Dynamics of water use under annual legume pastures in a semi-arid mediterranean environment

Abstract

The persistence of annual pastures in dryland farming depends on the development and maintenance of a substantial seed bank in the soil. Successful seed-set and production depends on a favourable soil moisture supply for a minimum period after anthesis starts in spring. The present studies evaluated the scope for increasing the probability of satisfactory soil water supply over this critical period in the southwestern Australian cereal belt by modifying the pattern of water use (ET) during the season. In the first trial, in 1987, weighing lysimeters were used to monitor ET which was partitioned into soil evaporation under canopy ( E SC) and transpiration ( T), in a first year pasture ( Medicago polymorpha) on soils of contrasting texture. On the fine textured soil rapid early growth promoted water-use and prematurely depleted stored soil water, compared to a coarse textured soil on which slower early growth was associated with more stored water later in the season. However, poor adaptation by M. polymorpha on the light textured soil led to 51 % of ET being lost to E SC, compared to 39% on the fine textured soil, and this prevented the pasture from exploiting the better water supply. As a result, water-use efficiency was significantly higher on the fine (8.2 kg ha −1 mm −1) than on the coarse (5.6 kg ha −1 mm −1) textured soil. In the second trial, in 1989, growth and pattern of ET by regenerated pasture was evaluated on a fine textured soil in response to three management treatments: (1) Control, (2) Mowed and (3) Grazed. Mowing and Grazing both reduced early water use, and this was translated into greater dry matter production in the mowed treatment in the last 4 weeks of the season. However, there was no difference in dry matter produced over the full post-anthesis period in the Control and Mowed, and this was attributed to the early start to the growing season (2–3 weeks), which meant that anthesis was also earlier than usual. In a season opening at the normal time, it is expected that the post-anthesis period would have been shorter and later and that dry matter, and seed yields would probably have benefited from the additional water stored and the resulting delay of senescence at the end of the season. It was concluded that there was scope for manipulating water-use, by measures such as grazing or mowing, to benefit seed yields on heavy textured soils, but that on light textured soils further evaluation was warranted using a better adapted pasture species.