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Providing the correct amounts of bioavailable trace minerals in diets is necessary for healthy, productive dairy cows. Negative impacts relative to the cow, environment, and profitability can occur when inadequate or excessive amounts of bioavailable trace minerals are fed. The 2001 Dairy NRC (Nutrient Requirements of Dairy Cattle, 7th Revised Edition) established requirements for cobalt (Co), copper (Cu), iodine (I), iron (Fe), manganese (Mn), selenium (Se), and zinc (Zn); since 2001, substantial research has been conducted regarding chromium (Cr) supplementation of dairy cow diets. The mineral requirement in most, if not all, U.S.-based nutrition models comes directly from the NRC.
The requirement for a trace mineral can be defined as the amount that must be absorbed daily to keep the cow healthy, maintain and optimize milk production, allow for efficient reproductive performance, and at the same time, maintain proper body stores of the mineral. Although this definition is widely accepted, quantifying actual requirements is extremely difficult, and substantial errors (both over- and underestimating requirements) can exist. Milk yield is often not useful in determining trace mineral requirements because it is often insensitive, at least in the short term, to extreme changes in dietary trace mineral supply. Measuring changes in body stores of trace minerals can be difficult (e.g., changes in liver copper concentrations). Quantifying dietary effects on cow health and reproduction is imprecise and usually requires a very large number of animals.
Another major area of uncertainty regarding trace mineral requirements is the bioavailability coefficients used to calculate absorbed trace minerals. Measuring the bioavailability of trace minerals is extremely difficult. Many of the values that are used were determined a long time ago using isotopes under very limited conditions. Because some of the absorption coefficients are extremely small (e.g., ~5% for several sources of Cu and 0.75% for many Mn sources), small differences in absorption coefficients can have substantial effects on the calculated dietary requirements. For example, if the actual absorption coefficient for Cu under a specific situation was 2.5 percentage units lower than the assumed 5%, the diet would need to contain twice as much Cu to provide adequate absorbed Cu. The difference between an absorption coefficient of 5% and 7.5% may not even be detectable using our current ability to measure absorption.
Lastly, several common dietary conditions can greatly influence absorption of trace minerals. For example, high dietary or water sulfur can reduce Cu and Se absorption markedly. Using the standard absorption coefficients in that situation may lead to Cu and Se deficiencies.
Because of the substantial uncertainties associated with trace mineral requirements and supply, nutritionists need to consider the costs of underfeeding versus overfeeding trace minerals when formulating diets. Underfeeding trace minerals can result in increased health problems, such as retained placenta and mastitis, poorer reproduction, and reduced milk yields. Overfeeding trace minerals can increase feed costs, increase the amount of trace minerals in manure (an environmental issue), cause excessive concentrations of minerals in animal products consumed by humans, interfere with absorption of other minerals, and result in mild to severe toxicity. Because of the potential problems associated with both under- and oversupplementation of trace minerals, most diets should not deviate greatly from NRC requirements.
Table 1. Approximate 2001 NRC requirements for lactating cows and suggested safety factors for trace minerals.
|Trace Mineral||NRC Requirement1||Safety Factor2||Comment|
|Chromium||Not established||NA||May increase milk yield in early lactation at ~0.5 ppm.|
|Cobalt||0.11 ppm||2 to 4 X||NRC recommendations may not maximize vitamin B-12 status.|
|Copper||10-12 ppm||1.2 to 3 X||1.2 X NRC should be fed to reduce the risk of deficiency because of uncertainty in supply and requirements. The safety factor must be increased as dietary (includes minerals from water) sulfur and Mo concentrations increase about 0.25% and 1 ppm, respectively. Cu should not exceed 3 X NRC.|
|Iodine||3.3 mg/100 lb BW||1 X||No new data justifying need for a safety factor.|
|Iron||15 to 18 ppm||1 to 1.2 X||No evidence that NRC level is not adequate; most basal diets contain more than adequate Fe.|
|Manganese||12 to 18 ppm||2.5 to 3.5 X||Studies have shown that NRC level is not adequate; studies suggest that 35 to 50 ppm is adequate.|
|Selenium||0.3 ppm (supplemental)||1 X||FDA regulations prohibit greater supplementation rates.|
|Zinc||43 to 50 ppm||1.2 X||1.2 X NRC should be fed to reduce the risk of deficiency because of uncertainty in supply and requirements.|
|1 Requirement assumes typical absorption coefficients and typical dry matter intakes.|
|2 Values expressed relative to NRC (2001) requirement. For example, if requirement is 12 ppm and safety factor is 1.25, diet should contain 12 * 1.25 = 15 ppm.|
Bill Weiss and Matthew Faulkner
The Ohio State University