Water use by lucerne and effect on crops in the Victorian Wimmera

Abstract

The effect of the perennial legume lucerne on the soil water regime of a farming system in the Victorian Wimmera was investigated by a combination of direct field measurements and the use of simulation modelling. In the field, soil profiles under lucerne-based pastures (also containing annual medic and Wimmera ryegrass) remained consistently drier throughout the year compared with continuous annual cropping. The amount of plant-available soil water (0–2.0 m) after 3–4 years of lucerne pasture was on average 48 mm less than after annual crops (wheat, field pea), most of which (81%) was extracted at depth (1.0–2.0 m). A wheat simulation study evaluated yield responses post-lucerne under a range of environmental conditions likely to be experienced within the region. It was predicted that the yield penalty for the first wheat crop after lucerne would vary from 0 to 0.87 t/ha, depending upon seasonal conditions. Overall, the analysis predicted a median yield penalty of 0.4 t/ha (15%) for wheat after lucerne. The risk of a large yield penalty (>0.8 t/ha) was low (5 years in 100). In the simulation, the risk of yield loss decreased during a cropping phase with each year after lucerne as the soil profile progressively became recharged with water. The time taken to recharge the soil prof ile fully after lucerne to levels equivalent to that under continuous cropping was not resolved in the field, but simulation studies suggested that it would be within 5 years.