The relationship between applied nitrogen, nitrogen concentration in herbage and seed yield in perennial ryegrass (Lolium perenne L.). I. Cv. Grasslands Nui at five sites around the globe

Potential environmental problems following the use of applied nitrogen (N) in cropping have resulted in research on indicators which attempt to ensure that the amount of N applied matches the specific requirements for the crop in question. The research described in this paper was conducted at Lincoln and Palmerston North (New Zealand), Corvallis (USA), Aberystwyth (United Kingdom) and Roskilde (Denmark), to test a relationship between the concentration of N in herbage at 14 days after spikelet initiation, and seed yield in perennial ryegrass (Lolium perenne L.) cv. Grasslands Nui. A range of applied N (between 0 and 250 kg ha⁻¹ of N) was used to create a range of herbage N in spring. Dry matter yield (DMY) was assessed 14 days after spikelet initiation and dry matter sent to New Zealand for analysis of combined N. Seed yield and thousand seed weight were analysed at harvest. At all sites adding N resulted in a signicant increase in seed yield. Maximum seed yields ranged from 300-2180 kg ha⁻¹, indicating that at some sites N was not the major limiting factor. The concentration of N in herbage accounted for 32-96% of the variability in seed yield, depending upon site. Results from combining all data suggested that for 80% relative seed yield (i.e., relative to the maximum seed yield for the individual trials), 100kg ha⁻¹ N and an herbage N concentration of 3% in spring are required, and that applying an additional 50 kg ha⁻¹ of N would increase the concentration of herbage N by 0.45%. The correlation between actual seed yield and N content (DMY x N %) 14 days after spikelet initiation (r = 0.78) indicated that for a seed yield of 1600 kg ha⁻¹, an herbage N content of over 90 kg ha⁻¹ would be necessary in early spring