A comparative study of diagnostic tests for the assessment of herd magnesium status in cattle.
In nine of ten pre-calving dairy herds we found that herd magnesium (Mg) status could be determined with 95% confidence by measurements of serum Mg (S-Mg), urinary Mg fractional clearance ratio (FCR) or creatinine corrected urinary Mg concentration (CUM) from 10 representative cows. Our findings support clinical impressions that FCR and CUM are more sensitive than S-Mg as predictors of a positive production response to Mg supplementation of dairy cows. The available clinical data is not adequately controlled and there is a need for controlled production-response trials to refine interpretation and to confirm the predictive value of these tests. Pending such trials we suggest that mean FCR >10% or CUM >1.0 mmol/l indicate that a positive response is likely. In seven lactating herds the act of milking was not associated with depression of S-Mg and we conclude that sampling cows immediately after milking Gould not result in spurious elevations of FCR. FCR and CUM show close correlation with each other but not with S-Mg.