Abstract:
Animal Production in Australia THE POSSIBLE CAUSE OF THE ABSENCE OF HYPOMAGNESAEMIA IN CATTLE GRAZING TROPICAL PASTURES - A REVIEW D.J. MINSON* and B.W. NORTON* SUMMARY To determine the possible reason why hypomagnesaemia is not found in cattle grazing tropical pastures, a review was made of the mineral composition of pastures. Published information on the level of magnesium, calcium and potassium content of tropical and temperate grasses and legumes have been used to calculate the mean and frequency of each level of these elements. The mean magnesium content of tropical grasses was 0.36% (g/100 g dry matter) compared with 0.18% in temperate grasses. The corresponding values for tropical and temperate legumes were 0.40% and 0.29% respectively. Tropical grasses and legumes contained less calcium than temperate grasses and legumes whereas potassium levels were similar in all species. This information suggests that the absence of hypomagnesaemia in cattle grazing tropical pastures is a direct result of the higher magnesium level and that this advantage is not offset by a difference in potassium content. INTRODUCTION leve rece cone fora pota WC magn graz been 1970 Hypomagnesaemia is a metabolic disorder of ruminants associated with 10~ 1s of magnesium in the blood. The deficiency is caused by the animals iving insufficient utilizable magnesium, either because of low magnesium entrations in the forage or because the availability of the magnesium in the ge is reduced by some factor (Mayland and Grunes 1978). A high level of ssium can have an adverse effect on magnesium absorption and the ratio of a + Mg) has been suggested as a guide to the relative quantity of available .esium (Mayland and Grunes 1978). Hypomagnesaemia often occurs in cattle ing temperate pastures and losses of up to 3% in dairy and beef herds have . reported in New Zealand and the USA (Baker and Gould 1976; Grunes et al. ; Mansfield et al. 1975). m- Animals grazing tropical pastures may be deficient in phosphorus, suiphur, nitrogen, iodine, copper, cobalt or selenium (Conrad and McDowell 1976) but there is an absence of any reports of hypomagnesaemia. In an attempt to determine the possible cause of this absence of hypomagnesaemia in cattle grazing tropical pastures the information was reviewed on the quantity of magnesium in tropical legumes and grasses and the existence Because of factors believed to reduce the availability of the magnesium. hypomignesaemia is often found in animals grazing temperate pastures, values for these have been included for comparative purposes. MATERIALS The magnesium, calcium and potassium content of temperature and tropical Agricultural grasses and legumes were taken from McDowell et al. (1974) Research Council (1976) and (Minson 1977) with a few values comidg from a large number of other publications. The mean concentrations of magnesium, calcium, .~ -k - * CSIRO, Division of Tropical Crops and Pastures, Cunningham Laboratory, St. Lucia, Qld 4067. Agriculture Department, University of Queensland, St. Lucia, Qld 4067. 357 Animal Production in Australia TABLE1 Mean concentration of magnesium, calcium, and potassium in tropical and temperate grasses and legumes (g/100 g dry matter). The number of samples analysed are shown in parenthesis. potassium and potassium to calcium + magnesium ratio were calculated for the tropical and temperate grasses and legumes (Table 1). The frequency distribution of the concentrations of the three elements were also calculated (Fig. 1). RESULTS Magnesium The mean magnesium content of the tropical grasses was twice that found in the temperate grasses whereas that of the tropical legumes was 38% higher than for temperate legumes (Table 1). Within each of the four pasture categories there was a range of values for magnesium but the differences between mean values could nut be attributed to this variation (Fig. 1). Calcium Temperate grasses contained 48% more calcium than tropical grasses whereas temperate legumes contained 54% more calcium than tropical legumes (Table I). These changes appeared to be a direct result of the higher magnesium content of the tropicals since the total quantity of divalent cations in both tropical and temperate species were similar. Legumes contained three times as much calcium as the grasses. Potassium Tropical and temperate grasses contained similar quantities of potassium but temperate legumes contained more potassium than tropical legumes (Table 1). In both temperate and tropical pastures, the legume tend to contain less potassium than the grasses. DISCUSSION Tropical grasses had twice as much magnesium as temperate grasses. It is suggest that the higher magnesium content of tropical grasses may explain the absence of hypomagnesaemia in lactating cattle grazing tropical pastures. In the Netherlands hypomagnesaemia occurs mainly when the temperature is between 8 and 14OC ('t Hart 1960). This has been attributed to the depressing effect of low temperatures on the magnesium content of the grass, with a 6OC difference in temperature changing the magnesium content by 0.12% (Grunes et al. 1968). Therefore it is possible that the higher mean magnesium contenT oFtropical grasses is due to the higher temperature at which they are grown and is not the result of genetic differences in the ability to accumulate magnesium. 358 Animal Production in Australia Fig. 1 Magnesium, calcium and potassium content of tropical and temperate grasses and legumes. - Tropical ------ Temperate. The recommended level of magnesium in the diet of lactating cattle is 0.20% (National Research Council, 1978). For the temperate grasses, 70% of the observations were below the recommended magnesium level compared with 29% of the tropical grass samples. Kith the temperate legumes 18% of the samples contained less than the recommended level of magnesium compared with only 2% of the tropical legume samples. It appears that the higher magnesium level in tropical forages leads to a much lower chance of hypomagnesaemia occurring. It has been suggested that a high level of potassium can have an adverse effect on magnesium absorption. When the K/(Ca + Mg) ratios were calculated for the four pasture categories there were only small differences in the ratio between temperate and tropical species (Table 1) indicating that the absence of 359 Animal Production in Australia hypomagnesaemia was not related to lower levels of potassium in the tropical species. There are three other possible reasons for the absence of hypomagnesaemia Tropical grasses have a lower mean in cattle grazing tropical pastures. nitrogen content than temperate grasses (Minson 1976) and this might increase the availability of the magnesium since a high level of forage nitrogen was thought to have an adverse effect on the absorption of magnesium (Kemp 1960; Daniel et al. 1969). However, a recent review on hypomagnesaemia suggested that high le%sof dietary nitrogen 'do not adversely affect magnesium utilization more than high potassium levels alone+' (Mayland and Grunes 1978). The level of milk production of cows grazing tropical pastures is very much lower than would be obtained with temperate pastures (Stobbs 1971). This lower level of production would reduce the quantity of magnesium required for milk Another possible production and hence the possibility of hypomagnesaemia. reason for the low incidence of hypomagnesaemia is the absence of cold stress in the tropics which has been shown to cause a depression in blood plasma magnesium (Sykes - .- 1969). et al. It was concluded that the lack of hypomagnesaemia in cows grazing tropical pastures was probably caused by the higher magnesium content of tropical pasture temperature. grasses associated with the higher growing and legumes Furthermore, it seems that the advantage of the higher magnesium level was not offset by differences in potassium content. REFERENCES 'The Nutrient Requirements of Farm AGRICULTURAL RESEARCH COUNCIL (1976). Livestock. No. 4 Composition of British Feedstuffs. Technical Review and Tables'. (Agricultural Research Council: London). BAKER, D.O., and GOULD, R.D. (1976). N.Z. J.Exp.Agric. 4: 51. CONRAD, J.H., and MCDOWELL, L.R. (1976). 'Latin Ameri?an Symposium on Mineral (University of Florida: Nutrition Research with Grazing Ruminants++. Gainesville). DANIEL, K., REID, R.L., and JUNG, G.A. (1969). J.Anim.Sci. 22: 179. GRUNES, D.L., THOMPSON, J.F., KUBOTA, J., and LAZAR, V.A?-(1968). Int. Congr. Soil Sci. Trans. 9th II: 597. GRUNES, D.L., STOUT, P.R., and BROWNWELL, JR. 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