Soil water dynamics, herbage production and water use efficiency of three tropical grasses: Implications for use in a variable summer‐dominant rainfall environment, Australia

Abstract

In the moist mid‐latitudes of eastern Australia, soil water dynamics, herbage production and water use efficiency (WUE) were monitored during 2006–2008, for five perennial pastures: digit grass (Digitaria eriantha), Rhodes grass (Chloris gayana), forest bluegrass (Bothriochloa bladhii), native grass (Bothriochloa macra and Rytidosperma bipartita dominant), lucerne (Medicago sativa); and two forage crops: oat (Avena fatua) and sorghum (Sorghum bicolor). Ground cover formed more quickly in Rhodes grass and lucerne (>70% ground cover in 120 and 175 days after sowing [DAS] respectively) than in forest bluegrass and digit grass (245 and 365 DAS respectively). Values of maximum extractable water (MEW) for Rhodes grass and lucerne were similar (180–242 mm), while values for digit grass and forest bluegrass (129–175 mm) were equal to or greater than those for native grass, and two annual forage crops (77–144 mm). Lucerne expressed the maximum root depth (1.46 m), while values for the tropical grasses (0.96–1.39 m) were greater than native grasses and forage crops (0.87–0.96 m). Native grasses (6.5–12 t DM/ha) had the lowest herbage production, which resulted in values of WUE that were significantly less than most other treatments (16–21 vs. 23–43 kg DM ha−1 mm−1). Digit grass (33–34 kg DM ha−1 mm−1) had higher WUE compared with the other tropical grasses (20–27 kg DM ha−1 mm−1). The data collected here suggest that a forage system comprising digit grass, lucerne and forage oat would provide high production and WUE in this environment.

Optimum plant density of Digitaria eriantha for herbage accumulation and hydrological performance in a summer dominant rainfall zone

“…year indicated that higher plant densities did not refill to near field capacity and so movement of water and/or nutrients below the root zone was not likely in these years. (Murphy, Boschma, & Harden, 2018;Murphy et al, 2008a), which is comparable with rooting depth of digit grass with a density of 16 plants per m 2 at the same age in this experiment. By year 4 when the initial stored soil water resources had been utilized and the pasture was dependent on winter and summer rainfall for soil water accumulation and plant growth, maximum plant root depth was restricted to <0.7 m (Table 2).…”

“…Maximum rooting depth of digit at all plant densities was achieved during year 3, with the exception of the highest density treatment (16 plants per m 2 ) which achieved a maximum and similar rooting depth during years 2 and 3. Digit grass has been reported to have an active rooting depth of 1.2 m during year 2 on a red Chromosol soil (Murphy, Boschma, & Harden, ; Murphy et al., 2008a), which is comparable with rooting depth of digit grass with a density of 16 plants per m 2 at the same age in this experiment. By year 4 when the initial stored soil water resources had been utilized and the pasture was dependent on winter and summer rainfall for soil water accumulation and plant growth, maximum plant root depth was restricted to <0.7 m (Table ).…”

Rapid soil development in response to land use change: case studies from the northern New South Wales slopes, plains and New England Tablelands

“…The observed greater abundance of roots and deeper penetration of tropical grass pasture roots is likely to be an initiating factor in a series of feedback mechanisms which subsequently changed soil characteristics. The findings in this paper are supported by other comparative studies that showed greater root abundance, soil penetration by roots, and water storage in deep subsoil by tropical grass pasture species in comparison with native pastures in north west NSW (Murphy et al, 2010;Murphy, 2014;Murphy et al, 2018). McCallum et al (2004) have shown that the increased abundance of roots under introduced temperate pastures, such as phalaris, has increased porosity of sodic subsoils in southern Australia.…”

“…The use of African pasture species in improved pasture systems is relevant to soil formation processes because of their much greater feed production (Harris et al, 2014) and therefore their potential to accelerate biological, physical and chemical processes. In addition, their greater root access to deep subsoils (Murphy et al, 2010;Murphy, 2014;Murphy et al, 2018) offers increased potential for cycling of subsoil elements compared with native pastures. The greater root densities and depths of African pasture grasses may also create a different internal soil hydrology through the creation of a greater number of soil macropores.…”

“…Production advantages of tropical grass pastures include dry matter production of 11 -19 t/ha/yr compared with 4 -9 t/ha/yr for native pastures and a greater proportion of digestible protein (Harris et al, 2014). Tropical grass pastures typically have significantly deeper roots (Murphy, 2014;Murphy et al, 2018). Murphy et al (2010) showed that rooting depth under sown tropical grass pastures was related to deeper soil water storage and use which coupled with greater water use efficiency (herbage produced per unit of rain) and feed value has enhanced production in grazing systems.…”