WATER-USE EFFICIENCY OF GRASSES AS AFFECTED BY SOIL, NITROGEN, AND TEMPERATURE

Forage grass production in the northeastern USA is limited by soil water availability, but little is known about water use by forage grasses in this region. The purpose of this study was to assess the effects of soils, temperature regime, and N fertilization on water-use efficiency of a C3 and a C4 forage grass. The study was conducted from 1987 to 1989 on four sites in central Pennsylvania selected for soil water-holding capacity and temperature regime. Plot areas of orchardgrass (Dactylis glomerata L. cv. Pennlate) were treated with a split application of 0, 84, and 168 kg N ha-1 of N-15-depleted NH4NO3. Plot areas of switchgrass (Panicum virgatum L. cv. Cave-n-Rock) were treated with a single application of 0 and 84 kg ha-1 unlabeled NH4NO3. Soil water was measured with a neutron probe on each plot biweekly during the growing season and monthly during the rest of the year. At heading stage (approximately 31 May), the orchardgrass plots were harvested for yield at a 7.5-cm stubble height and all herbage removed from the plot. These plots were refertilized and sampled again on 31 October. Switchgrass was harvested at a 15-cm stubble height at heading stage (approximately 15 July). Water-use efficiency was 126 kg ha-1 cm-1 H2O for spring-growth orchardgrass and 44 kg ha-1 cm-1 H2O for summer-fall-growth orchardgrass and was affected by N fertilization and temperature regime. Water-use efficiency was 250 kg ha-1 cm-1 H2O for summer-growth switchgrass and was also affected by N fertilization and temperature regime, but the effect of temperature was not as pronounced as in the case of orchardgrass.