Spring Nitrogen Rate and Timing Influence on Seed Yield Components of Perennial Ryegrass

Spring application of N generally increases seed yield of perennial ryegrass (Lolium perenne L.). Understanding the influence of N rates and timings on seed yield components can be used to improve N use efficiencies and seed yield. Fidd experiments during 1982 and 1983 at Oregon State University studied the effects of 60, 90, and 120 kg N ha⁻¹, applied either at double-ridge stage (DR) or at spikelet initiation stage (SI), on seed yield and yield components of the cultivar Pennfine. Control plots with no spring-applied N were also included. Little lodging was observed in 1982, but all plots severely lodged by maturity in 1983. Timing of N application did not affect seed yield in either year. Compared with the control, spring N increased seed yield 43% in 1982 and 39% in 1983 when averaged across rates. In 1982, seed yield did not vary with N rates applied at DR, but 120 kg N ha⁻¹ produced greater seed yield than 60 kg N ha⁻¹ when applied at SI. In 1983, seed yield was maximized by 90 kg N ha⁻¹ applied at DR, and by 120 kg N ha⁻¹ applied at SI. Nitrogen application did not affect the number of spikes per unit area, but increased seed number per spike in 1982 and weight per seed in 1983. Averaged across rates and timings, N increased total number of seeds produced per unit area 43% in 1982 and 30% in 1983. Seed yield was correlated with the number of seeds per spike in both 1982 (r² = 0.68) and 1983 (r² = 0.72). In 1983, seed yield was also correlated with weight per seed (r² = 0.55). We concluded that in the absence of severe lodging, 60 kg N ha⁻¹ applied at DR is adequate for achieving maximum seed yield. If fertilization is postponed until SI, however, higher rates of N may be required. Results also suggest that modifications in N fertilization should focus on improving seed set per spike and seed growth rather than increasing the number of spikes per unit area or floret sites per spike. Contribution from Oregon Agric. Exp. Stn., Tech. Paper no. 10809.