Salinity, sodicity, acidity, and phytotoxic levels of chloride (Cl) in subsoils are major constraints to crop production in many soils of north-eastern Australia because they reduce the ability of crop roots to extract water and nutrients from the soil. The complex interactions and correlations among soil properties result in multi-colinearity between soil properties and crop yield that makes it difficult to determine which constraint is the major limitation. We used ridge-regression analysis to overcome colinearity to evaluate the contribution of soil factors and water supply to the variation in the yields of 5 winter crops on soils with various levels and combinations of subsoil constraints in the region. Subsoil constraints measured were soil Cl, electrical conductivity of the saturation extract (ECse), and exchangeable sodium percentage (ESP). The ridge regression procedure selected several of the variables used in a descriptive model, which included in-crop rainfall, plant-available soil water at sowing in the 0.90–1.10 m soil layer, and soil Cl in the 0.90–1.10 m soil layer, and accounted for 77–85% of the variation in the grain yields of the 5 winter crops. Inclusion of ESP of the top soil (0.0–0.10 m soil layer) marginally increased the descriptive capability of the models for bread wheat, barley and durum wheat. Subsoil Cl concentration was found to be an effective substitute for subsoil water extraction. The estimates of the critical levels of subsoil Cl for a 10% reduction in the grain yield were 492 mg cl/kg for chickpea, 662 mg Cl/kg for durum wheat, 854 mg Cl/kg for bread wheat, 980 mg Cl/kg for canola, and 1012 mg Cl/kg for barley, thus suggesting that chickpea and durum wheat were more sensitive to subsoil Cl than bread wheat, barley, and canola.

This study measured forage biomass production, diet quality, cattle liveweight gain, and economic performance of six forage types at 21 sites across 12 commercial beef cattle properties in the Fitzroy River catchment of Queensland during 2011–2014 (28 annual datasets in total). The forages were annual forage crops (oats (Avena sativa), sorghum (Sorghum spp.) and lablab (Lablab purpureus)), sown perennial legume-grass pastures (leucaena-grass (Leucaena leucocephala spp. glabrata + perennial, tropical grass (C4) species) and butterfly pea-grass (Clitoria ternatea + perennial, C4, grass species)), and perennial, C4, grass pastures. The sown forages resulted in 1.2–2.6 times the annual cattle liveweight gain per ha than perennial grass pastures. Annual cattle liveweight gain per ha, forage establishment and management costs, and cattle price margin (sale price less purchase price, $/kg liveweight) all influenced gross margin, however, none was an overriding factor. The average gross margins ($/ha.annum) calculated using contractor rates, ranked from highest to lowest, were: leucaena-grass pastures, 181; butterfly pea-grass pastures, 140; oats, 102; perennial grass, 96; sorghum, 24; and lablab, 18. It was concluded that the tendency towards greater average gross margins for perennial legume-grass pastures than for annual forage crops or perennial grass pastures was the result of the combined effects of lower average forage costs and high cattle productivity.

Productivity of grain crops grown under dryland conditions in north-eastern Australia depends on efficient use of rainfall and available soil moisture accumulated in the period preceding sowing. However, adverse subsoil conditions including high salinity, sodicity, nutrient imbalances, acidity, alkalinity, and high concentrations of chloride (Cl) and sodium (Na) in many soils of the region restrict ability of crop roots to access this stored water and nutrients. Planning for sustainable cropping systems requires identification of the most limiting constraint and understanding its interaction with other biophysical factors. We found that the primary effect of complex and variable combinations of subsoil constraints was to increase the crop lower limit (CLL), thereby reducing plant available water. Among chemical subsoil constraints, subsoil Cl concentration was a more effective indicator of reduced water extraction and reduced grain yields than either salinity or sodicity (ESP). Yield penalty due to high subsoil Cl was seasonally variable, with more in-crop rainfall (ICR) resulting in less negative impact. A conceptual model to determine realistic yield potential in the presence of subsoil Cl was developed from a significant positive linear relationship between CLL and subsoil Cl: Since grid sampling of soil to identify distribution of subsoil Cl, both spatially across landscape and within soil profile, is time-consuming and expensive, we found that electromagnetic induction, coupled with yield mapping and remote sensing of vegetation offers potential to rapidly identify possible subsoil Cl at paddock or farm scale. Plant species and cultivars were evaluated for their adaptations to subsoil Cl. Among winter crops, barley and triticale, followed by bread wheat, were more tolerant of high subsoil Cl concentrations than durum wheat. Chickpea and field pea showed a large decrease in yield with increasing subsoil Cl concentrations and were most sensitive of the crops tested. Cultivars of different winter crops showed minor differences in sensitivity to increasing subsoil Cl concentrations. Water extraction potential of oilseed crops was less affected than cereals with increasing levels of subsoil Cl concentrations. Among summer crops, water extraction potential of millet, mungbean, and sesame appears to be more sensitive to subsoil Cl than that of sorghum and maize; however, the differences were significant only to 0.7 m. Among pasture legumes, lucerne was more tolerant to high subsoil Cl concentrations than the others studied. Surface applied gypsum significantly improved wheat grain yield on soils with ESP >6 in surface soil (0–0.10 m). Subsurface applied gypsum at 0.20–0.30 m depth did not affect grain yield in the first year of application; however, there was a significant increase in grain yield in following years. Better subsoil P and Zn partially alleviated negative impact of high subsoil Cl. Potential savings from improved N fertilisation decisions for paddocks with high subsoil Cl are estimated at ~$AU10 million per annum.

Keynote paper presented at the International Leucaena Conference, 1‒3 November 2018, Brisbane, Queensland, Australia.Leucaena (Leucaena leucocephala ssp. glabrata) is a highly palatable and productive forage used mainly by beef producers on extensive properties in northern Australia. When sown into native or sown grass pastures, leucaena provides significant production, economic, environmental and social benefits. Adoption of leucaena was slow initially due to a range of technical, agronomic and landscape factors. These have now been largely overcome through extensive research, development, producer experience and other advances, resulting in around 130,000 ha of cultivated leucaena being utilized across northern Australia.A range of aspects will need to be addressed if the adoption of leucaena is to be accelerated into the future. These include environmental concerns, especially potential weediness, and a range of technological needs, including soil nutritional requirements, grazing and toxicity management, opportunities for companion fodder systems and conservation options. Advances in technology and the ongoing need for a high-quality, profitable and sustainable perennial forage will ensure the continued adoption of leucaena across northern Australia for the foreseeable future.

Keynote paper presented at the International Leucaena Conference, 1‒3 November 2018, Brisbane, Queensland, Australia.Leucaena (Leucaena leucocephala ssp. glabrata) is a highly productive tropical perennial legume used primarily in extensive beef grazing systems across northern Australia. Its productivity provides substantial benefits to grazing businesses and economically significant areas of leucaena have been established in Queensland, with much smaller areas in both the Northern Territory and Western Australia. Specific environmental conditions (particularly soil type) and management practices are required to obtain reliable establishment and high productivity from leucaena-grass grazing systems. Significant research, development and extension have been undertaken in northern Australia, particularly in central Queensland, resulting in management packages which ensure establishment reliability and long-term productivity. However expansion into new areas can be constrained by regionally specific establishment issues. Adaptation of known establishment and management practices together with research and development are required for leucaena-grass grazing systems in new regions.