The mineral element content of spring pasture in relation to the occurrence of grass tetany and hypomagnesaemia in dairy cows

Abstract

1. A survey was carried out on dairy farms in the south of Scotland during the period 1958–59 to obtain information on: (a) the mineral element composition of spring pastures associated with grass tetany or staggers and hypomagnesaemia, and (b) the influence of magnesium supplements on the incidence of these conditions and on serum magnesium levels.2. Pastures associated with grass tetany had a significantly lower average magnesium and sodium content and a higher potassium content than those which were classed as normal. These differences were magnified in the values of the ratios K/(Ca + Mg) and (K + Ca + P)/(Mg + Na) which were considerably higher in tetany pastures. The incidence of tetany was positively correlated with the value of the former ratio. These results are similar to those obtained in Holland.3. The analytical results for pastures associated with hypomagnesaemia in clinically normal cows were not significantly different from those for normal pastures and there was no evidence of any association between the composition of the pasture and the incidence or severity of the hypomagnesaemia or the serum magnesium levels.4. The magnesium and sodium contents of the majority of the pastures examined would be judged as inadequate in the light of available information on the requirement of these elements.5. There was no evidence of any correlation between the magnesium content of the pasture and that of any of the other elements studied.6. Cases of grass tetany occurred in 25% of the 108 herds studied, the mean incidence in affected herds being 4·5 ± 0·65%. The overall incidence was 1·1 ± 0·25% and was significantly lower when a magnesium supplement was given as a prophylactic measure.7. Hypomagnesaemia was detected in clinically normal cows after turning out in 41·2% of the herds, the overall incidence being 8·7 ± 1·0%. On tetany free farms the feeding of calcined magnesite or magnesium-rich mineral mixtures did not appear to reduce the incidence of hypomagnesaemia or the fall in serum magnesium levels which occurred after turning out.8. These results emphasize the wide individual variation in the susceptibility of different cows to the factors causing hypomagnesaemia and grass tetany and also the differences between the two conditions in their dietary relationships. It is suggested that dietary factors may be responsible for producing the clinical signs of grass tetany in hypo-magnesaemic cows.

Hypomagnesemia Tetany in Cattle

“…In young spring grass, extremely low concentrations of sodium and high levels of potassium leads to sodium deficiency, a decreased Na:K ratio in the rumen, and consequently an increased level of potassium in ruminal fluid, which further exacerbates the imbalance by decreasing the absorption of magnesium from the rumen [8]. This result is a consistent with the observation which confirms the presence negative correlation between Na content of grass (Na deficiency-the author) and the emergence of symptoms of tetany [46]. Finally, adding NaCl into feed cows could be prevented grass tetany [47].…”

Chemical composition of pastures in relation to grass tetany in beef breeding cows

“…while a little below some of those quoted in the European work, are nevertheless fairly high, particularly for pastures which have not, so far as is known, received any potassic fertiliser. By comparison, Kemp and CHart's (1957) figures averaged 3.03% and 3.67% for non-tetany and tetany pastures, while those of Butler et al (1963) were 3.02% and 3.39%. In Norway.…”

“…A rather sharp rise in the incidence rate occurred at a ratio of about 2.2, and this figure has since been employed by various authors as an index of the tetany-proneness of pastures. Butler et al (1963) found an average ratio of 2.03 for tetany pastures as compared with 1.62 for normal pastures. Kemp and t'Hart's comparable figures were 2.37 and 1.67.…”

“…Some authors have found, on the average, somewhat lower Mg values for tetany than for non-tetany pastures, where large numbers of farms were surveyed (Kemp and t'Hart 1957;Butler and and Guegen 1963), although the differences have been small, and much overlapping occurred. Other authors have been unable to differentiate tetany from normal pastures on the basis of magnesium content (Rook and Wood 1960;Dishington 1965), and it is now tacitly and generally agreed that hypomagnesaemic tetany is not normally caused by an absolute deficiency of magnesium, per se.…”

“…Several authors have suggested that low sodium intake could possibly contribute to the development of hypomagnesaemic tetany (Frens 1955;Paterson and Crichton 1960;Ross 1961;Lehr 1962;Butler et al 1963;Kemp 1964), while others have discussed its effect on animal physiology ( van Weerden 1961;Bott, Denton, Goding, and Sabine 1964). Smith and Aines (1959) induced severe sodium deficiency in dairy cows by prolonged feeding of rations containing about 0.02% Na, and suggested a requirement of 21 g per day Na for lactating dairy cows.…”