Everything, including required nutrients, is toxic when consumed in great enough quantities. Signs of toxicity range from the mild (slightly reduced milk yields) to the most extreme (death). The maximum tolerable level (MTL) is the highest "dose" of something that does not cause any adverse effects on an animal. Dose can be defined as quantity consumed over time or per unit of body weight (e.g., grams/day or grams/lb) or concentration in the diet or drinking water (e.g., % or ppm). In 2005, the National Research Council published a book that summarized the scientific literature on mineral tolerances of animals. The book includes information on 39 minerals (although information for some is extremely limited) that could be consumed by animals via a "normal diet," a contaminated diet, and/or drinking water. The committee who wrote the report established MTL for those 39 minerals (plus nitrate) based on changes in production, intake, and adverse health effects. Some of the MTLs that are of practical importance include copper at 40 ppm (all values are on a dietary dry matter basis), selenium at 5 ppm, sulfur at 0.3 to 0.6%, and sodium chloride (salt) at 3%. However, numerous factors influence the MTL, and these MTLs should only be considered as guidelines. This paper will discuss the MTLs of some of the minerals that are of practical significance in the tri-state area (Michigan, Indiana, and Ohio) and provide an overview of some of the more important points in the publication.
Excessive consumption of a mineral or minerals is not uncommon. A cow can consume too much mineral because of errors in feed mixing at the farm or mill (for example, 100 lb of mineral premix added to the TMR instead of 10 lb), the use of feedstuffs that have high concentrations of a mineral (for example, some distillers grains can have very high concentrations of sulfur), overzealous supplementation of minerals, and contaminated drinking water. For most minerals, slight or even moderate overconsumption (relative to requirement) does not cause any adverse effects on the animal. As overconsumption becomes greater and perhaps of longer duration, cows can start showing signs of toxicity. Adverse effects to excessive consumption of minerals include:
The NRC subcommittee who wrote the mineral tolerance book was charged with reviewing the scientific literature on the effects of overconsumption of minerals on animals and to establish MTLs for as many minerals as possible and for different species or classes of animals. The book is 496 pages long, and this paper will not attempt to review the entire book. Mixing errors usually result in excessive consumption for only a short period of time (perhaps only one feeding or one day), and the adverse effects, if any, will be acute; unless the effects are severe, they will likely not be ascribed to excessive mineral intake. Therefore, this paper will emphasize chronic exposure caused by longer-term (weeks) consumption of diets and/or water with high concentrations of the mineral. In addition, this paper will discuss only minerals in which overconsumption is likely and which cause significant problems to cattle (Table 1).
The maximum tolerable level (MTL) of a mineral is defined as the maximum dietary level of a specific mineral that will not cause any adverse effects (including reduced productivity and health) when fed for a specific period of time. Level is a purposely ambiguous term that can mean concentration in diet or water (e.g., %, ppm, or g/L) or intake (e.g., grams/day). Numerous factors affect MTL, and, because of the almost infinite number of possible combinations, MTL cannot be established for all situations. Readers of the book must be aware of these factors and make educated estimates of what the MTL might be for specific situations based on available data.
The major factors that affect the dietary MTL include:
The MTL does not include a "safety factor." The measured concentrations of many of the minerals of interest (especially trace minerals) in diets are extremely variable because of analytical problems, sampling errors, and natural variation. This means that the average concentration of a mineral may be substantially different from the value obtained from a single sample or even the mean of a few samples. Because of this uncertainty, the MTL may be too high for minerals that cause substantial adverse effects on productivity and animal health.
The NRC established MTLs for numerous minerals (Table 1), but dairy cows in the tri-state area are extremely unlikely to be exposed to excessive levels of 14 of those, and those minerals will not be discussed. Of the remaining 14 minerals, four (cobalt, iodine, manganese, and magnesium) are extremely unlikely to cause any significant adverse effects. The MTL for cobalt, iodine, manganese, and magnesium are about 200, 100, 80, and three times greater than NRC (2001) requirement for dairy cows. The MTL for magnesium (0.6% of diet dry matter) is lower than concentrations shown experimentally to cause adverse effects (1 to 1.5%), but because no data were available between 0.6 and 1%, the MTL was set at 0.6%. The most likely adverse effect to feeding Mg at rates slightly above the MTL is diarrhea.
|Mineral||Health Concern2||MTL3||Mineral||Health Concern2||MTL3|
|Excessive exposure possible||Excessive exposure is rare|
|Calcium (Ca)||Medium||1.5%||Aluminum (Al)||Low||1000 ppm|
|Cobalt (Co)||Low||25 ppm||Boron (B)||Medium||150 ppm|
|Copper (Cu)||High||40 ppm||Bromine (Br)||Medium||200 ppm|
|Iodine (I)||Low||50 ppm||Cadmium (Cd)||High||10 ppm|
|Iron (Fe)||Medium||500 ppm||Chromium (Cr)||Low||100 ppm|
|Magnesium (Mg)||Low||0.6%||Lead (Pb)||High||100 ppm|
|Manganese (Mn)||Low||2000 ppm||Lithium (Li)||Low||25 ppm|
|Molybdenum (Mo)||High||5 ppm||Mercury (Hg)||High||2 ppm|
|Phosphorus (P)||Medium||0.7%||Nickel (Ni)||Low||100 ppm|
|Potassium (K)||Medium||2%||Silicon (Si)||Low||0.2%|
|Selenium (Se)||High||5 ppm||Strontium (Sr)||Low||2000 ppm|
|Salt (NaCl)||High||3.0%||Tin (Sn)||Low||100 ppm|
|Sulfur (S)||High||0.4%||Tungsten (W)||Low||20 ppm|
|Zinc (Zn)||Medium||500 ppm||Vanadium (V)||Low||500 ppm|
1The NRC established MTL for antimony, barium, bismuth, rare earth elements, rubidium, silver, titanium, and uranium for some species of animals but not cattle.
2Concern considers both the likelihood of a toxic exposure (including accidental) and severity of animal response.
3The MTLs are for cattle and are on a dry matter basis. Numerous factors affect MTLs, including bioavailability of mineral, duration of exposure, animal factors, and water concentrations. Data in this table should not be the sole source of information; readers should consult the appropriate section of NRC (2005).
Five minerals (calcium, iron, phosphorus, potassium, and zinc) cause moderately adverse effects when overfed for longer periods of time.
The MTL for calcium is 1.5% of dietary DM (approximately two times the NRC requirement for dairy cows). The only adverse effect reported at that concentration is slightly reduced feed intake. As the calcium concentration increases, a greater depression in intake is expected, which should result in reduced milk yields.
The MTL for iron is 500 ppm, which is about 10 times the NRC requirement. The most likely adverse effect of feeding more than 500 ppm of Fe is a secondary copper deficiency and perhaps a secondary deficiency of selenium and vitamin E. The MTL for Fe is probably lower than 500 ppm when high Fe is fed for long periods of time and if dietary copper is marginal (the high Fe causes depletion of liver copper, eventually leading to copper deficiency). The chemical form of the Fe must be considered when evaluating Fe. Silages and hays can have extremely high concentrations of Fe (> 500 ppm), but this Fe is almost always caused by soil contamination and is likely Fe oxide which is almost unavailable to animals and will cause few, if any, adverse effects. Iron from sources such as Fe sulfate and Fe chloride are readily available, and the MTL was established from experiments in which those types of Fe were fed.
The MTL for phosphorus is 0.7%, or approximately twice the NRC requirement. Adverse effects of feeding P at concentrations slightly higher than the MTL include reduced magnesium absorption, and in some studies, slightly reduced milk yields. Overall, adverse effects on the cow are unlikely when P is fed at rates slightly higher than the MTL. Phosphorus pollution caused by overfeeding P was considered by the committee when it established the MTL for P.
The MTL for potassium was set at 2%, or approximately twice the NRC requirement. The main reason for setting the MTL at 2% was because of the negative effect that high K diets have on magnesium absorption and because high K diets increase the prevalence of milk fever. Diets with more than 2% K can be fed to lactating cows without adverse effects if magnesium supplementation is increased.
The MTL for zinc was set at 500 ppm, or about 10 times the NRC requirement. High concentrations of dietary Zn can lead to a secondary copper deficiency, and rates of gain in cattle were reduced when diets contained greater than 500 ppm of Zn.
Overconsumption for longer periods of time of copper, molybdenum, selenium, salt (sodium chloride), and sulfur can cause substantial health problems in cattle, and diets should contain less than the MTL for those minerals.
The MTL for copper is 40 ppm, or about four times the NRC requirement. Chronic consumption of excessive amounts of Cu causes Cu to accumulate in the liver. Copper can continue to accumulate over months or even years until it reaches a critical concentration (approximately 1000 mg/kg of liver dry matter). Then a triggering event (unclear what this actually is) causes the liver to release massive amounts of Cu which causes breakdown of red blood cells and other cells and usually results in death. Long-term feeding of diets with 40 ppm of Cu are potentially fatal, but it requires months of overfeeding. Certain breeds of cattle are more likely to accumulate liver copper (Jersey cows accumulate more liver Cu than Holstein cows). The MTL for Cu will be higher for diets with high concentrations of sulfur and molybdenum. The form of Cu also must be considered. Copper oxide is essentially unavailable to cows, and diets can have very high concentrations of Cu from that source. The MTL was established from studies using available sources of Cu, such as Cu sulfate and Cu chloride.
The MTL for molybdenum was set at 5 ppm (no dietary requirement for Mo has been established). Although the NRC set the health concern for excessive Mo intake as "high," in reality, this MTL was set because of the negative effects that Mo has on copper absorption which often leads to a copper deficiency. True Mo toxicity probably occurs at dietary concentrations around 100 ppm. Copper deficiency caused by excessive consumption of Mo is not uncommon in the field, and the adverse effects of a copper deficiency can be substantial. However, in most situations, simply increasing copper supplementation will eliminate the negative effects of feeding diets with high concentrations of Mo.
The MTL for selenium was set at 5 ppm, which is about 17 times its requirement. Selenium can be extremely toxic to animals, often leading to death within hours of consuming a toxic dose. Chronic exposure to much lower dietary concentrations of Se also can lead to death (blind staggers). The problem with the original research claiming extreme toxicity of selenium was that the actual toxicity was not caused by selenium per se but rather by very specific selenium compounds found only in selenium accumulator plants (these plants are extremely rare to nonexistent in the tri-state area). The toxicity of selenium is much lower when Se is provided by sodium selenite, sodium selenate, and selenium yeast. When those sources of Se are fed to cattle, Se concentrations greater than 10 ppm have not caused adverse production or health effects in most studies. The MTL was set at 5 ppm mostly because of data from animals consuming Se accumulator plants.
Salt (Sodium Chloride)
The MTL for salt was set at 3% of dietary DM. Based on the sodium requirement of cows, this is about five times its requirement. Diets with high concentrations of salt can substantially limit feed intake, and when dietary concentrations of salt exceed the MTL, feed intake by dairy cows is often reduced, leading to reduced milk production. However, if low-salinity water is readily available, cows can tolerate higher dietary concentrations of salt with little adverse effects. Cattle are much less able to tolerate saline water. Water with 0.25% salt caused significant reductions in milk yields.
The MTL for sulfur was set at 0.3% for high-concentrate diets and 0.6% for high-forage diets. The requirement for dietary sulfur is 0.2%. High dietary concentration of sulfur can cause a secondary deficiency of copper and selenium, but the primary reason for the health concerns is polioencephalomalacia (PEM). In the rumen, several different forms of dietary sulfur can be converted to sulfide. When hydrogen sulfide is belched out, some is inhaled and absorbed through the lungs. Hydrogen sulfide has severe adverse effects on the central nervous system, causing seizures, convulsions, and death. In some cases (but not commonly), sulfur can lead to destruction of thiamine, producing a thiamine deficiency which causes similar clinical signs. High concentrations of sulfur are more likely to cause PEM when fed in a high-concentrate diet because the bacterial population in the rumen is more likely to produce sulfide than when a high-forage diet is fed.
The 2005 NRC book on mineral tolerances is an important reference for animal nutritionists. It contains several tables listing responses various animals have had to high dietary concentrations of numerous minerals. The book also points out the large gaps in knowledge we have regarding overfeeding of minerals. An overriding conclusion that can be drawn from the book is that all minerals can cause adverse responses, and for some minerals, the difference between adequate and deleterious concentrations is not large. For minerals, as for all nutrients, feed enough but not too much, and twice as much does not mean twice as good. Ordering information for this book can be found at the National Academies Press.
William P. Weiss
Department of Animal Sciences
Ohio Agricultural Research and Development Center
The Ohio State University
National Research Council. 2001. Nutrient Requirements of Dairy Cattle. 7th rev. ed. Natl. Acad. Press, Washington, D.C.
National Research Council. 2005. Mineral Tolerance of Animals. 2nd rev. ed. Natl. Acad. Press, Washington, D.C.