Editor’s note: The following information is provided by Mike Socha, research and nutritional services manager with the Zinpro Corp.; Jerry Torrison, veterinarian with the Zinpro Corp., and Neil Michael, veterinarian and technical field consultant with the Vita Plus Corp.
When dairy cattle health and performance fall below expectations, dairy consultants often begin looking at blood and tissue samples to provide a snapshot of the nutritional status of the herd.
Consultants are cautioned to not confuse results from individual diseased animals when attempting to make whole herd assessments of nutritional status. Several factors contribute to accurate assessments, including type and number of cattle sampled, sample type, mineral evaluated and sample handling.
Following are five key recommendations for pulling blood and tissue samples to assess the mineral adequacy of the diet.
- The type of tissue sampled is dependent on the mineral being analyzed (see chart below). For example, blood samples are a poor indicator of zinc and copper status because blood concentrations will not drop until these minerals are depleted.
- When sampling for mineral adequacy, at least 20 cows or 10 percent of the herd (whichever is less) should be sampled. Ideal candidates are cows 75 to 175 days in milk that are healthy and metabolically stable. Cows that are sick or stressed will not give accurate measurement of mineral status and should not be included in the sampling process.
- Collecting a clean sample is critical to the accuracy of the results. Possible sources of contamination include dirt or other foreign materials for tissue and, surprisingly, collection tubes for blood samples. For example, the red rubber stoppers on blood collection tubes typically use a lubricant containing zinc that can result in erroneously high zinc concentrations when the sample is analyzed.
- Proper handling of blood samples is crucial in obtaining a valid sample. Freezing whole blood samples increases the risk of hemolysis and results in elevated concentrations of iron, manganese, potassium, selenium and zinc in the serum. In addition, serum should be separated from the red blood cells within one to two hours of collection to avoid artificially high concentrations of potassium and zinc.
- Once a sample is accurately prepared, it is important to use the appropriate reference range to assess the mineral status. Animal parity, as well as life cycle stage of the animal, can affect the validity of the sample analysis. For instance, copper concentrations vary with age — older cows have higher concentrations than young cows — and the copper status of cows is generally lowest right around calving due to the transfer of copper from the cow to the calf. Vitamin B12, an indicator of cobalt supply, drops throughout lactation while selenium concentrations vary depending on the source of selenium in the diet, whether organic or inorganic.
Although blood and tissue samples can be an accurate measure of the mineral status of the herd, careful sampling and correct testing processes are critical. In addition, it is important to note that concentrations in tissues may not be indicative of whether or not cows will respond to additional minerals or minerals in different forms.