Carbon isotope discrimination as an index of tall fescue–endophyte association response to light availability and defoliation

Abstract

Tall fescue [ Lolium arundinaceum (Schreb.) S.J. Darbyshire] is adapted to the diverse growing conditions of humid, temperate agro-ecosystems. A mutualistic endophytic symbiont ( Neotyphodium coenophialum, [Morgan-Jones et Gams] Glenn, Bacon, and Hanlin) contributes to this resilience, but at the same time compromises grazing livestock health. Novel endophytes that do not produce ergo-alkaloids were developed to circumvent livestock health issues, but the response of novel associations to site conditions and management practices, such as partial shade and repeated defoliation, is unclear. We conducted a field experiment with tall fescue to determine if carbon isotope discrimination ( Δ) could be used to characterize host–endophyte association responses to site and management. Plants infected with either the native or novel non-ergogenic endophyte, or that were endophyte free were clipped to typical pasture (5 cm) or silvopasture (10 cm) residual sward heights each time mean canopy height reached 20 cm. The Δ of tall fescue was inversely related to light availability. Greater variation in Δ occurred when plants were clipped to a 5 rather than a 10 cm residue height. This trend persisted throughout the growing season and was similar whether Δ was expressed relative to potential evapotranspiration (ET o), leaf dry matter content (LDMC) or an index of herbage nutritive value (expressed as energy relative to crude protein). Host–endophyte association did not affect Δ. This might be attributable to relatively mild growing conditions occurring in the central Appalachian region of the USA, irrespective of site conditions, or the minimal cost of hosting endophyte. Carbon isotope discrimination responses for tall fescue agree with previous observations on Δ and light availability, and might be a useful means to assess silvopastoral management practices.