Fertiliser Usage and the Mineral Requirements of Grazing Livestock

Keywords: NPK fertilisers, Herbage, Cattle, Major mineral requirements, Crude protein/phosphorus correlations, Hypomagnesaemia

Nitrogen fertiliser consistently increases phosphorus, magnesium, potassium, sodium and calcium contents of growing herbage but towards maturity both crude protein and phosphorus contents decline in parallel. Fertiliser nitrogen has a larger positive effect than fertiliser phosphorus on herbage phosphorus contents. Any enforced reduction in nitrogen fertiliser use could have serious implications for the phosphorus intake of grazing cattle. Any proposed reduction in dietary phosphorus intake by high yielding dairy cows to below currently advised amounts requires extensive confirmatory experimentation before acceptance. Magnesium contents of grazed herbage are markedly lower in the spring than in the autumn. Most experiments concerned with hypomagnesaemia have involved groups of less than 5 cows of relatively low milk yield with well-above normal potassium fertiliser application rates. None of the results is statistically significant. Dusting pastures immediately prior to grazing in the spring with relatively small amounts of calcined magnesite may be more immediately effective than spreading larger quantities of more granular material in the winter. Nitrogen fertiliser beneficially increases copper and decreases molybdenum contents in grass. Long-term experiments have demonstrated that very high annual nitrogen fertiliser applications have no detrimental effects on the production and health of dairy cows provided basic amounts of phosphorus and potassium are given.

Prof. R. G. Hemingway and Prof. J. J. Parkins, Glasgow University Veterinary School, Bearsden, Glasgow, UK.

36 pages, 15 tables, 88 refs.