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Supplementing magnesium (Mg) to dairy cows is widely practised in New Zealand herds. Various methods are used including drenching, pasture dusting, water trough treatment and adding to hay/silage. Following hypomagnesaemia problems on a Northland farm, a capital input of 124 kg /ha of magnesium oxide (MgO) was applied in April 2000 followed by annual maintenance rates of 25-30 kg Mg/ha (all as MgO). Soil, pasture and animal samples were taken regularly over a 3 year period and in the second year, on-farm management practices were detailed along with climatic conditions. Results indicated a very good lift in soil quick test Mg (from 16 to 29) with treated pasture averaging 0.21-0.25 % Mg. In the first year blood serum Mg levels of the cows (10 sampled on 4 occasions) were all close to optimum at 0.90 mmol/l but in the second year (10 sampled on 6 occasions), there were occasions when levels were lower and in some cases at or below the critical level (0.62 mmol/l). When farm management practices and climatic conditions were documented in 2001, there were instances when other feedstuffs low in Mg were fed as a high proportion of the diet leading to less Mg intake than from treated pasture alone. Also very wet conditions from August through to November led to much pugging damage and presumably poor pasture utilisation and lowered Mg intake. Although increased soil and herbage Mg status is useful as a background for maintaining animal performance, there will be times when other Mg supplementation will be necessary. In the Northland case this occurred in the second year after capital Mg fertiliser had been applied. In separate field trials in Northland and Rotorua, kieserite (magnesium sulphate) was shown to be a quicker acting Mg fertiliser than Mg. The trials indicated that 25 kg Mg/ha as kieserite generally gave a significant short term lift in pasture Mg status. If the cost was comparable a "little and often" philosophy of Mg fertiliser application may be a better alternative to a capital plus maintenance approach allowing pasture Mg to be boosted more quickly at critical periods of the year. Keywords: dairy cows, kieserite, magnesium fertiliser, magnesium oxide, magnesium supplementation

Magnesium supplementation of the dairy herd: a case study in Northland and a comparison of two magnesium fertilisers; kieserite and magnesium oxide