Carbon Isotope Discrimination in Orchardgrass and Ryegrasses at Four Irrigation Levels

Abstract

Availability of irrigation water is often the limiting factor in the establishment of improved pastures in the U.S. Intermountain Region, and grasses used for pasture should have enhanced water-use efficiency (WUE). Objectives of this study were to describe (i) relationships between carbon isotope discrimination (Δ) and dry matter (DM) yield, and (ii) trends in Δ across four water levels (WL 2, wettest, to WL 5, driest) and two harvest dates under frequent defoliation in cultivars of orchardgrass (Dactylis glomerata L.) perennial ryegrass, (Lolium perenne L.), intermediate ryegrass (L. ×hybridum Hausskn), and festulolium [×festulolium braunii (K. Richt.) A. Camus.]. Within all WLs, orchardgrass cultivars exhibited higher WUE than the ryegrasses, which includes intermediate ryegrass and festulolium. Significantly more variation for Δ within WLs was observed in the ryegrasses than orchardgrass. For orchardgrass, DM yield and Δ were not correlated at WLs 2 to 4. However, they were negatively correlated at WL 5. Correlations between Δ and DM yield were not significant at WLs 2 and 3 for the ryegrasses, but were positive at WLs 4 and 5. To avoid possible declines in DM yield in breeding programs to increase WUE in orchardgrass and the ryegrasses, DM yield should also be monitored.