Nitrogen utilization of perennial ryegrass in dairy cows

Abstract

In intensive dairy farming, perennial ryegrass is among the main forages used because of its rapid response to nitrogen (N) and its high herbage yield. Due to a high N fertilization and cutting at a young and leafy stage, grass has a high crude-protein (CP) content that is rapidly degraded by rumen microbes. The supply of N often exceeds the supply of energy available to rumen microbes, and almost 80 % of this excess N is excreted in the urine, giving rise to a low efficiency of N utilization by grass-fed dairy cows. Improving the efficiency of grass N utilization in dairy cattle is possible by including specific traits in grass-breeding programmes. Perennial-ryegrass cultivars differ in the water-soluble-carbohydrate (WSC) content, and WSC is a consistent and heritable trait. An increase in WSC is related with only a slight increase in total carbohydrate (CHO), with a decrease in neutral detergent fibre (NDF) and (or) the non-determined residual fraction, whereas the CP content is hardly affected. This shift in the source of energy in cultivars with an increased WSC content may better match the rapidly degradable CP in the rumen, viz. synchronization, and improve the efficiency of N utilization in the rumen. However, cultivars with an increased WSC content did not result in a significantly increased microbial protein flow to the duodenum and efficiency of microbial protein synthesis, nor in a reduced NH3 content in rumen fluid or milk urea N (MUN), and did not result in an increased efficiency of N utilization in dairy cows. Moreover, a substantial proportion of N is recycled within the rumen and in the animal, and this may nullify the positive effect of an improved synchronization on the efficiency of microbial protein synthesis. With increased N-fertilization levels, the CP content in grass increased and this resulted in an increased N intake in dairy cows. The increased N intake was related with increased MUN, a slight increase in N excreted in milk but a strong increase in urinary N excretion, and thus a decrease in efficiency of N utilization in dairy cows. Thus there is little scope to improve the efficiency of N utilization in dairy cows by increasing the WSC content in cultivars with a high CP content, but decreasing the CP content in grass or grass-based diets to around 150 to 160 g/kg DM may result in substantial improvement