farm circumstances in Northern Ireland. Supplementation via concentrates was preferred by most dairy farmers, but reliance on magnesium fertilizers increased as farm size increased. Beef farmers relied heavily on the magnesium block and made little or no use of fertilizers.
The value of applying various magnesium compounds, such as calcined magnesite (MgO), kieserite (MgSO4.H2O) or dolomitic limestone (CaCO3. MgCO3), to pastures to raise their magnesium concentrations has been exten- sively studied (Allcroft, 1961; McIntosh et al., 1973). The magnesium lime- stone was less effective than the other sources, which increased pasture values from 2 to about 3 g Mg kg21 DM. Treatment of lucerne (alfalfa) with kieserite at four rates, up to 4480 kg ha21, increased concentrations from 2 to 3.2 g Mg kg21 DM at the highest application, with no effect on availability of the magnesium present (Reid et al., 1979). Application rates of 1125 kg ha21 for magnesite or kieserite and 5600 kg ha21for magnesium limestone are rec- ommended, but this method of control has limitations on many soil types.
Foliar dusting of pastures with fine calcined magnesite (MgO) before or during tetany-prone periods has proved effective, but effects are short-lived, regardless of the amount applied (Fig. 6.8; Kemp, 1971). Rogers (1979) rec- ommended that at least 17 kg ha21 be used and repeated after heavy rainfall
Pasture fertilizers
Fig. 6.8. Control of hypomagnesaemic tetany by dusting pastures with magnesite (MgO) is short-lived because the method relies on surface adherence (Kemp, 1971):
repeated applications are necessary and a single treatment with a suspension of magnesium hydroxide may be more effective.
and at not more than 10-day intervals; treatments can be integrated with a
‘strip-grazing’ system. Spray application of magnesium hydroxide in fine sus- pension can produce short-term increases in herbage and serum magnesium from low application rates (5 kg Mg ha21: Parkins and Fishwick, 1999). Foliar dusting and spraying use the pasture as a short-term carrier for the dietary magnesium supplement.
For calves and dairy cows which are being fed on concentrates, the provision of sources such as MgO in the concentrate mixture can prevent the disorder.
Alternatively, incorporating magnesium into mineral mixes, drenches, salt- and molasses-based, free-choice licks, adding magnesium sulphate (MgSO4) to the drinking-water or even sprinkling the mineral on feeds, such as cereals, chopped roots or silage, have all been used as methods of supple- mentation. Magnesium must be given to all stock continuously during the tetany-susceptible period, since the disease can occur within 48 h of with- holding the supplement while cows are at grass. ‘Secure’ prophylactic doses have been given as 50–60 g MgO or its equivalent day21 (25–30 g Mg) for adult dairy cattle, 7–15 g day21 for calves and 7 g day21 for lactating ewes. In a study with calving beef cows, 56.8 g MgO day21 maintained serum magne- sium to a mean of 1.0 mmol l21, compared with 0.5 mmol l21 in untreated cows grazing similar pastures (Boling et al., 1979). However, these recom- mendations seem far too generous in the light of even the worst estimates of requirement (Tables 6.4 and 6.5) and should be halved once the immediate risk has passed. Conservative use of MgO is indicated, because the amounts previously recommended will shift acid : base balance and raise urine pH (Xin et al., 1989): such changes increase the risk of milk-fever (see Chapters 4 and 8). Excessive feeding of magnesium prior to parturition did nothing to allevi- ate the hypomagnesaemia in beef cows associated with onset of grazing in spring (Ritter et al., 1984).
In New Zealand, the method of choice is to give 10 g Mg as MgCl2 daily from the milking platform to each individual, in bloat control drenches. This method assures uniformity of magnesium intake, which free-choice methods cannot give. The use of magnesium alloy bullets, lodged in the rumen, has not proved a reliable means of preventing hypomagnesaemia in dairy cows (Kemp and Todd, 1970), because they cannot always release sufficient mag- nesium daily throughout the risk period, but they may be slightly more effec- tive in the beef cow, which requires less magnesium (Stuedemann et al., 1984). A recent survey showed that the provision of magnesium-containing blocks to beef suckler herds in Northern Ireland did not reduce the incidence of hypomagnesaemia (McCoy et al., 1996). Addition of a soluble magnesium salt, such as the chloride, sulphate or acetate, to the water-supply has been extensively studied, with mostly beneficial results (see Rogers and Poole, 1976). However, water intake by drinking varies widely between individuals and with weather and pasture conditions, and it is not easy to ensure by this means the necessary intake. The value of soluble carbohydrates, as a source Dietary supplements
of energy which concurrently lowers rumen pH and improves magnesium absorption, has received some attention (Metson et al., 1966; Giduck et al., 1988; Schonewille et al., 1997b) and may contribute to the efficacy of liquid mixtures of molasses and magnesium.
In view of the evidence that magnesium metabolism and susceptibility to
‘grass tetany’ are subject to genetic variation, it would seem worthwhile to explore the possibility of selecting for resistance to the disease via indices which indicate high efficiencies of magnesium utilization. The strategy is attractive in that it targets the vulnerable minority of any population, whereas preventive supplementation methods involve treatment of the whole herd or flock, regardless of individual levels of risk. Ratios of Mg : creatinine in the urine of artificial insemination (AI) sires on a standard ration might allow the Table 6.4. Factorially derived estimates of the average magnesium requirements of grazing and housed sheep (g kg21DM).
Growth/ Dietary requirement
Live weight production (g kg21DM) DMIb
(kg) (kg day21) At grassa Houseda (kg day21)
Growth 20 0.1 1.0 0.50 0.50
0.2 0.9 0.45 0.76
30 0.1 1.0 0.50 0.67
0.2 0.9 0.45 1.00
0.3 0.8 0.40 U
40 0.1 1.0 0.50 0.83
0.2 0.9 0.45 1.23
0.3 0.7 0.35 1.80
Fetuses (212 weeks to term)
Pregnancy 40 1 0.9 0.45 0.64–0.96
2 0.9 0.45 0.74–1.25
75 1 1.1 0.55 1.03–1.51
2 0.9 0.45 1.17–1.93
Milk (kg day21)
Lactation 40 1 1.2 0.60 1.18
2 1.2 0.60 1.90
75 1 1.4 0.70 1.48
2 1.3 0.65 2.18
3 1.3 0.65 2.90
aAbsorbability values were 0.20 for sheep grazing pasture high in potassium (> 30 g K kg21 DM), 0.40 for housed sheep.
bDry-matter intakes for diets of intermediate nutritive value, q = 0.6 (see ARC, 1980).
U, unattainable performance on such diets.
Genetic selection
selection of resistant lines (Suttle, 1996). Progress can also be made by select- ing for plants rich in magnesium. Thus hypomagnesaemia was controlled in sheep by grazing them on a grass cultivar high in magnesium (Moseley and Baker, 1991).
The effectiveness of different chemical forms of the element have mostly been compared in ruminants (Henry and Benz, 1995). Magnesium phosphate, a calcium magnesium phosphate and magnesium ammonium phosphate are all satisfactory sources for growing sheep given dry diets (Fishwick and Hemingway, 1973) and magnesium phosphate is satisfactory for lactating cows on spring pasture (Ritchie and Fishwick, 1977). Magnesium retentions were similarly increased in growing wether sheep by magnesium oxide and two forms of magnesium phosphate (Fishwick, 1978), but the oxide and hydroxide were given low average values of 0.75 (n = 18) and 0.60 (n = 3) relative to MgSO4 for sheep by Henry and Benz (1995). Commercial sources of calcined magnesites (i.e. MgO) vary widely in particle size and processing – particularly the calcination temperature – and hence in nutritive value (Xin et al., 1989; Adam et al., 1996). Coarse particle sizes (> 500 µm) and low calcination temperatures (< 800°C) are associated with low estimates of apparent absorption. Magnesium chloride may be better than acetate or sulphate salts, in view of the anion effects described earlier. The AMg value Table 6.5. Factorially derived estimates of the average magnesium requirements of grazing and housed cattle (mg kg21DM).
Growth/ Dietary requirement
Live weight production (g kg21DM) DMIb
(kg) (kg day21) At grassa Houseda (kg day21)
Growth 100 0.5 1.4 0.91 2.1
1.0 1.3 0.85 3.2
200 0.5 1.5 0.98 3.3
1.0 1.3 0.85 4.7
400 0.5 1.6 1.04 5.2
1.0 1.3 0.85 7.3
Weeks to term
Pregnancy 600 12 2.0 1.00 5.8
4 1.7 0.85 7.3
Milk (kg day21)
Lactation 600 10 2.2 1.40 9.4
20 2.0 1.33 14.0
30 1.9 1.26 18.8
aBased on absorbability coefficients of 0.15 and 0.23 for grass high in potassium (> 30 g K kg21DM) and hay + concentrates, respectively.
bAssuming a diet of moderate digestibility, with a q value of 0.6.
Mineral sources
for magnesium chloride was nearly twice as high when added to cereals than when added to hay (0.3 vs. 0.16: Suttle, 1987) and MgO was a consistently better source when added to a maize-based rather than a dried grass diet (Adam et al., 1996). A source such as magnesium-mica, which releases its magnesium postruminally, may be less susceptible to the rumen-based antagonism from potassium (Hurley et al., 1990). Using weight gain and magnesium in the tibia ash as criteria, it was shown that the oxide, carbonate and sulphate are all sources of highly available magnesium for broiler chickens (McGillivray and Smidt, 1975). The AMg value for reagent grade MgSO4.7H2O was 0.57 at 0.4 g Mg kg21 DM in a semipurified diet for chickens (Guenter and Sell, 1974), confirming the contrast with ruminants.
The dietary magnesium requirements of livestock vary with the species and breed of the animal and its age and rate of growth or production, but mostly with the absorbability of the mineral in the diet of ruminants. The concurrent dietary levels of calcium and phosphorus influence requirements in the non- ruminant. Vitamin D does not directly affect magnesium absorption, but it may do so indirectly through effects on the rates of accretion or mobilization of bone salts (Richardson and Welt, 1965). The magnesium status of the animal has been said to influence requirements (McAleese et al., 1961), because of the magnesium reserves in the skeleton. However, bone reserves provide only limited and temporary protection, particularly when the fall in absorbed magnesium is abrupt. Requirement figures used in ration formula- tion should therefore be sufficient to maintain the skeletal reserves in all species, although failure to provide them will not necessarily cause disease.
The variation in absorbability of magnesium is so wide that it must be taken into account if meaningful requirements are to be calculated. The mean absorbability values in Table 6.2 have therefore been used to generate