Lab Tests to assess reliably the Mineral Status of Cattle & Sheep:

Mineral Tests on Milk Samples & Bulk-Blood Samples are not as reliable as tests on 7-10 Individual Blood Samples
 

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BLOOD is the standard tissue used internationally for accurate routine assessment of the mineral status of cattle & sheep. For milk to replace blood as the standard sample for this purpose, levels of the relevant minerals in blood & milk should correlate very highly; unfortunately, in most cases, the correlation for a given mineral are very poor. One exception to this is the Milk Iodine (MI) test.

Veterinary practitioners are responsible for the diagnosis of the cause(s) of animal disease. Blood testing is a valuable diagnostic support service. As the signs shown by affected animals can range from clinical to sub-clinical, the testing of blood samples can be used to verify the clinical diagnosis or to monitor the effectiveness of control measures.

To reduce / avoid losses in cases where imbalances in nutrients & minerals affect livestock, problems need to be detected quickly, diagnosed accurately & corrected effectively. Modern farming practices place increasing demands for production on animals. Highly productive animals are under heavy metabolic stress. Consequently, the requirements for nutrients & minerals in animal diets have increased. 

Blood Iodine tests: We examined many blood tests in earlier attempts to assess iodine-nutrition of cattle: total blood I (TOTI), protein-bound I (PBI) & thyroid hormones (T3, T4). We abandoned ALL of those tests in the late 1970s. They were not (and still are not) reliable to assess iodine status of ruminants. A highly accurate test of iodine status was developed in the mid 1980s - the plasma inorganic iodine (PII) test.

To assess iodine nutrition of cattle & sheep accurately, we advise assay of PII levels in 7-10 INDIVIDUAL blood samples. The PII test is more reliable than the MI test, but is time-consuming, labour-intensive & more expensive to do. Unfortunately, no lab in the State des the PII test now. The Veterinary Research Laboratory in Stormont (Belfast) used to run PII tests for vets in the Republic, but have discontinued that service because they could not cope with the numbers of samples.

Because the PII test is not available any more, veterinary diagnostic labs in the Republic offer only the MI, T3 and T4 tests. Consequently, because the wrong tests are being used, genuine outbreaks of iodine deficiency are being missed. Also, in herds with high inputs of iodine supplements, use of the wrong tests (MI, T3, T4, etc) is diagnosing iodine deficiency when such deficiency could not exist. The result is that feeds and supplements with a very generous iodine content are being wrongly blamed as providing inadequate iodine for the cattle.

In summary, analysis of 7-10 INDIVIDUAL BLOOD samples provides the best practical assessment of the mineral status of a herd or flock.

If one needs an ACCURATE assessment of iodine status, one should test 7-10 INDIVIDUAL samples from the problem group for PII. The T3, T4 and MI tests are NOT recommended for accurate nutritional assessment.

Milk Iodine (MI) Tests: These tests must be done in specialised labs on milk samples free of iodine contamination. The tests must be interpreted with caution. An MI value <50 ug/L may indicate a low iodine intake. Iodine-contamination of milk is common because many farmers use iodine-rich teat-dips & disinfectants. Thus, an MI value >50 ug/L can arise in cows whose bloods are severely low in iodine.

Milk tests can NOT adequately replace blood tests for routine & accurate assessment of mineral nutrition of cattle & sheep. Bulk milk is easy to obtain; it can be analysed for multiple components. Analysis of bulk-milk can help to assess protein & fat levels, urea, ketones, antibodies, mastitis risk, etc. Some commercial Labs offer analysis of milk samples as an aid to investigation of mineral deficiencies in cattle & sheep. However, milk tests for nutritional major elements (such as Ca, Mg, Na & P), or trace elements (such as Cu, Se or I etc) are unreliable to assess the animals' status of these nutritional elements.

In 1997, Rogers did an exhaustive search of published veterinary literature (including Focus-on-Veterinary-Medicine) & online databases (including Medline). He also e-mailed several thousand members of the American Association of Veterinary Laboratory Diagnosticians, Beef List, Dairy List & colleagues in State Veterinary Diagnostic Labs abroad. He failed to find reliable scientific evidence that milk (whether from individual cows, or from the bulk-tank) can be used to assess accurately the mineral status of cattle & sheep.

Thus, in line with the advice from State Veterinary Labs abroad, we do not recommend mineral tests on bulk samples of blood or milk for that purpose.

The MI test has no relevance to problems in calves, drystock, or cows in late pregnancy; such animals do not produce milk! In lactating cows, the MI test must be used with caution because iodine contamination occurs easily if there are iodine-containing compounds in the dairy. MI simply is NOT as reliable as PII to assess iodine nutrition of cows.

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Phil Rogers MRCVS, TAHIP Developer. Copyright 2005-2008. All Rights Reserved. Updated: 05 Dec 2005