Tropical Grasslands (2005) Volume 39, 921
An evaluation of three tropical ley legumes for use in mixed farming systems on clay soils in southern inland Queensland, Australia
A. M. WHITBREAD, B.C. PENGELLY AND B.R. SMITH
CSIRO Sustainable Ecosystems/APSRU, Brisbane, Australia
In the mixed grain-livestock producing subtropical regions of northern Australia, declines in the fertility of cropping soils combined with an increasing demand for high quality forage have prompted many producers to consider greater use of ley or phase pastures in their farming systems. The recently released tropical legumes, Lablab purpureus cv. Endurance and Macroptilium bracteatum cv. Juanita, and a previously commercially available legume cultivar, Clitoria ternatea cv. Milgarra, were compared at 3 onfarm sites across southern inland Queensland. L. purpureus produced its highest biomass in the first (range 1595–7037 kg/ha DM) and second years (range 4717–6150 kg/ha DM) but high plant mortality and an absence of seedling regeneration resulted in poor dry matter production in the third year. While C. ternatea produced low biomass in the first season (<1277 kg/ha DM), yields increased progressively, reaching 4047 kg/ha DM by the third year. Virtually all of the original plants of this species survived and 15–28 seedlings/m2 established from high soil seed reserves, ensuring a persistent pasture. Biomass production of M. bracteatum in Years 2 and 3 was relatively stable on the sites with the deeper soil profiles (Downfall Creek and Brigalow) and ranged from 3382 to 4940 kg/ha DM. At one site, cropping activities over the previous 20 years had depleted soil nitrate-N content measured to 1.2 m prior to the legume phase, to 27–34 kg/ha N. Following the 3 seasons of legume production and a 12-month fallow, soil nitrate-N increased to 168–223 kg/ha N. At another site, soil nitrate-N contents of 177–180 kg/ha N were measured in the soil profiles to 1.5 m following the legume leys compared with only 74 kg/ha N in a continuous-wheat treatment. Management strategies to capitalise on this additional soil N are discussed.