Solubility of mineral elements present in ruminant feeds

Abstract

SUMMARYEight feeds were treated with seven solvents and the proportion of seven mineral elements (Ca, Mg, P, Na, K, Cu, Zn) released was assessed. Six of the feeds were from Sri Lanka (Panicum maximum ecotype Guinea A, Glyricidia maculata, Artocarpus heterophyllus, untreated and urea-treated rice straw, and rice bran) and two from the Netherlands (maize silage and wheat straw). The solvents were water, tris buffer, rumen fluid from a cow deprived of (RF -) or fed (RF +) mineral supplements, neutral detergent solution with (NDS +) or without (NDS -) EDTA, and acid detergent solution (ADS).Both the type of feed and the solvent significantly influenced (P < 0·01) the amount of dry matter loss and the proportion of minerals released. Maize silage released over 80% of its minerals, except Cu, in water and tris buffer, probably because of the low pH (3·7) during ensiling. The other feeds differed widely in their ability to release minerals. In general, P, Na and K. were more soluble in water than Ca, Mg and Zn.Mineral concentration in RF influenced not only the amount of minerals released, but also the extent of sorption by the feed. The latter effect was more pronounced in feeds with low mineral concentration, maize silage being no exception.Treatment with NDS+ and ADS removed all minerals except Cu. With all feeds, 12–34% and 5–34% of the Cu remained in the ND and AD residues, respectively, indicating its association with the cell wall. Results of the NDS- treatment showed that some of the Ca and Mg may be associated with the cell wall.Comparison of the feeds across the different solvents tested indicated that, in terms of absolute quantity of mineral released, G. maculata could be a good source of Ca, Mg, K and Cu, and that rice bran is a good source of P and Zn. The variety of rice straw tested released high amounts of Na. A. heterophyllus is rich in available Ca.