Effects of pH, temperature and osmolality on the level and composition of soluble N in feedstuffs for ruminants

Solubility of N is an important parameter in many protein evaluation systems for ruminants. The influence of different rumen conditions, such as pH, osmolality and temperature of solvents, on solubility of N compounds in various animal feed ingredients was examined in two experiments. In the first experiment, 17 feedstuffs were evaluated including legumes, oilseeds, grains, ryegrass and silages. Solvents were tap water at 18 and 38 °C, borate-phosphate buffer of pH 6.8 and sodium acetate buffer of pH 5.0, both at 38 °C. In the second experiment, N solubility of soybeans, soybean meal, peas, faba beans and lupins was evaluated in buffers at 4 pH values (5.0, 5.6, 6.2 and 6.8) and two osmolality values (300 and 400 mOsm/l) at 38 °C. The N solubility in feedstuffs was affected by the pH and osmolality of the solvent used, with the largest effects related to pH. Nitrogen solubility decreased upon reduction in pH from 6.8 to 5.0 for many feeds investigated, except for grass silage, maize silage, formaldehyde treated feeds, rapeseed meal and maize. The largest effect was for untreated legumes, where the average solubility as a fraction of the total N decreased from 0.43 at pH 6.2 to 0.26 at pH 5.6. The pH did not affect the NPN expressed as a fraction of total N, but NPN as a fraction of soluble N increased from 0.35 at pH 6.2 to 0.67 at pH 5.0. SDS-PAGE electrophoresis of the soluble N fraction of lupins, peas and soybeans showed that the decline in pH increased the ratio between albumins and storage globulins, especially 11S, in this fraction. The reduction in pH also decreased the proportion of 11S globulins of the soluble N fraction in faba beans. The effect of osmolality on N solubility was less pronounced than that of pH. Increasing osmolality from 300 to 400 mOsm/l generally increased N solubility. However, a significant pH–osmolality interaction for beans, lupins and peas occurred because osmolality did not affect N solubility at the highest pH level. Results show that pH of the solvent, and to a lesser extent osmolality, affect N solubility, and suggest that evaluation of CP degradation in the rumen requires consideration of rumen fluid pH and osmolality.