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Trace Tissue Testing

By Kathy Winters, Red Horse Valley, LLC

Successful balancing of diets for our captive caprines is not necessarily an easy task! Forages and hay vary in quality and nutrient content with the season, weather, fertilization practices and even the time of harvest. With the largest component of our herds’ diet being so variable, we often resort to adding concentrates and supplements to ensure minimum daily requirements are met.

Unfortunately, it is not simple on many different levels. Needs vary with life-stage. The absolute amount required of any given nutrient may depend on the amount of another nutrient in the diet. For instance, the “optimal” ratio of zinc to copper is 3:1 to 4:1 under normal conditions, depending on whether the diet is hay vs fresh herbage. Additionally, the maximal tolerated daily dose of copper in the diet of goats is currently unknown (NRC 2005, 2007) and likely varies with goat type and breed. “Zinc, iron, cadmium, and molybdenum in diets or supplements all interact with copper metabolically. For this reason, it is impossible to give a maximum or minimum tolerable dietary copper level based on copper content alone. The level of protein and the level of the interacting metal ion and sulfate all can influence the absorption and utilization of copper. Consequently, any statement about copper levels in the diet as related to toxicity has to be made in the context of the presence of these other substances in the diet.” George K Davis and Walter Mertz. Trace Elements in Human and Animal Nutrition, 5th Edition. 1987.

So what is a producer to do? Well, in my experience, we just have to do the best we can, perhaps consulting small ruminant nutritionists, and then monitor the results with tissue testing and adjust as needed. Some information may be gleaned from serum trace mineral testing (be sure to collect into navy blue top collection tubes if zinc is of interest), however the body is very good at homeostatic mechanisms to keep levels in a fairly tight range even when other tissues may be deficient or toxic. The gold standard is liver trace mineral testing, and this is an option even in the living animal!

Current technology allows for a very, very small amount of tissue to be assayed for a panel of several trace elements. Michigan State Veterinary Diagnostic Laboratory only requests 50 mg of tissue from liver biopsy or necropsy. Age of the animal is requested at submission, as the reference ranges are species-specific and age-dependent since concentrations are reported as ppm on a dry weight basis.

There are many diagnostic labs across the nation. Some allow producers to directly submit samples while others will only take samples submitted through your veterinarian. Given that veterinary time is expensive, I have personally set up accounts with MSU-VDL and TVMDL for direct submission. Note that your veterinarian may charge you a consultation fee to go over the results with you if you don’t know what to do with the results you receive directly. If they don’t, it would be nice if you offered anyway, IMHO.

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