Over 95% of a range cow’s diet is comprised of forage. As range plants pass through various stages of growth and maturity, their nutrient (protein, energy, vitamin, and mineral) content varies greatly. Rangeland varies tremendously in plant species, soil type, moisture level, and length of growing season. These environmental factors also contribute to the variation in nutrient content of forages.

It is commonly recognized by producers and nutritionists that this variation occurs. However, it is not commonly recognized that the normal variation in energy, protein, and macro minerals (primarily calcium, phosphorus, and magnesium) are relatively small compared to the variation in trace mineral concentration (copper, zinc, iodine, manganese, selenium, iron, etc.). A discussion of factors that contribute to nutrient variation and suggestions for working within the normal variation that occurs in native grasses follows.

Trace mineral concentrations of forages can be affected by several factors:

Some native forage species concentrate certain minerals at a higher rate than others. A striking example would be the concentration of selenium by certain species of Astragalus (such as milk vetch and locoweed) where concentration can become toxic. In contrast, other grass species (such as buffalo and grama grasses) in the same environment remain safe for domestic livestock. Research has shown that even when grown on similar soils, wheat and oats will often contain two to three times the level of manganese compared to barley, which in turn, will often contain twice the level found in corn.

Legumes are generally higher than grasses in calcium, potassium, magnesium, copper, zinc, and cobalt. In contrast, grasses tend to be higher than legumes in manganese and molybdenum. Similar differences in concentration of individual trace minerals can be measured among varieties of the same species (an example is rye grass). Clearly, forage and grain species take up and store various trace minerals at different levels. These are genetic and physiological differences among plant species and families within species.

Plants, like animals, have nutrient requirements for growth and reproduction. Plants react to inadequate trace minerals in the soil either by reducing plant growth or reducing concentration of the minerals in their tissue. Optimal trace minerals for plant growth may yield feeds (parts of the plant that are removed by the animal) that are deficient to support acceptable animal performance. In general, trace mineral levels which will support optimal plant growth are much below the trace mineral requirements of animals for normal growth and function. Mineral concentration in plants can be affected by mineral concentration in the soil. This frequently exerts its importance through mineral impact on soil pH, which can enhance or restrict, the ability of plants to incorporate mineral in its tissue (leaf, stem, and seed).

Plant maturity affects mineral concentration. There is generally a rapid uptake of mineral during early growth and a gradual dilution as the plant matures. For example, copper level in forages may be reduced by one-half as the plant progresses from early vegetative to full bloom stage. The seed of plants will concentrate phosphorus at significantly higher concentrations than will the leaf or stem fraction.

Seasonal and/or climatic conditions can affect trace mineral concentration. Some minerals (the more water soluble ones—phosphorus and potassium) can be leached from the plant during the wet season. However, copper, zinc, and manganese tend to be bound in plant tissue and are less susceptible to leaching. As forages stand through the fall and winter, leaves and seed heads are usually lost. These portions of the plant often contain greater concentration of minerals than do stems. Thus, a progressive change in leaf to stem ratio during the winter also contributes to a change in mineral concentration of the plant.

The bottom line of this discussion is the realization that trace mineral concentration in forage can vary tremendously due to time of the year, stage of maturity, soil type, mineral level in the soil, forage species, rainfall, and sampling error. When mineral supplements are custom made for a specific ranch, one must consider what location(s) on the ranch were sampled, what forage species did the sampler assume the cow would eat, what season of the year was included in the formulation, from what soil types that occurs within a pasture and/or ranch, from well-drained or poorly drained areas, and was it a dry or wet year when the sampling was done. The point being—there are a tremendous number of variables that affect trace mineral levels in forages. Can we formulate for each variable; or, do we average out over all the variables and come up with one mineral supplement that provides insurance?

These data indicate trace mineral concentrations are highly variable among common forages consumed by grazing range cattle. Also, several average values are below National Research Council (NRC) requirements; and, all values are low when one standard deviation is applied to the average. This fact indicates that, in most situations, grazing cattle would need mineral supplementation to provide the rumen bacteria and the host animal optimal minerals for efficient utilization of forage and for animal growth and performance.

Producers and/or nutritionists who choose to cut corners by eliminating trace mineral packages are taking a risk. Native grass forages are extremely variable in trace mineral concentration and the majority of the time fail to provide levels needed to support optimal performance and efficiency of cattle on range. Failure to meet cow trace mineral needs could easily express itself in depressed efficiency in forage utilization, poor reproductive performance, slower growth, and lower immune function. Should cow/calf and stocker operations that depend on grazing provide a free-choice, balanced, quality mineral supplement? Research data indicates that it would be a sound investment.

References:

Berger, L.L. 1996. Variation in the trace mineral content of feedstuffs. Prof. Anim. Scientist 12:1-5.