Response of Populations of Lolium perenne cv. S 23 with Contrasting Rates of Dark Respiration to Nitrogen Supply and Defoliation Regime: 1. Grown as Monocultures

Simulated swards of two populations of perennial ryegrass cv.S 23 selected for contrasting rates of mature leaf tissue respirationwere grown in a glasshouse. From establishment, the swards weresubject to three levels of nitrogen supply (14.5, 32 and 173.5ppm N) and from the first harvest 7 weeks after sowing, to threecutting frequencies (at intervals of 1, 3, and 6 weeks). Throughout the 18-week experiment, the herbage yield and themean tiller weight of the slow-respiring population were 10%greater on average than those of the fast-respiring population.Increasing nitrogen supply (from 14.5 to 173.5 ppm N) enhancedthe yield advantage of the slower-respiring population—moreso under infrequent cutting (from nothing to 22%) than underfrequent cutting (from 6 to 13%). Both maximum absolute yields,and the greatest yield advantage of the slow-respiring populationover the fast, were achieved when high nitrogen was combinedwith infrequent defoliation. Lolium perenne, perennial ryegrass cv. S23, respiration, nitrogen defoliation, simulated sward, dry-matter production, monoculture, population