Grass tetany is a magnesium (Mg)
deficiency in cattle and other ruminants that has
caused many livestock deaths in the United States
and throughout the world. The occurrence of grass
tetany depends greatly on forage constituents that
reduce the availability of dietary Mg. Recent research
findings are increasing our understanding
of this difficult nutritional problem

Grunes, D.L.

Mayland, H.F.

Northwest Irrigation & Soils Research Laboratory Publications

Forage Crops: Grass Tetany

USDA - ARS - NWISRL

GRASS TETANYby H. F. Mayiand and D. L. G7101.38Grass tetany. Grass tetany is a magnesium (Mg)deficiency in cattle and other ruminants that hascaused many livestock deaths in the United Statesand throughout the world. The occurrence of grasstetany depends greatly on forage constituents thatreduce the availability of dietary Mg. Recent re-search findings are increasing our understandingof this difficult nutritional problem.Forage low in Mg. When cattle are grazing for-age containing less than 0.2% Mg, grass tetanymay occur. The type of forage eaten by the animalis very important. For example, grasses and le-gumes contain an average of 0.14 and 0.26% Mg,respectively. Mature timothy hay may contain aslittle as 0.07% Mg. However, hay is less likely tolead to tetany than is fresh forage because the Mgin hay is more available to cattle.Forage Mg levels vary because of cultural prac-tices that affect the actual amount of soluble Mg inthe soil or its availability to plants. Fertilizing withhigh levels of potassium (K) often depresses Mguptake by plants, but fertilizing with nitrogen (N)may increase Mg uptake and concentration. How-ever, fertilizing grass-legume pastures with N may20aeaiasas20	 2	 4	 6forage N, %Fig. 5. Relationship between total nitrogen content offorage and "availability" to cattle of forage magnesium.(From A J. Matson et Chemical composition of pasturesin relation to grass tetany in beef breeding cows, N. Z J.Apr, Res., 9:410-436, /966)increase the proportion of grass. Thus the Mg con-centration in the overall forage mixture may de-crease, since grasses generally contain muchlower Mg concentrations than do legumes.Acid soils should be limed for optimum forageyields. Continued application of calcitic limestone,however, causes a gradual depletion of soil Mgsources and may ultimately lead to lower forageMg levels and to increased incidence of grass tet-any. Calcium (Ca) ions may depress Mg uptake byplants if soil Ca concentrations greatly exceedthose of Mg. This effect on Mg uptake may also beobserved in calcareous soils.Organic manuring also may affect forage Mgconcentrations. Poultry manure, for example, maycontain 3.6% N, 1.5% K, and only 0.35% Mg. Ferti-lizing with poultry manure increases forage Mgconcentrations. However, applying more than 4metric tons of poultry manure (dry-weight basis)per hectare-year to pastures would be expected togreatly reduce the Mg availability to ruminantsbecause of the high forage N and K levels pro-duced. Thus, applying large amounts of animal lit-ter or inorganic N and K fertilizer increases thetetany hazard.Magnesium uptake and subsequent concentra-tions in grasses are temperature-related. Perennialryegrass, when grown at constant K and Mg levels,contained 0.13% Mg when exposed to a day-nighttemperature of 20-14°C. At a higher day-nighttemperature of 26– 23°C, the forage contained0.24% Mg. Temperature may also control the typeof forage available for grazing. Cool-season grassergrow better at lower temperatures than do mostlegumes. Thus forage in a mixed grass-legume pas-ture may consist primarily of grasses during theearly spring when tetany usually occurs.Reduced availability of forage Mg. The reduc-tion in dietary Mg availability is most often asso-ciated with rapidly growing grass forage. Highconcentrations of K in the forage may reduce die-tary utilization of Mg by cattle. Tetany has oftenoccurred in pastures where the forage K concen-tration is 3% or greater, and values approaching4% K should certainly cause concern. The inci-dence of tetany increases greatly when the forageK/(Ca + Mg) values exceed 2.2 on an equivalentbasis. Magnesium in forage having high N levelsalso is less available to cattle (Fig. 5).Availability to livestock can also be reducedwhen the Mg is complexed with certain organiccompounds. Such tricarboxylic acids as citric andtrans-aconitic may complex Mg, but these acidsare not likely to be the most important factor inreducing Mg availability to animals. Long-chainfatty acids may also complex with Mg, formingwater-insoluble soaps that are excreted in thefeces. The concentration of long-chain fatty acidsin forage is directly proportional to the N concen-tration. Therefore, the decreased Mg availabilityattributed to N (Fig. 5) may be partly due to theeffect of organic complexing agents whose levelsare associated with forage N concentrations.A high ratio of N to soluble carbohydrates in theforage may be related to reduced Mg availability.Forage N values may increase under conditions ofrapid forage growth and N fertilization. Solublecarbohydrates are reduced by N fertilization, and0.8aR0.6.? 0.43.o0.2z0	April	 May	 JuneFig. 6. Grass tetany coincided with an increase in ratioof nitrogen to soluble carbohydrate in forage. Fertilizingwith 150 kg N/ha amplified the ratio increase and wouldlikely increase the tetany hazard. (H. F. Mayland and D. L.Grunes, unpublished data)concentrations are greatly decreased during rapidplant growth. Thus, environmental conditionsconducive to rapid forage growth have coincidedwith the occurrence of grass tetany (Fig. 6).The Mg in forage produced under low solar ra-diation levels may be less available to cattle. Forexample, when the average daily radiation levelsfor three consecutive spring tetany periods were437, 547, and 551 langleys per day, the incidence ofgrass tetany was severe, light, and a trace, respec-tively.Alternatives to reduce tetany. Several agronomicmanagement alternatives are available for reduc-ing the incidence of grass tetany. Pastures can berenovated to include a larger proportion of legume.Dusting forage with 30 kg/ha MgO will providetemporary protection from tetany, until the Mg islost from the forage by wind or rain. A spray of aMg•bentonite clay-water slurry will adhere more tothe foliage and last somewhat longer than the dust.Either the dust or spray will provide Mg to theanimals as they graze the treated forage.Magnesium fertilization will increase the Mgconcentration of forage grown on coarse-texturedacid soils, but responses, if any, will be muchsmaller on fine-textured soils. Using dolomiticlimestone, rather than calcitic sources, will assuresome replenishment of Mg in acid soils. The do-lomite solubilizes more rapidly if mixed into thesoil.Another helpful practice is to apply only a por-tion of the N and K fertilizer in early spring andapply the remainder later in the season. Soil testrecommendations should be followed to avoid ex-cessive N and K fertilization. Where soils are to belimed, or Mg fertilization is economical, mainte-nance of a balance of soil nutrients should be at-tempted. For Mg this includes sufficient exchange-able soil Mg to give 10 to 15% of the cation ex-change capacity, Mg/K values of 2 or greater, andMg/Ca values of 0.2 or greater (all values on anequivalent basis).Alternatives in livestock management shouldalso be considered in reducing the severity of grasstetany. Grazing of forage likely to produce tetanyshould be postponed until the danger passes. Or,such forage might be grazed by livestock havinglower Mg requirements, that is, nonlactating ani-mals. The more direct method of reducing Mg tet-any is to supplement the animals with Mg. Thisshould decrease the incidence of grass tetany, al-though some cases may still occur if some animalsdo not accept the Mg supplement.For background information see ALKALOID;FESCUE; GRASS CROPS; LEGUME FORAGES in theMcGraw-Hill Encyclopedia of Science and Tech-nology.	 [it F. MAYLAND; D. L. GRUNES]Bibliography: R. E. Barker and A. W. Navin,Crop Sci., 14:50-53, 1974; R. F. Barnes and D. L.Gustine, L. P. Bush and R. C. Buckner, D. L.Grunes, and G. C. Marten, in A. G. Matches (ed.),Anti-quality Components of Forages, Crop Sci.Soc. Amer. Spec. Publ. no. 4, 1973; L. P. Bush etal., Crop Sci., 12:277, .1972; D. L. Gustine et al.,Agron. J., 66:636-639, 1974; G. C. Marten andR. M. Jordan, in Proceedings of the 12th Interna-tional Grassland Congress, Moscow, 1974, in press;G. C. Marten, A. B. Simons, and J. R. Frelich,Agron. J., 66:363-368, 1974; H. F. Mayland et al.,Agron. .1., 66:441-446, 1974; H. F. Mayland andD. L. Grunes, J. Range Manage., 27:198-201,1974; J. A. Stuedemann and S. R. Wilkinson, /974Beef Cattle Short Course Proceedings, Athens, GA,1974; FL H. Whittaker and P. P. Feeny, Science,171:757 –770, Feb. 26, 1971; S. R. Wilkinson et al.,International Symposium on Livestock Wastes,ASAE Publ. Proc., 271:321 – 324, 328, /971.

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Forage Crops: Grass Tetany

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