New Zealand Journal of Agricultural Research abstracts
The magnesium requirements of pastures in New Zealand: A review
D. C. Edmeades
agKnowledge Ltd
P.O. Box 914
Hamilton, New Zealand
Abstract Magnesium (Mg) is an essential nutrient for plants and animals, and hypomagnesaemia, a disorder associated with low blood Mg in ruminants, is a major problem in the New Zealand pastoral industry. This paper reviews the research conducted in New Zealand in the last 30 years to develop diagnostic criteria to predict, and strategies to manage, soil, plant, and animal Mg. The Mg status of New Zealand soils is examined and it is concluded that most New Zealand topsoils have significant reserves of Mg, at least for optimal pasture production, for the foreseeable future. There is evidence, however, that soil Mg levels are slowly declining because, in the absence of fertiliser Mg inputs, most pastoral farms are in a negative balance with respect to Mg. The soils most vulnerable to developing Mg deficiency are the coarser textured soils used for dairying and under high rainfall (>1200 mm). Extreme deficiency resulting in the loss of production in legume-based pastures occurs if the soil Mg concentration is < 4-5 Quick Test Mg (QT Mg) units and the mixed herbage Mg concentration is < 0.10%. Achieving and maintaining soils at > QT Mg 8-10 ensures that the probability of Mg deficiency in respect to pasture production is small. The evidence shows that even the most extreme deficiencies can be eliminated with inputs of about 25 kg Mg ha-1 yr-1, and balance studies indicate that soil Mg levels can be maintained with inputs of between 5 and 20 kg Mg ha-1 yr-1, depending particularly on the type of operation (dairying or sheep and beef) and the leaching regime of the soil. The factors that affect soil Mg levels, plant uptake and pasture concentrations, such as inputs of potassium and lime, season, and plant species are discussed. Overcoming Mg deficiency in pastures does not, however, eliminate the risk of hypomagnesaemia in lactating ruminants. The evidence indicates that feed intake is the most important determinant of animal Mg status and milk production, but other factors can affect the ability of the animal to utilise Mg. Assuming an adequate feed intake, animal Mg requirements during early lactation can only be met if the pasture Mg concentration is > 0.20%. To achieve such concentration in spring requires that the soil Mg level is QT Mg 25-30 or above. On those soils in New Zealand with low soil Mg status such levels can only be achieved with large capital inputs of fertiliser Mg (>100 kg Mg ha-1). Strategies for managing pasture and animal Mg requirements are discussed in relation to the agronomic effectiveness of Mg fertilisers. The importance of Mg in the long-term sustainability of the New Zealand pastoral system is emphasised and weaknesses in current knowledge are identified and highlighted for future research.
Keywords animals; fertiliser; hypomagnesaemia; magnesium; nutrient budgeting; pastures; soil fertility; sustainability
A03010; Received 20 February 2003; accepted 27 February 2004; Online publication date 17 August 2004
New Zealand Journal of Agricultural Research, 2004, Vol. 47: 363-380
0028-8233/04/4703-0363 © The Royal Society of New Zealand 2004
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