Micronutrients limiting pasture production in Australia

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Abstract
Low levels of plant-available micronutrients were an inherent feature of many agricultural soils in Australia, mostly due to the prevalence of highly weathered soil parent materials. The diagnosis and correction of the widespread deficiencies of micronutrients, especially copper (Cu), molybdenum (Mo) and zinc (Zn), were prerequisites for the development of productive, legume-based pastures in southern Australia. In subtropical and tropical regions, Mo deficiency commonly limited pasture-legume production. Soil treatments involving micronutrient fertiliser incorporated in soils, or applied as additives to superphosphate, were generally effective in alleviating micronutrient deficiencies. In the low-output dryland pasture systems, the annual removal of micronutrients in wool and meat is small compared with rates added in fertiliser. Hence, in general, the residues of soil-applied micronutrient fertilisers remain effective for many years, for example, up to 30 years for Cu. By contrast, shorter residual values occur for manganese (Mn) fertiliser on highly calcareous soils, and for Zn in high-output pasture systems such as intensive dairy production. In the last two decades since the recommendations for micronutrient management of pastures were developed, there have been many changes to farming systems, with likely implications for micronutrient status in pastures. First, increased cropping intensity and low prices for wool and meat have meant lower nutrient inputs to pastures or to the pasture phase of rotations with crops. However, when pastures have been rotated with crops, ongoing small additions of Cu, Zn and Mo have been common. In cropping phases of farming systems, lime application and no-till may have altered the chemical and positional availability of micronutrients in soils to pastures. However, there has been little study of the impacts of these farming-systems changes on micronutrient status of pastures or profitability of the production system. The intensification of dairy production systems may also have altered the demand for, and removal rates of, micronutrients. Soil tests are not very reliable for Mn or Mo deficiencies, and well-calibrated soil tests for boron, Cu and Zn have been developed only for limited areas of pasture production and for a limited range of species. There is limited use of plant tests for nutrient management of pastures. In conclusion, there is limited knowledge of the current micronutrient status of pastures and their effects on animal health. Pasture production would benefit from targeted investigation of micronutrients status of pasture soils, pasture plants and micronutrient-linked animal-health issues.

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