LONG-TERM PRODUCTIVITY OF LEUCAENA (LEUCAENA LEUCOCEPHALA)-GRASS PASTURES IN QUEENSLAND

Hedgerows of the fodder tree legume Leucaena leucocephala (Lam.) de Wit ssp. glabrata (Rose Zárate) (leucaena) planted with companion grass (leucaena-grass pasture) form a productive, profitable and sustainable tropical pasture in northern Australia. Leucaena is renowned for its longevity (>30 years) and productivity under regular grazing, and this is a key factor in its profitability. To-date graziers and researchers have not been concerned about the sustainability of commercial leucaena-grass pastures, which are rarely fertilized. However, nutrient depletion could be expected after many years of nutrient removal under heavy grazing without replenishment, even on soils of moderate initial fertility. This study investigated the long-term productivity of leucaena-grass pastures in relation to nutrient depletion in Queensland. Experimental trials were conducted at 3 research stations and 6 commercial cattle properties. Prior to conducting field trials, a postal survey of leucaena growers ascertained perceived changes in leucaena, grass and livestock productivity over time. Physical and management factors influencing long-term pasture productivity were also explored. Graziers reported that leucaena productivity had declined in 58% of aging pastures. Lower livestock productivity was associated with declining leucaena growth, even though grass growth remained vigorous. Leucaena growth decline was more frequent on soil types of marginal initial fertility. Maintenance fertilizer was not applied to most (98%) leucaena pastures surveyed despite significant amounts of nutrient removal, particularly phosphorus (P) and sulphur (S), occurring over prolonged periods of moderate to high grazing pressure. It was predicted that under current management practices large areas of commercial leucaena pasture will be affected by soil nutrient depletion over the next 10 years. The effect of age of leucaena plants on pasture productivity was investigated in pastures aged from 8 to 38 years. Leucaena growth, expressed as rainfall use efficiency (RUE), declined with age (from 4.0 to 1.9 kg total dry matter (DM)/ha/mm), as did leaf nitrogen (N), P and S concentrations. Leucaena productivity decline was attributed to P and S deficiency restricting growth and symbiotic dinitrogen (N2) fixation. Composition of interrow grass changed from native grass dominance before leucaena establishment to green panic (Panicum maximum var. trichoglume) dominance in the aging leucaena pastures, particularly adjacent to leucaena hedgerows. This was attributed to increased soil Navailability. Leucaena and grass roots were concentrated in the topsoil; however, leucaena roots did extend beyond 1 m depth while grass roots did not. Changes in topsoil organic carbon (OC) and total nitrogen (TN) resulting from the planting of leucaena hedgerows into native grass pastures and previously cropped soils were studied. Topsoil OC and TN contents increased significantly under leucaena pasture (OC from 81-290 kg/ha/year and TN from 12-24 kg/ha/yr). Since TN and OC showed similar trends, there was no significant effect on carbon:N ratios. Leucaena contributed to soil OC both directly via plant part decomposition, and indirectly, via enhanced grass growth in the inter-row. Lower topsoil OC accumulation rates (81 kg/ha/yr) were observed in the older leucaena-grass pastures related to the decline in yield and vigour of leucaena in these aging pastures. The amount of carbon dioxide equivalent (CO2-e) accumulated in soil OC in productive leucaena-grass pasture was estimated to be higher than the amount of CO2-e emitted in methane from beef production from these pastures, thus positively impacting on their greenhouse gas balance. Leucaena responses to P and/or S applications were evaluated in a 30 year-old leucaenagrass pasture. Leucaena RUE and symbiotic N2 fixation were restricted by S deficiency. Sulfur concentration in leaf tissue and high N:S ratio were useful indicators of S deficiency. Although leucaena growth and its nutritional status were little affected by P application, symbiotic N2 fixation did respond significantly to P application. Leucaena and grass responses to fertiliser applications were further evaluated at a variety of soil types and environments on 8 sites in Queensland. Increased leucaena RUE (from 3.1 to 4.6 kg total DM/ha/mm) and enhanced nutritional status at most sites showed that leucaena plants were restricted by P and/or S deficiency. The major factors contributing to the P and S deficiencies were: a) inherent low soil P and/or S fertility, b) nutrient removal by cropping and grazing, c) shallow soils, d) acid soils, and e) grass competition for available water and nutrients. Inter-row cultivation (with or without fertiliser) had little effect on leucaena growth but significantly increased grass RUE (from 4.7 to 7.0 kg total DM/ha/mm) at some sites probably due to enhanced mineralization of N. Leaf P and S concentrations were not reliable indicators of deficiencies of these nutrients, possibly due to inadequate leaf sampling conditions. The effects of ambient temperature, water stress and phenological development of plant on nutrient concentrations in leucaena leaf was investigated to determine whether leaf tissueanalysis can reliably predict nutrient deficiencies. The youngest fully expanded leaf (YFEL) was established as the most appropriate leaf tissue for predicting nutritional status of leucaena plants since the YFEL: (a) was an easily identifiable tissue in which nutrient shifts were at a minimum; (b) provided information for readily mobile (N, P and potassium) and variably mobile (S, copper and zinc) nutrients, thus simplifying leaf collection; and (c) facilitated comparison of data from leaves of similar physiological age in different growing conditions and sites. Nutrient concentrations in YFEL were significantly influenced by water stress and phenological stage of plant development (particularly flowering and pod filling) through the mechanism of rate of leaf appearance. Chronological age of the YFEL, an indicator of leaf appearance, varied from 12 to >70 days depending upon plant phenological stage, being >140 days under prolonged water stress. It was found that nutrient concentrations in leucaena YFEL can only be interpreted against critical concentrations if plants are actively growing (October-April) in a vegetative stage and YFEL are 0.7% DM should be discarded as they are likely to be too old (>20 days). The research program has identified that leucaena established on non-alluvial soils need to be provided with regular maintenance P and S fertiliser to promote symbiotic N2 fixation and to maintain high RUE. At present, many leucaena pastures are likely to be suffering undiagnosed nutrient deficiencies that will be limiting pasture and animal productivity and enterprise profitability. Youngest fully expanded leaf analysis can be used as a predictive tool to diagnose nutrient deficiencies provided the recommended protocol is followed. Further investigation is required to: a) assess the duration of responses to applied fertiliser to determine frequency of application; b) investigate the rate of maintenance fertiliser P and S that has to be applied to maintain leucaena symbiotic N2 fixation and RUE at a desired level to benefit both forage quality and quantity, and soil fertility; c) study methods of fertiliser placement for adequate and timely supply of nutrients, particularly P, to leucaena roots; and d) confirm the use of Ca concentration in YFEL as a predictor of optimum leaf age for the range of soils and areas where leucaena is grown