Nitrogen nutritional status in perennial grasses under defoliation: Do stubble height and mixed cultivation matter?

Abstract

AbstractBackground and aims: The concepts of critical nitrogen (N) concentration and nitrogen nutrition index (NNI) provide effective tools for assessing the N nutritional status of pastoral systems. However, no conclusive data exist concerning the effects of stubble height at harvest on the N dilution curve [N concentration (N%) = a(biomass)−b] and mixed species cultivation on N nutritional status of forage plants with contrasting growth strategies. To evaluate these effects, three cool‐season perennial grasses (exploitative, Arrhenatherum elatius L.; moderately exploitative, Dactylis glomerata L.; conservative, Festuca arundinacea Scherb.) were grown under different cultivation methods either as pure stands or as a mixture and conducted two experiments.Methods: In experiment I, canopies of different grass species pure stands and mixtures were subjected to defoliation managements aimed to not exceed the critical LAI at harvest. Forage samples were collected from October 2015 to August 2016 to determine the N nutritional status throughout the seasons. In experiment II, half of each pure stand was mowed to 10 cm in height, and the other half to ground level. Thereafter, herbage samples were taken during regrowth to determine N dilution curves in both scenarios (from ground level and from stubble height of 10 cm).Results: We verified an NNI above 0.8 during autumn and winter for A. elatius, D. glomerata, and the mixture but only in winter for F. arundinacea. Dactylis glomerata grown in the mixture had a lower N% than its pure stand during autumn and winter, whereas A. elatius and F. arundinacea showed no differences irrespective of cultivation method throughout the year. The three forage species presented classical dilution curves when regrowth was initiated from ground level, but quadratic trends were observed when regrowth was initiated from stubble heights of 10 cm.Conclusions: Overall the N nutritional status of cool‐season perennial grasses was not affected by cultivation method when pastures were managed in conditions of non‐limiting N supply and with pre‐cut/grazing height targets derived from critical LAI measured at the full vegetative growth stage. Moreover, the N nutrition status in pastures managed under moderate/lenient defoliation should not be evaluated during periods of less intense canopy renewal and/or right after the cut/grazing events.