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

Abstract

Pastures from 27 sites (19 farms), mainly in the Hawke's Bay district, were sampled during an outbreak of grass tetany in beef breeding cows. Pastures were generally ryegrass-dominant, with little clover at grazing height, and were mainly closely grazed. Chemical analyses gave the following results (mean value and range of values. %) :-

Mg, 0.19 (0.14–0.25): Ca, 0.39 (0.29–0.61); K. 3.29 (2.0–4.0); Na. 0.06 (0.00–0.19); P, 0.50 (0.35–0.69); N. 5.28 (4.2–6.3); non-protein N. 0.94 (0.75–1.17); non-protein N as a percentage of total N, 18 (16–22); soluble carbohydrate. 9.7 (3.8–15.5); ratio K/(Ca + Mg). 2.45 (1.42–3.63).

The magnesium levels fall within the range where occurrence of hypomagnesaemic tetany may be considered a possibility, and the over-all results suggest that the deficiency of magnesium in the animal is induced by a high-protein—low-energy intake. Possible contributory factors in the pasture are high potassium, low calcium, and low sodium. Since no fertiliser nitrogen or potassium was used, the high level of these elements is attributed mainly to luxury consumption from high-fertility patches resulting from the return of animal excreta.