Investigation of urinary magnesium as an on-farm indicator of magnesium status in pasture-fed dairy herds

Abstract

This study measured parameters of magnesium (Mg) metabolism in two herds in order to assess the suitability of urinary Mg as an on-farm indicator of Mg status in pasture-fed dairy herds. Changes in serum, milk and urinary Mg concentrations were monitored fortnightly in a Mg-supplemented herd (S) and weekly in an unsupplemented (U) herd during the period animals are typically most at risk in seasonal-milk supply herds.

The Mg concentrations in the pasture consumed by both herds were similar and ranged from 1.5 to 2.2 g/kgDM and 1.6 to 1.8 g/kgDM for the S and U herds, respectively. Potassium concentrations in herbage during this period were greater than 25 g/kgDM except in late November for the U herd and above the concentration at which significant impairment of Mg absorption can be anticipated.

The mean (±SEM) serum Mg concentrations for both herds were not significantly different except during mid September to mid October, and had a similar range; 0.72 (± 0.03) to 0.96 (±0.02) mmol/l and 0.77 (±0.04) to 0.92 (±0.02) mmol/l in the S and U herds, respectively. Serum Mg concentrations from 10 of the 35 Mg-supplemented cows were below the accepted critical clinical threshold of 0.6 mmol/l on one or more sampling days compared to 4 of the 21 heifers in the unsupplemented herd.

Urinary Mg excretion was estimated from urinary Mg and creatinine concentrations, and live weight data according to the formula of Windisch et al. (1995). The estimated mean daily urinary Mg excretion was significantly (P < 0.05) different between the two herds on 4 of the 7 sampling days, and ranged from 0.55 (±0.11) to 4.80 (±0.31) g/d and 0.97 (±0.20) to 2.89 (±0.33) g/d for the S and U herds, respectively. During the same period, the range in mean urinary Mg concentration for the S herd was 0.9 (±0.21) to 5.9 (±0.55) mmol/l compared with 1.1 (±0.26) to 8.7 (±0.79) mmol/l for the U herd.

Milk yield was significantly different (P < 0.05) between the two herds throughout the trial. The herd mean milk Mg concentrations were low; 106 and 109 mg Mg/l for the S and U herds, respectively. In assessing Mg requirements of cows under New Zealand pasture-based systems, a lower mean milk Mg concentration than used in international nutrient requirement citations (120 mg Mg/l) could be appropriate.

At a herd mean urinary Mg concentration range of 2 - 4 mmol/l it is estimated that at least 5% of the herd could be at risk of hypomagnesaemia. It is suggested that at an average herd urinary Mg concentration of less than 4 mmol/l Mg supplementation is required. Under conditions when herd mean urinary Mg concentration is above 4.0 mmol/l, additional Mg supplementation is not advocated even if one animal has a urinary Mg concentration < 1.0 mmol/l, the latter probably reflecting an individual which manages Mg homeostasis at low rates of Mg absorption.

In conclusion, results from this research suggest that average herd urinary Mg concentration has the potential to be used as an on-farm indicator of herd Mg absorption, and to assess if the dietary supply of Mg is adequate in pasture-fed dairy herds.