high in intrinsic K were fed. The two high-K rations induced similar increases in ruminal K concentrations both before and after feed consumption. The feeding of KHCO3 did not influence ruminal pH. The intake of extra K may raise ruminal K concentrations, which increases the transmural potential difference so that Mg transport across the rumen epithelium becomes depressed. Thus, intrinsic and added K had identical effects on ruminal K concentrations and on Mg absorption. Feeding trials with ruminants in which K intakes are manipulated with the use of KHCO3 may reflect those cases when concentrations of K intrinsically present in feedstuffs may vary.

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.

Impaired absorption of magnesium in the aetiology of grass tetany

Magnesium is absorbed mainly from the reticulo-rumen and there are a number of factors reducing its absorption. The chief of these is the increased potential difference across the rumen epithelium caused by increased intraruminal potassium concentration. A significant amount of magnesium leaves the extracellular fluid each day as saliva. As only a portion of it is reabsorbed the rest is lost through the endogenous faecal excretion of magnesium. Thus, during impaired magnesium absorption, saliva could play an important role in the aetiology of hypomagnesaemia especially during dietary sodium depletion and the resultant increase in the potassium content of the saliva.