A lucerne-digit grass pasture offers herbage production and rainwater productivity equal to a digit grass pasture fertilized with applied nitrogen

In recent years, livestock producers have widely sown tropical perennial grasses, such as digit grass (Digitaria eriantha), in the frost-prone summer dominant rainfall zone of eastern Australia. Tropical grasses require substantial nitrogen inputs to maintain water productivity and forage quality. Perennial legumes offer a potential nitrogen source for these perennial grasses when sown in a mixed sward. Lucerne (Medicago sativa) is the perennial legume most widely sown in grazing systems in south-eastern Australia. In this study, conducted over the period 2014–2018, we compared the soil water dynamics and rainwater productivity of pure stands of digit grass (fertilized with applied nitrogen), lucerne, desmanthus (Desmanthus virgatus) and leucaena (Leucaena leucocephala) and binary mixtures of digit grass (not fertilized with applied nitrogen) with each legume. We found that often growing season actual evapotranspiration (ETₐ) was similar among the treatments (P > 0.05), but rainwater productivity (kg DM/ha.mm) and proportion (%) of legume herbage mass were not (P < 0.05). Our results showed that the lucerne-digit grass mix was equally productive and efficient as fertilized digit grass (P > 0.05), particularly in the latter three seasons. However, lucerne dominated the herbage mass (c. > 67% legume). Leucaena production was delayed by frost and severely impacted by competition with digit grass, and generally underperformed both lucerne and desmanthus. This observation did always not hold when alfalfa mosaic virus (AMV) impacted desmanthus in specific seasons. Our experiment has shown that a mixed sward of lucerne-digit grass is highly productive and offered high rainwater productivity. Both desmanthus and leucaena, in mixes with digit grass, provided useful contributions of legume herbage mass, in specific seasons and under specific conditions, but both underperformed overall compared with lucerne.