Abstract

Two groups of 10 mature, crossbred cows received either a single daily dose of 10 g Mg or no supplemental Mg for two 5 day periods. Blood was sampled twice daily and urine once daily. On 3 days blood and urine were sampled four times. Serum Mg levels of 3 unsupplemented animals fell from 0.74 ± 0.034 to 0.25 ± 0.020 mmol/1 within 5 days while other animals within this group maintained serum Mg levels despite the absence of Mg supplementation. At any given sampling time serum Mg levels of individuals within the unsupplemented group varied from 0.22 to 0.88 mmol/1 and those of supplemented animals from 0.41 to 0.83 mmol/1. Mean serum Mg, Ca, Na and K concentrations did not differ between the groups. Unsupplemented animals had significantly (P<0.001) lower urinary fractional clearance ratios of Mg and Ca. The within and between animal variation in serum Mg levels was high for both Mg supplemented and unsupplemented animals. The fractional clearance of Mg in urine indicated a likely benefit from Mg supplementation for 100% of the unsupplemented animals while serum Mg concentrations only identified 40% of these animals. Fractional clearance of Mg in urine has potential as an indicator of animal Mg status.

DE, Dalley

Proceedings of the New Zealand Society of Animal Production, Volume 54, , 27-30, 1994