Release of nitrogen, sulphur, phosphorus, calcium, magnesium, potassium and sodium from four tropical hays during their digestion in nylon bags in the rumen

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Abstract
The rate of release of N, S, P, Ca, Mg, K and Na from four milled tropical hays in nylon bags in the rumen of cattle over a 168 h period were measured. The hays were: Medicago sativa, Stylosanthes humilis, Chloris barbata and Heteropogon contortus. Three ways were used to express results: (a)element as % of element initially present; (b) element as % of dry matter initially present, and (c) element as its concentration in residual dry matter. The proportions of elements removed during digestion were positively related to initial concentration of the elements in the hays, with the exception of P in C. barbata hay. High proportions of Mg and K were released within 48 h in all hays. Only about 60% of N, P and Ca initially present was removed even after 168 h in the rumen. Amounts of elements remaining became constant for each element after 48 h and were for N, 6.0; Ca, 3.0; K, 1.0; P, 0.7; Na, 0.7; S, 0.5 and Mg, 0.3 g element kg−1 DM initially present. Concentrations of elements in the residual dry matter after decreasing in the first 24 h, increased in the latter stages of digestion, even though cell wall digestion continued. The four hays released the seven elements differently. Generally, the elements in Medicago were most soluble and those in Heteropogon the least. However, the extent of solubilisation in Stylosanthes and in Chloris varied with the element concerned. In Heteropogon hay, which had low element concentrations initially, less than half of the N, S, P, Ca and Na were released even after 168 h digestion.