Mineral nutritional status and botanical composition of pastures. II. Effects on nitrogen concentration and digestibility of herbage

Abstract

To study the effects of herbage nutritional status and botanical composition on herbage nitrogen content and digestibility, we carried out an experiment in which : (i) four plots had similar herbage nutritional status (N and P), but different botanical compositions, (ii) three pairs of plots each with similar botanical composition but different herbage nutritional status. Nitrogen content and digestibility patterns were studied from the start of vegetative growth to the time of flowering of the main grasses. To explain variation in nitrogen content and in vitro organic matter digestibility (IVOMD), we studied the leaf-stem ratio and the percentage of senescent material in the herbage.At the start of vegetative growth, the IVOMD was stable but it was lower for P deficient plots in relation to botanical composition (plots dominated by Festuca rubra). However, the temporal decrease of IVOMD was less rapid in these plots because they had a higher leaf-stem ratio and a lower percentage of senescent material. Thus, at the flowering stage of grasses, the IVOMD was the highest for P deficient plots.At the beginning of spring, the nitrogen content of the plot with non-limiting P herbage nutritional status was the highest, but it became the lowest after the heading stage of the main grasses. Differences between plots were related to P herbage nutritional status rather than to botanical composition : before stem elongation of the grasses, deficiency of P reduced N absorption, subsequently the rate of decrease in nitrogen concentration of the herbage was educed.