Trace Minerals for California Beef Cattle 

University of California, Cooperative Extension

How to evaluate mineral status

Cattle require protein, energy, fiber, minerals, vitamins, and water. These are the nutrients used in body maintenance, growth, lactation and reproduction. Each nutrient has unique chemical characteristics and specific functions in the animal. Some are interchangeable (through metabolism) and others are required in specific amounts or forms. Minerals, and particularly trace minerals, are required in very small amounts. The addition of only a tiny amount may have dramatic effects on animal performance and health. As we learn more about each nutrient and how to do a better job of feeding that nutrient, some other nutrient becomes limiting. This is the concept of the "first-limiting" nutrient. Animal performance will be limited by the first-limiting nutrient just as a barrel made of individual wooden staves will hold no more liquid than its shortest stave. As the shortest stave is lengthened, some other becomes shortest, and limiting.

Phosphorus, copper, zinc and selenium are particularly important due to their impact on productivity and for the propensity to mineral deficiency in grazing cattle. Molybdenum, sulfur and iron are also discussed as they often interact with and influence status of phosphorus, copper, zinc and selenium.

Because only small amounts are needed people tend to either completely ignore trace minerals assuming such a small amount could not have a major effect, or, treat them as magic elixirs. Neither attitude is correct. As we better understand the role of energy and protein and more adequately satisfy those requirements, minerals may become more important and limiting. In addition, as soils are continuously cropped, and plant yields are increased, trace minerals may become less available or more diluted. Cattle frequently obtain their feed from a single source such as one pasture or hay from one specific area. When the site of feed production is deficient in minerals, then cattle will be deficient since they have no alternate source for the deficient minerals.

Historical information provides information about the mineral status of cattle. Conclusive clinical symptoms of trace mineral deficiencies establish a definite history of the mineral status. Knowledge of soil origins and soil mineral levels also helps to identify potential mineral deficiencies. Animal responses to mineral supplementation can also identify areas of mineral deficiencies. In all circumstances, even with extensive historical information, evaluating and monitoring mineral status must be conducted at the herd or individual animal level.

A combination of cattle tissue and forage samples is best to determine mineral status. Forage samples should be analyzed for phosphorus, selenium, copper, zinc, sulfur, iron, and molybdenum. Water sources can be analyzed for iron and sulfur. Whole blood samples can be used for selenium status, with serum samples for phosphorus, copper and zinc. Copper evaluations should also include liver biopsy samples.

Values for interpreting these samples are given, but many factors interact which raise or lower values that then may be considered deficient, marginal, adequate or excessive. Mineral status is not black or white, rather shades of gray. Monitoring change in mineral status through periodic sampling is strongly recommended. Measuring performance or response, such as increased rate of gain, can also be used to determine mineral status and the value of supplementation. However, some performance factors such as feed efficiency or improved immune system are difficult to measure.

Selenium

Both plant (feed) and animal samples can be used to estimate selenium status. However, feed samples are less precise due to possible feed selectivity, interfering substances, and variable biological availability of dietary selenium. Whole blood samples are easy to obtain and provide adequate precision. Serum or plasma are inferior to whole blood samples and should not be used.

Generally accepted adequate levels for selenium are 0.08 ppm whole blood or higher. Cattle should be supplemented when blood selenium levels are below 0.08 ppm. Long-term whole blood selenium levels of 2.0 ppm have not been excessive but due to possible antagonisms with other trace minerals, whole blood selenium levels are ideally less than 1.00 ppm. Typically, 10 samples from a herd are adequate to assess herd status. Cattle in different management groups, such as by feed, age, or stage of production, should be considered separately.

Cattle moved to different locations may need sampling after 60 or more days at each location. Different locations may have different Se levels in their forage and would be reflected in cattle blood levels.

Copper

Laboratory analysis of blood (serum) samples is the most common method of determining copper status. However, liver tissue samples are superior, but more difficult to obtain. Typical laboratory results will report copper amount in serum or plasma (the liquid part of the blood sample, not the cells). Generally serum copper levels should be 0.7 to 1.2 ppm. Serum copper levels below 0.5 ppm are considered deficient, while values between 0.5 and 0.7 ppm are marginal. Copper levels in plasma samples are about 5 percent higher than serum samples.

Diets containing 5 to 10 ppm copper are usually adequate. Forage copper of 3 to 5 ppm is usually marginal, with levels less than 3 ppm deficient. High dietary molybdenum (5 to 6 ppm dietary molybdenum) or high sulfate levels will decrease copper absorption or availability and necessitate higher dietary copper levels. Forage copper levels of 12 to 27 ppm have been associated with copper deficiency when molybdenum levels are excessively high. In some cases of high molybdenum, serum copper levels will be normal but deficiency occurs. In those cases, additional cooper supplementation is required to overcome interference by high molybdenum levels.

Serum copper levels do not measure storage of copper in the liver. Therefore, liver samples provide more information about copper status and are the preferred sample. Liver samples are obtained by liver biopsy under practical working conditions by a veterinarian. Liver samples can also be obtained from animals that have been slaughtered. Liver copper levels between 100 and 200 ppm on a 100 percent dry matter basis indicate adequate copper. Levels above 250 ppm are not unusual in supplemented cattle. Cattle are considered markedly deficient when liver copper is below 35 ppm.

Zinc

Laboratory analysis of blood samples is the most common method of determining zinc status. Typical laboratory results will report the amount in serum or plasma (the liquid part of the blood sample). Generally, serum zinc levels should be 0.8 to 1.4 ppm. Recommended dietary zinc levels are 9 to 18 mg./lb. of diet dry matter.

Supplements with zinc have been found to lower copper levels. Thus, whenever zinc is supplemented copper status should be monitored. If copper levels are marginal before zinc supplementation, it is likely that copper will become deficient leading to a need to supplement copper.

Mineral status will vary depending on specific local conditions. Historical information provides evidence of typical mineral status for numerous specific areas. If cattle in these specific areas consume feeds from other areas, then historical mineral status will not be appropriate or germane. In all circumstances, evaluating and monitoring mineral status must be conducted at the herd or individual animal level.

Phosphorus

Laboratory analysis of feed is the most frequent method of assessing phosphorus status. Requirements for phosphorus in the diet vary but generally range from 0.20 to 0.25 percent, cattle with faster rates of gain and lighter body weight have higher needs.

Heavily milking beef cows (20 pounds or more milk per day) need 28 or more grams of phosphorus while average milking cows require about 22 grams daily. As a guideline, one pound of alfalfa has about 1 gram of phosphorus. Therefore, beef cows on full feed of alfalfa will have adequate phosphorus only if they are average or below in milking ability. Specific requirements can be found in the National Research Council, Nutrient Requirements of Beef Cattle. Analyzing feed for phosphorus content can be adequate for determining phosphorus status of feedlot cattle. However, it is not practical for non-confined cattle. Laboratory analysis of blood samples provides a better method to determine phosphorus status of all cattle.

Normal blood plasma (the liquid part of the blood sample) phosphorus levels are 56 to 65 ppm. When plasma or serum levels are below 40 ppm, supplementation should be considered.

Mineral status will vary depending on specific local conditions. Historical information provides evidence of typical mineral status for numerous specific areas. If cattle in these specific areas consume feeds from other areas, then historical mineral status will not be appropriate or germane. In all circumstances, evaluating and monitoring mineral status must be conducted at the herd or individual animal level.

When interpreting laboratory values for tissue samples, recognize that numerical values for deficient, adequate and excess are not black and white, but rather shades of gray. Due to many potential interactions with other nutrients and dietary components, adequate levels are difficult to confine to specific values. As identified in the text, adequate levels of some trace minerals are dependent on the level of other trace minerals.

Numerical values for samples vary tremendously depending on the units or basis of the report. A whole blood selenium value of 0.04 ppm is equal to 0.018 mg/lb., 0.00004 percent or 0.04 mg/kg. The moisture level or reporting basis of the sample is also critical. Liquid samples such as whole blood or serum, and liver samples, for example, are usually analyzed and reported on a wet weight basis, or as received basis. Forage samples are frequently reported on both an as received basis and 100 percent dry matter basis. In the latter case, all of the moisture has been removed. The moisture basis will make a tremendous difference in most cases in the numerical values. Compare values only when they are expressed on the same moisture basis.

Guidelines for interpreting mineral concentrations are shown. Due to interactions between minerals, sampling for multiple minerals is preferred. Deficient and adequate values will vary depending on levels of other minerals. Compare values only when they are expressed on the same moisture basis and on the same units.