Small Things can have Big Impacts
Dr. Robert Van Saun, DVM, Penn State Extension Veterinarian
Trace minerals and vitamins comprise a very small portion of an animal’s diet (< 1% of dry matter), yet they can have a tremendous impact on animal health and performance. When it comes to essential nutrients to support the dairy animal, cobalt, copper, iron, iodine, manganese, selenium, and zinc comprise the required trace minerals. These mineral elements are only required at a level of milligrams (mg; one one-thousandth of a gram) or micrograms (µg; one one-thousandth of a milligram) per day. It is difficult to appreciate how nutrients required in such small amounts can have a significant impact on animal health and performance. Similarly, fat-soluble vitamins A, D, and E are also required in moderate amounts (mg/day). Collectively, these essential nutrients can impact most aspects of metabolism, antioxidant status, and, most importantly, immune function.
One would think that since these nutrients are only required in such small quantities (i.e., micronutrients), it should be easy to meet the animal’s nutritional needs. Unfortunately, this is not the situation. Forage, which forms the foundation of an organic diet, is highly variable in its trace mineral concentration. On the East Coast, our forages are predominantly deficient in selenium and zinc. There is potential for cobalt and iodine deficiencies in areas surrounding the Great Lakes region. East of the Mississippi River, forage copper content is generally good; however, many potential inhibitory factors can induce copper deficiency. Generally, grasses are high in iron and manganese, but these minerals can often interfere with availability of other trace minerals. The bottom line is that one should perform a wet chemistry analysis on forages to determine their trace mineral status. When doing this, be sure to request molybdenum analysis, as this mineral is highly variable in forages and is a very significant inhibitor of copper uptake.
When it comes to fat-soluble vitamins, both vitamins A and E are plentiful in fresh pasture, whereas vitamin D is not present in pasture but only in sun-cured forage. Unfortunately, when forage is harvested, dried or ensiled, and stored, these vitamins are rapidly oxidized due to exposure to moisture and air and depleted from the forage. Feeding stored forage, either as ensiled or dry hay, will result in negligible vitamin content requiring dietary supplementation.
Micronutrients and Disease
We come back to the question, why is this important? Many of these essential trace minerals and vitamins perform specific biologic functions; however, more importantly, they are important regulators of the immune response. Previous Ohio State University research has shown an important role of vitamin E in mastitis prevention. This research found a serum concentration of 3 µg/mL or less increased the risk for mastitis by 9-fold. Another field study from Canada found no direct association of vitamin E status with mastitis in early lactation; however, none of the blood samples collected had a vitamin E concentration above 3 µg/mL. Instead, cows with higher (>1.5 µg/mL) blood vitamin E concentration were at less risk for experiencing retained fetal membranes. This study did show that increasing blood vitamin A concentration had a significant effect on reducing mastitis risk in early lactation. Other research showed that a large quantity of vitamins A and E are lost through the mammary gland just before calving, as these vitamins are concentrated in colostrum. Without proper dietary supplementation, colostral loss of these vitamins can compromise the vitamin status of the cow, leading to increased disease risks. Besides the potential for compromised cow vitamin status, colostrum could also have less vitamin content if the cow is insufficiently supplemented. These vitamins do not cross the placenta efficiently; thus, the calf will be born with a low vitamin status and must rely on colostrum intake to provide sufficient vitamins to sustain their metabolism, growth, and immunity. A recent scientific review has suggested that beef calves are deficient in vitamin A and the current National Research Council supplementation guidelines are inadequate. Vitamin A deficiency has been linked to increased risk for stillbirths in both dairy and beef calves. Vitamin D may also be of concern for similar reasons. Newborn nursing animals may present with long bone deformities from vitamin D deficiency rickets. This situation is most serious in animals giving birth at the end of winter, as their vitamin D status will be reduced as a result of limited sunlight exposure in winter conditions.
In contrast to the vitamin situation, the trace minerals are efficiently transported across the placenta during gestation, with the exceptions of iodine and manganese. Mineral supplementation of the cow during gestation will determine the adequacy of fetal mineral status. The fetus is capable of storing these minerals in its liver at levels exceeding what may be found in the cow. The importance of this storage is that milk is inadequate in most trace minerals; thus, requiring mobilization of stored mineral in support of calf metabolism, growth, and immunity. Colostrum does contain a higher concentration of these trace minerals compared to milk; however, it is not sufficient to support early neonatal development and health.
There is a growing body of evidence to suggest trace mineral deficiencies during gestation can lead to fetal demise as an abortion or stillbirth. Marginal trace mineral status in the dam can compromise fetal liver mineral storage and result in mineral deficiencies in early postnatal life. Field observations would suggest these neonatal animals with compromised liver mineral storage are more susceptible to infectious disease conditions due to their immune response being compromised. Their growth may also be diminished. Again, field observations have suggested that weaned animals coming from geographic regions known to have issues with copper or selenium deficiencies will not perform as well in a feedlot system, leading to greater respiratory disease.
Rumen-Micronutrient Interactions
Supplementation of the pregnant dam is critical to ensuring adequate transfer of these essential micronutrients to the fetus and colostrum. Although we understand the importance of the rumen in allowing our production animals to consume forages and convert these feeds into highly nutritious human food, the rumen environment can be detrimental to mineral and vitamin nutrition. The rumen environment is anaerobic for facilitating microbial fermentation; however, this results in an abundance of reducing equivalents (i.e., negative charges) looking for a stable neutral status. As most minerals are positively charged, these negatively charged reducing agents will be attracted to the mineral atoms and ultimately alter their electrical valence. Why this is important is that mineral atoms are only absorbed in the small intestine based on a specific charge to the atom. It is hard to appreciate that a minor difference in the presence or absence of an electron can impact mineral nutrition. A good example is selenium. The selenium atom is only biologically available if in the +4 (selenite) or +6 (selenate) valence. In the rumen, the selenium atom will attract electrons, changing its charge to 0 (elemental selenium) or -2 (selenide), neither of which is biologically available for absorption.
Another example is related to sulfur, which can interfere with many minerals, preventing their absorption. Water, especially water draining a reclaimed strip mine, can contain high concentrations of soluble sulfates. Plants and some byproduct feeds can also have higher levels of sulfur. In the rumen environment, sulfur or sulfate can be rapidly converted to the sulfide (-2 charge) ion. Sulfides readily bind to any positively charged atom, forming an insoluble metal sulfide (i.e., copper sulfide, zinc sulfide). Insoluble mineral forms cannot be absorbed in the small intestine. For efficient mineral absorption, the metal ion must be dissociated and soluble in water. Only soluble metal ions present in the thin water layer above the intestinal epithelium can be absorbed.
In the situation with vitamins, these organic compounds have complex structures that often contain carbon double bonds or rings. These structures also attract reducing equivalents causing double bonds to be converted into single bonds. In the case of vitamin A, the compound contains three double bonds that are essential for its biologic function in the body. When these double bonds are hydrogenated, the biologic activity is lost. Upwards of 40% of dietary vitamin A can be destroyed in the rumen, especially when feeding a high-grain diet. It is believed that both vitamins E and D can also be altered in the rumen, reducing their biologic activity.
Supplementation Options
One way to address these issues is to feed more of the given nutrient to ensure an adequate amount is being absorbed. Unfortunately, this approach increases costs, may negatively impact other nutrients, and may increase environmental contamination. Another approach is to supplement these nutrients via parental injections. Again, this approach is more expensive, may lead to toxicities, and has short-term responses. A better option is to provide a protected form of minerals to prevent rumen alteration. This is where the feeding of organic-form minerals may play a role. There are several commercial mineral supplements where the metal atom is linked in some fashion to another compound (termed a ligand), making the metal atom less amenable to rumen alteration. What is important to note here is that the ligand must dissociate from the metal atom, typically under the low pH conditions of the abomasum, for the metal ion to be absorbed. Unfortunately, there is little available documentation for this with many of the commercial organic minerals. Various controlled research studies have not shown any conclusive outcome that organic minerals are any better than more inorganic forms, suggesting there are other dietary or animal factors confounding this issue.
There are a number of methods that can be used to improve mineral status of forage via fertilization and soil management. However, agronomists will indicate a need to treat soil to maintain near neutral pH to facilitate plant growth. Most of the microminerals are better absorbed by the plant in a slightly acidic soil pH. Obviously, one does not want to compromise forage yield in lieu of increasing forage micromineral content. This brings us back to paying attention to mineral status of forage and providing appropriately balanced mineral sources to meet the animal’s requirements. In addressing amounts needed, one also needs to address potential interfering minerals that reduce mineral availability. There are no readily available guidelines on how to modify mineral content relative to interfering issues, with the exception of copper and molybdenum. This interaction has been well documented in ruminant animals. The recommendation here is to maintain a 6:1 up to 10:1 dietary ratio of copper to molybdenum in the diet. For sheep, we suggest an upper limit of 8:1. If this ratio is greater than 16:1 the risk for copper toxicosis increases. A suggested dietary ratio between zinc and copper is 4:1, but this is not a hard number as copper and molybdenum.
In providing micronutrients, one should focus on supplementation during mid-to-late gestation through the early breeding period. This is when supplementation will provide the greatest return on your investment. Ideally, one should provide adequate minerals year-round. As for mineral sources, providing only organic minerals is expensive; thus, most nutritionists will recommend a blend of inorganic and organic mineral sources. Typically, I recommend between 30 and 40% mineral supply from organic sources and the remaining from inorganic sources. Salt should be available to animals at all times. Remember that animals do not have specific appetites for individual minerals, thus one cannot expect them to selectively consume minerals to meet specific mineral needs. Sodium is what an animal desires and will consume salt-based mineral to meet their sodium requirement. This situation becomes a challenge where forage might contain high (>0.1%) sodium, which will reduce the animal’s desire to consume salt sources.
Mineral and vitamin supplementation is best provided through the diet rather than through parenteral injections, as this is more physiologic. It would be best to provide these micronutrients in the diet on a daily basis and minimize the animal’s ability to choose to consume or not. Free choice mineral supplementation can be used, but one needs to appreciate there is tremendous individual animal variation in mineral intake. This will result in some animals consuming more than needed and others less than needed. The mineral concentrations in the mineral supplement need to be assessed relative to forage mineral content and observed mineral intake. Many commercial free choice mineral supplements suggest a higher intake than what is often observed. For cattle, typically we would expect a free choice mineral intake of 1.5 – 2 ounces per day. For sheep and goats, the expected intake would be 0.25-0.33 ounces per day.
Summary
The micronutrients of trace minerals and fat-soluble vitamins play critical roles in animal health, disease prevention, and productivity. Micronutrient nutrition is critically important during gestation and early lactation due to losses to the fetus and colostrum, and increased metabolic and immunologic needs in early lactation. Forage does not supply sufficient trace minerals, and conserved forages are deficient in fat-soluble vitamins; thus, requiring appropriate supplementation to ensure adequate intake to meet requirements. Mineral sources can be organic or inorganic in form, with organic forms being less affected by ruminal alteration. Dietary supplementation is the preferred method in supplying these micronutrients; however, free-choice mineral supplementation can be used, recognizing its limitations. Routine forage testing is essential to be able to appropriately define the needed micronutrients required to be provided in the diet to meet the animal’s requirements.
Dr. Robert Van Saun, DVM, Professor of Veterinary Medicine, Penn State University who can be contacted at rjv10@psu.edu, 814-867-2038, will be presenting at the 26th Annual NODPA Field Days in Leesport, PA, on September 24th and 25th, 2026.
Posted: to Organic Production on Sat, Jul 4, 2026
Updated: Sat, Jul 4, 2026