Survey of long-term productivity and nutritional status of Leucaena leucocephala-grass pastures in subtropical Queensland

A survey of the productivity and nutritional status of leucaena (Leucaena leucocephala subsp. glabrata) pastures of different ages was conducted in subtropical Queensland from 2006 to 2008. Four leucaena stands (aged 8, 20, 31 and 38 years) growing on the same Vertosol soil type at Brian Pastures Research Station were surveyed. In the higher rainfall season of 2007-2008, leucaena yields and rainfall-use efficiency were highest in 8-year-old stands [2128 kg total dry matter (DM)/ha or 4.0 kg DM/ha.mm] and lowest in 38-year-old stands (978 kg total DM/ha or 1.9 kg DM/ha.mm). The reduced productivity in the 38-year-old leucaena pasture was associated with a decline in stem number/plant and leucaena plant density due to observed plant mortality. The reduced yield and vigour of aging leucaena was associated with nitrogen deficiency related to declining phosphorus and sulfur availability for adequate symbiotic N-2 fixation and leucaena plant growth. Nutrient deficiencies of phosphorus and sulfur in leaf tissue were related to low to medium initial soil fertility (7-27 mg/kg of bicarbonate extractable phosphorus in the top 20 cm), coupled with inherent subsoil constraints (shallow soils, sodicity and high pH), and exacerbated by both long-term removal of nutrients by grazing animals and a reduction in soil phosphorus and sulfur availability over time. To maintain the productivity of leucaena pastures, plant nutritional status needs to be monitored in order to determine strategic fertiliser application.