Water use, competition and a temperate-zone C4 grass (Eragrostis curvula (Schrad.) Nees. complex) cv. Consol
Authors: Johnston W. H. , Koen T. B. , Shoemark V. F. 
Published: September 2002

The effects of a perennial C4 grass (Eragrostis curvula [Schrad.] Nees. complex cv. Consol) and winter-growing annual pasture on the soil water deficit (SWD) was assessed at 2 sites near Wagga Wagga, NSW. At Site A, newly sown E. curvula-Trifolium subterraneum L. pasture was compared with an annual pasture containing Lolium rigidum Gaud. and T. subterraneum. A mature E. curvula pasture sown 10 years previously was compared with an adjacent volunteer annual pasture at Site B.

The annual pasture at Site A had little effect on the SWD at 120 cm depth, but it developed an appreciable SWD at 60 cm on 2 occasions when spring rainfall was well below average. This deficit was not evident the following autumn. When E. curvula was present in the pasture, it developed a substantial SWD at 120 cm in summer and autumn. This represented a significant realignment of the water balance that had the potential to reduce deep drainage.

The size of the SWD in autumn determined how rapidly soils reached field capacity in winter. At Site B, E.�curvula consistently created a deficit of between 108 and 124 mm to 100 cm depth in autumn, compared with 2-106 mm for the annual pasture. The soil profile under E. curvula did not reach field capacity until 60-100 days later than the annual pasture. The annual pasture at Site B had a minimal effect on the SWD at 100 cm in 3 of the 4 years of the study.

The camping behavior of sheep at Site A caused E. curvula to succumb to competition from T. subterraneum, Hordeum leporinum Link, and Cryptostemma calendula (L.) Druce. Competitive interactions between T.�subterraneum cv. Junee and Eragrostis curvula cv. Consol were therefore investigated in a glasshouse study using the de Wit replacement series methodology. Pots were watered to weight and water usage was recorded under 2 levels of watering [low (restricted watering) and high (watered to field capacity)] that were applied after an establishment period of 6 weeks. Pots were harvested after a further c.12 weeks and above- and below-ground yield of each species determined.

T. subterraneum tended to use water up to the limit of its supply, and as water became less available and in response to increasing atmospheric water demand in spring, it frequently wilted. E. curvula rarely wilted; it exhibited water-saving strategies, including leaf waxing and leaf rolling in the heat of the day in response to increasing water demand without necessarily exhausting its water supply.

Both species used water more efficiently (dry matter/kg water used) when its supply was limited. The water use efficiency of monoculture T. subterraneum was greater than that of slowly growing E. curvula, and both species used water most efficiently when growing alone. T. subterraneum was highly competitive against E. curvula. Competition resulted from its out-of-phase growth compared with E. curvula.

E. curvula has the potential to markedly affect on the water balance. However, it is vulnerable to competition. Managing the flush of growth in spring is an important issue in maintaining a balance between E. curvula and winter-growing annual species.