by

Larry L. Berger, Ph.D.
Professor, Animal Sciences
University of Illinois

Animal nutritionists and livestock producers have recognize that variation in nutrient profiles of feedstuffs is a common occurrence. However, few producers realize that the normal variation in energy, protein or macrominerals is relatively small compared to what has recently been reported for the trace minerals.

The purpose of this review is two-fold. First, it is important to describe the variation in trace mineral profiles among common feedstuffs so it can be considered in supplementation programs. Secondly, identifying those factors contributing to this variation may be helpful to individual producers and nutritionist in preventing trace mineral deficiencies.

Describing Trace Mineral Variation:

Even when making allowances for extreme values due to sampling error, soil or other contaminations, analytical error, etc., variability in trace mineral concentrations are much greater than for protein and energy in common feedstuffs. For example, Adams (1975) reported that the variation within samples of legume-grass forage for total digestible nutrient and crude protein was 1.4 and 5.0 fold differences (maximum value/minimum value), respectively. In contrast, when the maximum value was divided by the minimum for copper, iron, manganese and zinc the ratios were 46, 260, 44 and 38, respectively. A 30-50 fold range in trace mineral concentration was common within a given forage. The extremely high variation in iron content is usually reflective of the amount of soil contamination.

It has been widely assumed that forages are much more variable than grains. Undoubtedly, this is true for energy and protein. However, the research of Adams (1975) suggests that this may not be the case for the trace elements. When comparing the percentage coefficient of variation for copper, iron, manganese, and zinc, the statistical means for forages were 57, 68, 75 and 63% respectively. The same values for corn were 62, 83, 116, and 39%, respectively. Coefficients of variation were greater for corn than forages for all minerals except zinc. It is true that grains generally have lower concentrations of the trace elements than forages. Consequently, smaller changes in actual amount may appear as a larger coefficient of variation.

Another common misconception is that feedstuffs grown in a given geographical region will have similar trace mineral profiles. The following table summarizes data taken from the Pennsylvania State Forage Laboratory between 1969 and 1973 (Spears, 1994). The data are expressed as a mean ± standard deviation for each trace mineral. These data are interpreted to suggest that even within a relatively small region of the United States, major variations can exist in forage mineral values. In many cases, the standard deviation is equal to or greater than the mean value.

Table 1.

Trace mineral content of forages analyzed at the Pennsylvania State forage testing service between 1969 and 1973.          Values are statistical mean ± standard deviation in mg/kg)

Factors Affecting Trace Mineral Concentrations:

Trace mineral concentrations are affected by four interdependent factors: 1) the genus, species or variety of crop, 2) type and mineral concentration of the soil, 3) stage of plant maturity, and 4) climatic or seasonal conditions. The following discussion is intended to give nutritionists and livestock producers a review of our current understanding on how these factors cause variation in trace mineral content of feeds.

Genus, species or varietal effects:

The most striking example of species difference occurs with selenium. Certain species of Astragalus growing on seleniferous soils contain 3,000-5,000 ppm selenium (Underwood, 1981). While other species may contain only 10-20 ppm selenium when grown on the same soil.

Differences between grains grown on the same soil are less dramatic, but just as significant. For example, wheat and oats will often contain 35-40 ppm manganese, while barley will only have 14-16 ppm and corn 5-8 ppm manganese when grown in the same environment. In these cases, changing energy source in the diet can have a dramatic effect on the amount of supplemental manganese required.

In general, legumes are higher in calcium, potassium, magnesium, copper, zinc, iron, and cobalt than grasses. In contrast, grasses tend to be higher in manganese and molybdenum than legumes when grown on the same soil. However, within grasses there is also a great deal of variation. Beeson et al. (1947) reported that copper ranged from 4.5 to 21.1 ppm and manganese from 96 to 815 ppm for 17 North American grass species grown on the same sandy loam soil and sampled at similar stages of maturity.

Even variety within a specie can have an effect. For example, Johnson and Butler (1957) reported that one strain of perennial ryegrass, selected and bred specifically for high dry-matter yield, had only one-tenth the iodine concentration compared to the most common variety. In this situation, botanical composition of the pasture was the primary factor affecting iodine intake. The point is that even subtle management changes can affect supplemental trace mineral requirements.

Mineral concentrations in soils:

Plants react to inadequate supplies of trace minerals in the soil either by reducing the concentration of the deficient element in their tissue or by reducing growth, or a combination of both. However, it should also be recognized that optimal trace mineral supplies for plant growth may yield feeds that are deficient. For example, the iodine, selenium and cobalt concentration needed for optimal plant growth are much below the requirements of animals.

Probably the mineral most affected by soil levels in selenium. Average values of 0.80 ppm and 0.05 ppm selenium have been reported for wheat grain grown on high and low selenium soils, respectively. Selenium concentrations as low as 0.005 ppm have been reported for wheat grown on selenium deficient soils in New Zealand (Underwood 1981).

Mineral concentration in soils has a great effect on pH, which in turn has major impact on mineral uptake by plants. For example, molybdenum uptake increases as soil pH increases. Plants containing potentially toxic concentrations of molybdenum are almost always from areas with a very alkaline soil. Likewise, plants that are deficient in molybdenum are usually from acid soils.

Changing soil pH can also affect other minerals. Mitchell (1957) monitored the change in mineral concentration of red clover and ryegrass when soil pH was increased from 5.4 to 6.4 by liming. Herbage cobalt concentrations were reduced from 0.22 to 0.12 ppm and 0.35 to 0.12 ppm, and manganese from 58 to 40ppm and 140 to 130 ppm on a dry basis, for the red clover and ryegrass, respectively. Zinc and copper also tend to decrease with increasing soil pH.

Stage of maturity:

Forage trace mineral concentrations are more affected by maturity than that of grains. Generally, there is a rapid uptake of mineral during early growth and a gradual dilution as the plant matures. Copper, zinc, iron, cobalt and molybdenum are the most common elements affected by plant maturity (Underwood 1981). For example, copper levels in Timothy hay decreased from 11 to 5 ppm as maturity increased from the early vegetative to the full bloom stage.

Climatic and seasonal conditions:

Although climatic and seasonal conditions are difficult to alter, management factors such as irrigation can be used to study the effects on mineral levels. In general, seasonal or climatic conditions that maximize yield often have no effect or decrease trace mineral concentrations. Leaching of soluble nutrients from forages during wet weather is well documented. However, copper, zinc, and manganese tend to be bound in plant tissues and are less susceptible to leaching than minerals like potassium and phosphorus.

As forages stand through the fall and winter, typically leaves and seed heads will be lost. These portions of the plant often contain greater concentration of the trace elements than the stem. Consequently, the mineral concentration of the standing herbage usually decreases due to a change in leaf to stem ratio.

Regions of the U. S Consistently Deficient:

The combination of soil type, plant species, and climatic conditions have resulted in certain regions of the country which are consistently deficient in one or more of the trace minerals. These are areas where clinical deficiency symptoms are common if the deficient mineral is not adequately supplemented. Besides these known areas, many other areas of the country may produce feedstuffs which are marginal in one of more of the trace elements.

The areas with the most severe copper deficiencies tend to be along the East and West Coast, Upper Midwest and Florida . Numerous research trials involving animals fed in the Midwest or High Plains regions have responded to copper supplementation. As demonstrated by the data from Pennsylvania, the average forage copper levels were near the requirement for ruminants, but the variation among samples was very high so that over 50% of the samples would be copper deficient. Because copper absorption is depressed by molybdenum, Figure 2 is included to show those area where molybdenum toxicity is a problem. In these areas, copper requirements may be increased 50-100% due to excess molybdenum.

Severe zinc deficient areas  include much of the Southeast, Texas, West Coast and portions of Nebraska and Wisconsin. Common signs of zinc deficiency include poor performance, parakeratosis, foot rot and slowed wound healing.

Iodine deficit areas includes most of the Northern-half of the U. S., parts of the Southeast, Kansas, Missouri, Louisiana and California. Iodized salt should be fed in all areas of the United States. Diets that contain soybean meal, cottonseed meal, canola meal, or members of the cabbage family have been shown to increase the iodine requirement.

Plants along both Coasts and in Upper Midwest are known to be deficient in Manganese. High levels of calcium and or phosphorus also increases the requirement for manganese. Animals deficient in manganese have reduce growth, feet and leg problems and impaired fertility.

Severe selenium deficiencies have occurred in the Northeast, Midwest, Southeast, Northwest and portion of Arizona and New Mexico. Common deficiency symptoms include white muscle disease, stiff lamb disease, poor fertility, and an impaired immune system.

Cobalt deficiency is most common in the Upper Midwest, Northeast, East Coast regions and in parts of Louisiana, Arkansas and Oregon . Cobalt is required for vitamin B₁₂ synthesis in ruminants. Deficiency symptoms include general unthriftiness, depressed appetite, and increased susceptibility to infections.

In summary, trace minerals concentrations in feed are the most highly variable of any of the nutrients required by animals. Feedstuff analysis is recommend to establish baseline levels for producers who raise most of their own feeds. However, because many producers buy a major portion of their feed, often from unknown origin, and feed it up before the feed analysis information is available, another approach is needed. Feeding a properly fortified trace mineralized salt to supply the majority of the trace mineral requirement, is the safest approach. In today's highly competitive livestock and poultry production environment, the risk are too great to do it any other way.

Literature Cited

Adams, R.S., 1975. Variability in mineral and trace mineral content of dairy cattle feeds. J. Dairy Sci. 58:1538

Beeson, K.C., L. Gray, and M.B. Adams. 1947. The absorption of mineral elements by forage plants. 1. The phosphorus, cobalt, manganese, and copper content of some common grasses. J. of the American Society of Agronomy 39:356.

Johnson, J.M. and G. W. Butler. 1957. Iodine content of pasture plants.1. Method of determination and preliminary investigations of species and strain differences. Physiologia Plantarum 10:100.

Mitchell, R.L. 1957. The trace element content of plants. Research. UK 10:357.

Owens, Fred. 1988. The haves and the have nots. Beef Magazine. May Issue, p. 10.

Spears, J. W., 1994. Minerals in forages. p.281. In George C. Fahey, Jr. (ed.) Forage quality, evaluation, and utilization. American Society of Agronomy. Madison WI. 53711

Underwood, E.J. 1981. The mineral nutrition of livestock. p. 10. Commonwealth Agricultural Bureaux, Slough, England