The science surrounding protein and energy nutrition, no matter the livestock species, is a couple of generations ahead of mineral nutrition – particularly trace mineral nutrition. Yet finally there are signs that mineral nutrition science might one day catch up. One sign of progress is industry and academic interests using ‘organic’ minerals in lieu of inorganic forms of zinc, manganese, copper and selenium. The initial focus is on reproductive issues and on enhancing immune response and milk quality – areas where trace mineral availability may be limiting. Another issue is environmental accumulation of minerals in manure spread on cropland.

Comparatively, little basic research has been done in animal nutrition for some time. For the past several decades, energy and protein received the lion’s share of research attention, and during this time we gained an understanding of the forms and functional nature of fat, fiber, starch and protein sources, and the importance of these factors in digestion, growth and production. It should therefore surprise no one that there is a relationship between form and function for other nutrients, including minerals. It should also be no great shock that herbivorous and omnivorous animals can often better utilize those mineral forms which they evolved to metabolize – in other words, forms that occur in plants or other biological tissues.

Mineral forms in plants are like those in animals: ‘Organic’

Ask a group of nutritionists what forms of copper or zinc occur in plants and few hands will be raised. That question just never came up in college. We were all trained in the ‘name-that-deficiency-syndrome’ school of mineral nutrition, which recalls the early experiments that first defined mineral requirements of livestock.

It is a better bet to ask a plant physiologist or an agronomist about the nature of minerals in plants. The answer is instructive because it turns out that copper, manganese, zinc and selenium exist in plants in about the same forms in which they exist in the animal – at the active site of an enzyme; in other words, linked to amino acids in a protein. The same is true for magnesium, a macromineral. Magnesium is at the active site of RuBisCo, the enzyme that ‘feeds the world,’ so-called because it fixes CO2 in the ‘dark’ reactions of photosynthesis.

Functions of the minerals in plants also parallel those in animals to a large extent. Copper, zinc, manganese and iron are involved in antioxidant enzyme systems, for example. The exception to this parallel in plant and animal mineral function is selenium.

The function of selenium (Se) in plants is unknown. Plants (non-Se accumulators) take up soil selenite and form selenomethionine using the same cellular machinery used to form methionine. This selenoamino acid is sufficiently similar to methionine that it is freely substituted in plant protein formation. Some have suggested the plant puts selenium into protein in self-protection against a very toxic element. Whether or not this is the case, this pathway certainly produces the safest form of selenium for animals. Selenomethionine is known as the ‘food form’ of the element.

Mineral proteinates, which contain trace elements chelated to different amino acids, are the mineral sources that most closely simulate the ways in which plant ingredients supply copper, manganese and zinc.

What advantage might plant mineral forms confer to the animal?

One nutrient source would be considered advantageous over another if it is in some way better able to meet the animal’s physiological needs; however, defining relative value of mineral sources is not always easy. While it is reasonable to say greater absorption and retention are important, mineral balance math does not always explain biological responses. When we see decreases in somatic cell count (SCC) or fewer days open in cattle given trace mineral proteinates, what explains why that animal(s) had more metabolically useful mineral when it was needed? Equally significant in terms of impact on animal health or conception rates may be its route through the digestive tract, or its metabolic path in the animal.

Ways in which ‘nutritive value’ of mineral sources differ

•Protection against toxicity

•Protection against oxidative stress

•Storage in tissue reserves

•Transferability to progeny

•Protection against interactions during digestion

•Absorption and retention rates

An important practical use of organic minerals on dairy farms is avoidance of inorganic mineral interactions that prevent absorption by the animal. While inorganic minerals are also a natural part of any animal diet, the main source of inorganics is usually water and would not be expected to provide more than minor amounts of the trace elements. In agriculture, the inorganic ion (and anion) content of water is more likely a problem we must find ways to feed around.

Unlike wild animals, who would consume water from either a flowing source or from more than one source, domestic stock consume a single point source, often groundwater with disproportionate amounts of iron, sulfur, salt or calcium. The result frequently is interactions between minerals in water and dietary inorganic supplements (e.g., iron with copper) that prevent absorption. Organic sources, as they are not subject to immediate release as free metal ions, are more likely to reach absorption sites and less likely to prevent absorption of another element.

Functional ingredients follow nature’s form

It is doubtful we will ever improve upon nature, but we must constantly improve our ability to approximate it if we expect to keep doing a better job of feeding high producers. Forages cannot take up enough trace mineral to meet the needs of modern animals, but trace mineral proteinates and selenium forms biosynthesized by yeast can make up the difference to both meet daily needs and allow storage against times of increased demand such as immune defense or reproduction. PD