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Thirty-five experiments are demonstrated in which high-lactating cows were exposed to abrupt changes from a normal dietary Mg supply to an extremely Mg-poor diet. The preliminary diet was adequate, supplying the cows with about 25 g Mg per day. The experimental feeding was individually calculated, containing 2.55 Mg for maintenance and 0.15 g for each kg of milk. Daily serum mineral analyses showed a pronounced hypomagnesaemia in all 35 experiments within 2 to 18 days. In 5 experiments there was a simultaneous fall in serum Ga, and the cows collapsed in tetany. The other 30 cases remained clinically healthy in spite of the low serum Mg level as long as the diet was given unchanged. In 25 cases these already hypomagnesaemic cows were exposed to a relatively high oral supplementation of Na2HPO4,Na2SO4 or both. In 9 cases the supply led to pronounced tetanic convulsions, in 4 cases a subtetanic state was observed, and in 4 cases the cows showed borderline symptoms, i.e., they reacted clinically as well as analytically immediately after the supply was given, but no attacks appeared, and the cows recovered as soon as normal fodder was given. In 8 cases the cows did not react to the “trigger” doses at all. The experiments are discussed and the reactions compared with those observed when high-lactating cows are let out on tetany-prone, (NH4)2SO4-dressed pastures. The conclusion is drawn that a high Mg content in the diet prior to a Mg-deficient diet has little, if any, influence on the tendency to precipitate tetany. Considerable decreases in content, or availability, of dietary Mg always lead to decreases in serum Mg. Occasionally, under special unfavourable circumstances, when the drop in serum Mg is extraordinary rapid, the serum Ca level may drop as well and tetany occur without further stressing elements. In most cases, however, the results seem to support the suggestion that tetanic couvulsions are not conditioned entirely by the reduced availability of Mg but are to a great extent dependent upon the serum Ca-reducing effect of co-operating, so-called trigger compounds, simultaneously present in the diet. In this connection the addition of Na2HPO4 and Na2SO4 to the Mg-poor diet has been studied and found to confirm the suggestion made in earlier papers (Ender et al 1957; Dishington 1965) that the high content of Na, K, P, and S found in (NH4)2SO4-dressed pasture grass seems to play a prominent part in making this grass tetany-prone.

Hypomagnesaemia and Hypomagnesae-Mic Tetany Induced in Lactating Cows by Changing the Diet