Salt and Trace Minerals for Livestock, Poultry and Other Animals
EXCESS MOLYBDENUM
Excess molybdenum causes a copper deficiency and results in extreme diarrhea, weight loss, decreased productivity, graying of hair in cattle, pigment formation, bone and joint disorders, problems with reproduction and heart function, loss of crimp in the wool, anemia, and many other problems with ruminants. Molybdenum is an essential nutrient because it is a constituent of the enzyme xanthine oxidase and other enzymes.
Molybdenum has been shown to be essential for lambs, chicks, and turkey poults fed highly purified diets. Molybdenum does not need to be added to practical diets yet, although that may be a possibility sometime in the future. In a few small, isolated areas that are very low in molybdenum, supplementary molybdenum in the diet may prevent copper toxicity in sheep. At this time, however, the Food and Drug Administration does not recognize molybdenum use as safe, and current regulations prohibit adding it to feed for sheep unless prescribed by a veterinarian for therapeutic purposes (155). Molybdenum is well known for its toxicity in areas of known excess molybdenum that occur in certain locations in the United States (California, Nevada, Oregon, Hawaii, Utah, Montana, Colorado and Florida) and in other countries of the world (England, New Zealand, Canada, Ireland, Argentina, Peru, Cuba and others) (128, 147).
There is considerable difference among animals in the tolerance of high levels of molybdenum. Cattle have the least tolerance, followed by sheep. Horses have failed to show any signs of toxicity in pastures that severely affect cattle. One study, however, showed that the presence of 5 to 25 ppm molybdenum in forages caused some disturbance in copper utilization in horses (98). Chick growth is inhibited at 200 ppm and turkey poult growth is depressed at 300 ppm molybdenum in the diet. Swine are the most tolerant to high levels of molybdenum. Pigs have been fed diets containing 1,000 ppm molybdenum for three months with no harmful effects (92).
Tolerance of non-ruminant animals to molybdenum is much higher than that of ruminant species. Because of this high tolerance, little research has been conducted to determine whether excess molybdenum might cause problems with non-ruminants under certain conditions. Tolerance to high levels of molybdenum varies with the age of the animal, quantity and form of ingested molybdenum, inorganic sulfate content in the diet and copper status. Molybdenum tolerance also varies with intake of methionine, cystine, and protein, which are capable of being oxidized to sulfate in the body. The level of sulfate in the diet can alter the absorption, retention, and excretion of absorbed molybdenum. The effect of excess molybdenum is a deficiency of copper, although there are many interrelationships involved that make it a complex matter. As discussed previously, the copper-molybdenum-sulfate interrelationship is very important, but other nutrients also can have a modifying influence on this interrelationship.
Under normal conditions, the liver contains 2 to 4 ppm molybdenum. This level can increase to 25 to 30 ppm with excess molybdenum intake. The level of molybdenum in the liver returns to normal when excess molybdenum intake ceases. If the excess molybdenum intake is prolonged, a depletion of tissue copper levels occurs and a copper deficiency results. High sulfate intake helps counteract the effect of high molybdenum levels in forage. The degree of molybdenum absorption and retention in the body tissues is decreased by the sulfates. This means that if the copper intake of the diet is about normal, sulfates can counteract a slight excess of molybdenum.
Molybdenum from dried forage may be less available than from green forage, because forages that interfere with copper metabolism when grazed do not interfere when fed as dry forage (157). The normal level of molybdenum in forages is 3 to 5 ppm. In areas of excess molybdenum, forages may contain from 20 to 100 ppm. Copper deficiency may occur when forages have copper levels below 5 ppm, and molybdenum levels above 3 to 5 ppm. Excess molybdenum levels in forages can usually be counteracted by increasing the level of copper in the total diet to two or three times normal. Sometimes even higher copper levels are needed, depending on the many interrelationships of copper-molybdenum-sulfates and other nutrients. To further complicate the matter, it is known that alkaline soils increase molybdenum availability but decrease copper availability to plant life. Therefore, alkaline soils increase the severity of the problem in excess molybdenum areas. Soil wetness in poorly drained soils may also increase the molybdenum level in the forages produced there (85, 91, 92, 93, 94, 96, 98).
In most excess molybdenum areas, a mineral mixture with two to three times the normal copper level is fed. In a few areas, a copper compound is injected. Livestock owners should follow the recommendations of a nutritionist or a veterinarian familiar with the excess molybdenum area in which they live. The recommendations made in each area may differ somewhat to meet the specific local excess molybdenum problem.
©Copyright 1974, 1979, 1983, 1987, 1993, 1999, 2001, 2006
