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

Abstract

Background 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.

A management proposition to preserve functional identity of grasses in mixed swards

“…Nitrogen was applied every 30–60 days, at the rate of 30 kg N ha −1 during the first experimental year and 50 kg N ha −1 during the second experimental year. The greater N fertilization during the second year was necessary to maintain the nitrogen nutritional index above the threshold of 0.8 (Américo et al, 2021) as recommended by Lemaire and Gastal (1997). More detailed data regarding how light measurements and fertilization were performed can be found in Duchini et al (2018; 2019).…”

“…The greater N fertilization during the second year was necessary to maintain the nitrogen nutritional index above the threshold of 0.8 (Américo et al, 2021) as recommended by Lemaire and Gastal (1997).…”

Root mass vertical distribution of perennial cool-season grasses grown in pure or mixed swards

“…Regarding to the cultivation method, a greater root biomass was observed in the mixture (655 g/m 2 ) when compared to the monocultures (averaging 493 g/m 2 ; Figure 1A). As interspecific competition was probably restrained by the high input of fertilizers (AMÉRICO et al, 2021) and tiller population density was not greater in the mixture (DUCHINI et al, 2018), an alternative explanation for such greater root biomass in the mixture could be the species-specific root interaction. In this context, root growth and distribution would be modulated by the identity of the neighboring roots, and non-nutritious cues, such as root exudates (BAIS et al 2006) or root rhizosphere biota (KARDOL et al, 2007), could be the mechanisms behind root recognition (MOMMER et al, 2010).…”

“…The soil of the experimental area is classified as Typical Aluminum Humic Cambisol and presented the following characteristics (0-20 cm): pH (SMP) 6.0; organic matter 1.4%; Ca 6.31 cmolc/ dm 3 ; K 180 mg/dm 3 ; P 8.7 mg/dm 3 ; Mg 4.04 cmolc/ dm 3 ; H+Al 4.6 cmolc/dm 3 ; cation exchange capacity (pH 7.0) 15.4 cmolc/dm 3 , base saturation 70.2%; clay 46%. Fertilizations were performed in order to maintain a rich-soil environment and nitrogen nutrition status above 0.8 (see AMÉRICO et al 2021 for more information). The treatments were A. elatius 'Santa Vitoria', F. arundinacea 'Quantum II', and D. glomerata 'Ambar' sowed as monocultures and as mixture composed of the three species in the same proportions.…”

“…This response is relatively common in grasslands, where root mass is concentrated near the soil surface, with about 60-80% of the total root mass concentrated up to a depth of 15 cm (SKINNER & DELL, 2016). Moreover, we hypothesized that there were no stimuli to plants deepen their root system, as fertilization (AMÉRICO et al, 2021) and rainfall (DUCHINI et al, 2019) were not limiting during the experimental period.…”