In formulating supplements for cows grazing winter forage, two often overlooked nutrients are minerals and vitamins. They have less impact than protein and energy on cow/calf performance and economics, but they should not be overlooked (Corah, 1990). While supplementation is important, over supplementation of minerals should be avoided to prevent possible environmental problems associated with runoff from waste or application of cattle waste to soil. Certain minerals can actually be toxic if supplemented in excessive amounts.
| Table 18. Mineral Requirements of Beef Cattle. | ||||
|---|---|---|---|---|
| Requirement | ||||
| Growing Cattle | Pregnant Cows Lactating Cows | Maximum Level | ||
| Calcium, % | 0.45 | 0.3 | 0.45 | 2 |
| Phosphorus | 0.3 | 0.2 | 0.3 | 1 |
| Magnesium | 0.1 | 0.12 | 0.2 | 0.4 |
| Potassium, % | 0.6 | 0.6 | 0.7 | 3 |
| Sodium, % | 0.08 | 0.08 | 0.1 | - |
| Sulfur, % | 0.15 | 0.15 | 0.15 | 0.4 |
| Iron, PPM | 50 | 50 | 50 | 1000 |
| Manganese, PPM | 20 | 40 | 40 | 1000 |
| Zinc, PPM | 30 | 30 | 40 | 500 |
| Copper, PPM | 10 | 10 | 10 | 100 |
| Iodine, PPM | 0.5 | 0.5 | 0.5 | 50 |
| Selenium, PPM | 0.2 | 0.2 | 0.2 | 2 |
| Cobalt, PPM | 0.1 | 0.1 | 0.1 | 10 |
| Molybdenum, PPM | - | - | - | 5 |
Forages do not contain adequate amounts of salt (sodium). Sodium can be supplemented as sodium chloride or sodium bicarbonate, and both forms are highly available. Iodized salt should always be used to avoid an iodine deficiency. Cattle fed maintenance rations while confined in drylot often consume high levels of mineral mixtures, perhaps from boredom (Sewell, 1990).
Calcium is the most abundant mineral in the body. Vitamin D is required for active absorption of calcium. Forages are generally good sources of calcium, and legumes are higher in calcium content than grasses. Alfalfa has relatively high levels of calcium but 20 to 33% is unavailable to the animal.
Drought conditions and increased forage maturity (e.g., stockpiled forage) can result in low forage-phosphorus concentrations. Phosphorus supplementation becomes far more critical in cases of winter grazing than feeding hay.
Reproductive problems are common if phosphorus is deficient. Plasma phosphorus concentrations consistently below 4.5 mg/dL are indicative of a deficiency, but bone phosphorous is a more sensitive measure of phosphorus status. The ideal diet calcium:phosphorus ratio is 2:1 but ratios of 7:1 are acceptable.
Cows around calving time should have free choice access to 10-12% phosphorus mineral. An example would be one-half salt and one-half dicalcium phosphate. Cows at other times of the year and stockers would need a mineral consisting of 25-35% dicalcium phosphate or 7-8% phosphorus. Varying the phosphorus level is one means of saving money.
Potassium levels of 0.6 to 0.8 percent of ration dry matter are considered adequate for cattle. In general, potassium levels of Ohio forages are adequate to excessive in potassium content. These high levels can be associated with reducing magnesium absorption and thus causing grass tetany problems. Therefore, always check potassium levels before any supplemental additions.
Low concentrations of potassium have been observed in stockpiled fescue during the winter (Clanton, 1980). Leaching during the winter may cause potassium levels in fescue pasture to drop as low as 0.24-0.3 percent of dry matter during winter months (Sewell, 1990). Potassium can be supplemented to cattle diets as potassium chloride, potassium bicarbonate, potassium sulfate, or potassium carbonate. All forms are readily available. If potassium is added for winter feeding, remove it from the mixture when fescue starts growing in the spring. Growing forages are usually high in potassium.
Grass tetany is most common in lactating cows grazing lush spring pastures. During the early spring, climatic and soil conditions are cool and wet; plants will not contain adequate levels of phosphorus or magnesium. While both these minerals may be in adequate amounts in the soil, plant uptake is slow due to the cool, wet conditions. Fertilizing pastures with nitrogen and potassium is associated with increased incidence of grass tetany. Cows depend on a frequent supply of magnesium from the feed since mobilization of magnesium from the bone is not very efficient.
Kemp and t' Hart (1957) observed that the K:(Ca + Mg) ratios of normal and tetanigenic pastures were 1.67 and 2.37, respectively. Metson et al. (1966) reported a mean K:(Ca + Mg) ratio of 2.45 in pastures of 19 farms that collectively had a 10% incidence of tetany in beef cattle.
Magnesium absorption has been improved by feeding grains and ionophores. Legumes are usually higher in magnesium than are grasses. Magnesium oxide and magnesium sulfate are good sources of supplemental magnesium. Including 15-20% magnesium oxide in the mineral mix should reduce the problem. Adding 6-10% molasses or soybean meal will assure intake.
As with all crops, proper pH is the most important factor in crop management (Munson and Joern, 1996). If the soil does need lime, use a dolomitic source if soil-test magnesium levels are less than 50 ppm. If the field has recently received manure, the importance of soil-test information cannot be overstated. At the rate used by some producers, a single manure application may supply several years worth of phosphorus, and sometimes potassium. Do not apply excessive rates of nitrogen early in the spring because high nitrogen levels can reduce magnesium availability in ruminants. Maintain relatively high soil-test phosphorus levels as some research has shown that phosphorus additions can increase tissue magnesium levels and potentially even decrease potassium uptake. Delay potassium application on grasses until late spring as high potassium fertilization decreases magnesium uptake. Consider interseeding clover since legumes are higher in magnesium than grasses.
Selenium deficiency will cause retained placentas, infertility, and white-muscle disease in calves. The normal cow requirement is 0.1 ppm. The maximum tolerable concentration of selenium has been estimated to be 2 ppm.
Selenium is generally supplemented in animal diets as sodium selenite, while seleno-methionine is the predominant form of selenium in most feedstuffs. Selenium from seleno-methionine or a selenium-containing yeast was approximately twice as available as sodium selenite or cobalt selenite in growing heifers (Pehrson et al., 1989). Availability of selenium from sodium selenate was similar to sodium selenite (Podoll et al., 1992). Vitamin E should be added to the diet along with selenium. Calves should be injected with a selenium-vitamin E solution at birth, where a problem exists. Alternate methods of supplementing selenium include injecting selenium every three to Four months or at critical production stages and using boluses retained in the rumen that release selenium over a period of months (Hidiroglou et al., 1985; Campbell et al., 1990).
Liver samples are the ideal way to determine a deficiency, with 0.25 to 0.5 ppm considered normal and 0.1 to 0.15 ppm considered deficient. Blood can also be used as an indicator with normal levels being 0.08 to 0.3 ppm with deficiencies considered being 0.002 to 0.025 ppm.
Requirements of sulfur for grazing cattle are not well defined (approximately 0.15%). In Australia, sulfur supplementation increased gain by 12% in steers grazing sorghum-sudangrass containing 0.08 to 0.12 percent sulfur (Archer and Wheeler, 1978). The sulfur requirement of ruminants grazing sorghum-sudangrass may be increased because of the need for sulfur in the detoxification of cyanogenic glucose found in most sorghum forages.
Dietary sulfur requirements may be higher when diets high in rumen bypass protein are fed because of the limitation of sulfur for optimal ruminal fermentation. When urea or other nonprotein nitrogen sources are fed, sulfur supplementation may be needed. Mature forage, forages grown in sulfur-deficient soils, corn silage, and sorghum-sudangrass can be low in sulfur. The typical nitrogen to sulfur ratio of a complete diet should be 10:1, nitrogen to sulfur.
Copper deficiencies can cause poor reproduction, broken bones, weak calves, and light color hair. Discoloration normally occurs first around the eyes and tips of the ears. Sometimes, changes in hair color are not noted and the effect of a copper deficiency simply occurs as reproductive problems, scours, or calves older than four months ceasing to perform. Simmental and Charolais cows and their calves were more susceptible to copper deficiency than Angus cows fed the same diet (Ward et al., 1995).
Unfortunately, with copper, the forage may contain an adequate level, but if the diet contains either high levels of molybdenum (2 ppm) or sulphur (0.25%), both of these tie-up copper, rendering a deficiency. Ideally, the copper to molybdenum ratio should be 5:1 or greater (Munshower and Neuman, 1979). Legumes were blamed for increasing the molybdenum levels on reclaimed strip ground in Montana (Munshower and Neuman, 1979). High concentrations of iron (Phillippo et al., 1987) and zinc (Davis and Mertz, 1987) also reduce copper status and may increase copper requirements.
Recent studies indicate that copper oxide is very poorly available relative to copper sulfate (Langlands et al., 1989; Kegley and Spears, 1994). Copper sources are copper sulfate, copper carbonate, copper proteinate, and copper lysine. Injectable forms of copper such as copper glycinate or copper EDTA have been given at three- to six-month intervals to prevent copper deficiency (Underwood, 1981). Although feed-grade copper oxide is largely unavailable, copper oxide needles, which remain in the gastrointestinal tract and slowly release copper over a period of months, have been used as a copper source for cattle (Cameron et al., 1989).
Sewell (1990) suggested that producers look at the commercial mineral supplement that they are using for cattle fed fescue and similar forages to see if the label shows approximately the following percent amounts of trace minerals: Selenium 0.0008 to 0.0016; cobalt 0.0008, copper 0.04, zinc 0.08; and manganese 0.08. Steeds (1991) recommended that Manitoba producers should only buy mineral supplements containing more than 2,000 ppm (0.2%) copper.
The iodine requirement is 0.2 to 0.3 ppm in the total diet. Goitrogenic substances in the feed may substantially increase the requirement (two- to Four-fold), depending upon the amount and type of goitergens present. Plant sources that can increase the iodine requirement are white clover and some Brassica forage such as kale, turnips, and rape. They impair iodine uptake but can be overcome by increasing dietary iodine.
A zinc deficiency can affect reproduction, the skin, and cause swelling of the bone joints or slow healing of wounds. Zinc deficiencies tend to impair sperm production and sperm quality in bulls. Cows require 30-40 ppm zinc with diets containing 2-10 ppm considered deficient. Legumes are generally higher in zinc than grasses.
In general, iron deficiency is unlikely unless parasite infestation or disease exists and causes chronic blood loss. Availability of iron from forage appears to be lower than from most supplemental iron sources (Raven and Thompson, 1959). Iron is normally supplemented in the diet as ferrous sulfate, ferrous carbonate, or ferric oxide. However, ferric oxide is basically unavailable (Ammerman et al., 1967).
Manganese can cause infertility, light hair color, and calves with weak pasterns. Manganese requirements are approximately 40 ppm. A deficiency has sometimes been noted with feeding corn-silage diets.
Cobalt affects reproduction, growth, and causes pale skin. Cobalt supplementation plus an injection of vitamin B12 should alleviate symptoms. Both cobalt and iodine requirements can be met by using blue salt free choice (Steeds, 1991). Red salt is plain white salt plus iodine. Blue salt is red salt plus cobalt.
Indications are that fescue is deficient in certain trace minerals (Sewell, 1990). Selenium, cobalt, copper, and zinc may be borderline or deficient in fescue and other forages, such as crop residue and mature grasses. Sewell (1990) suggested that producers look at the commercial mineral supplement that they are using for cattle fed fescue and similar forages to see if it has approximately the following percent amounts of trace minerals as stated on the label - selenium 0.0008 to 0.0016, cobalt 0.0008, copper 0.04, zinc 0.08, and manganese 0.08. Ohio may require somewhat high copper levels than this.