Optimal plant densities of lucerne (Medicago sativa) for pasture production and soil water extraction in mixed pastures in south-western Australia

This study aimed to determine the optimal plant density of lucerne for maximum pasture productivity combined with enhanced environmental outcomes in a Mediterranean climate. Lucerne was sown at different seeding rates (1, 2, 4 or 8 kg ha−1) at two sites (Esperance and Katanning); productivity and soil water extraction was compared with an annual pasture over 4 years. Lucerne plant densities 5–6 months after seeding in 2002 ranged from 8 to 39 plants m−2 at both sites. Initial lucerne plant densities at the Esperance site were stable from 2002 until late 2004, and then began to decline, especially at the higher densities. The decline was associated with an extended dry spell. At the Katanning site, the decline was slower but constant from mid 2004 onwards with similar rates of decline for each treatment. Lucerne densities >20 plants m−2 at both sites produced more dry matter than lower plant densities (10 plants m−2) in most years. At both sites the proportion of lucerne in the total biomass increased with higher plant densities but generally at the expense of the proportion of subterranean clover. Lucerne at densities of 10 plants m−2 at Katanning had up to 30–53 mm higher soil water content to a depth of 2.6 m during summer and early autumn compared with that of >20 plants m−2. However, the differences between lucerne plant densities were smaller than the difference between lucerne and annual pasture (up to 120 mm). At Esperance the plant available soil water was limited and there were no differences between lucerne treatments. We conclude that in the 430–450 mm annual rainfall zone of south-western Australia, the higher the lucerne plant density (up to 40 plants m−2) in the first summer after establishment the more productive the pasture is in most years, the higher the proportion of lucerne in the total biomass and depending on the site the drier the soil profile. A drier soil profile can potentially reduce the spread of salinity.