Effects of N fertilisation, leaf appearance and time of day on N fractionation and chemical composition of Lolium perenne cultivars in spring

New Zealand dairy systems are based on Lolium perenne pastures with low nitrogen use efficiency (NUE) and high urinary N losses. Modelling attempts to provide solutions to improve NUE are limited due to lack of data on pasture N fractions under a range of management situations. The effects of N application (0 or 25 kg N/ha), cultivar (standard and high sugar cultivars), regrowth interval (leaf appearance of 2, 3 and 4 leaf) and time of day (morning or afternoon) on chemical composition and N fractionation of L. perenne were investigated in a field study in early spring. Leaf appearance interval and time of day had the greatest influence on chemical composition and fractionation. Both fraction A (non-protein N) and water soluble carbohydrates increased by 60% between second and fourth leaf appearance (P<0.001) and by 20% between morning and afternoon (P<0.001). Total N and fractions B1 and B2 (soluble true protein) declined during the day and with leaf appearance. Addition of low fertiliser N rates did not alter N fractionation or chemical composition but improved herbage yield. The effects of using a high sugar cultivar were small and inconsistent and differences between cultivars were more closely linked with flowering date. Generally, the relative proportions of N fractions could be altered with management strategies but the overall solubility of N remained high at over 0.85. While management factors play an important role in influencing the quality of herbage and total N relative to carbohydrate, there is little scope to improve NUE by reducing the proportions of soluble N fractions.