28 Jun 2022
Tim Potter BVetMed, PhD, MBA, CMgr MCMI, MRCVS outlines the nutritional role these play, how to detect deficiencies and how to appropriately supplement them.
Tim Potter
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Image: © Anthony Brown / Adobe Stock
Many trace minerals have very specific, but often multiple roles.
For example, selenium is necessary for growth and fertility in animals, as well as the prevention of disease.
As well as individual trace minerals having several functions, several trace minerals may be involved in a single function.
For example, selenium, zinc and copper are all involved in immune function; this means that, rather than considering the role of each trace element, it is more helpful to consider the role of trace minerals as a whole in some of the more common problems encountered in livestock.
Twenty-two elements have been identified as essential for animal life (Underwood, 1981), with the functions they perform being classified in four broad types: structural, physiological, catalytic or regulatory.
The elements are usually further classified into macro or trace elements based on the quantities required. The commonly referenced macroelements include: calcium, phosphorous, potassium, sodium chloride and magnesium. Trace elements include: copper, cobalt, selenium, manganese, iodine, zinc, iron, molybdenum and chromium (National Research Council, 2001).
An animal’s exact trace element requirement varies depending on its physiological state, with factors such as age, reproductive status and growth rate all having an impact.
To avoid deficiencies, trace elements must be ingested, absorbed and retained in sufficient quantities to keep up with growth, development and reproduction, as well as to replace that “lost” as part of normal maintenance.
Nutrition is the largest expense in food animal production, and has the greatest impact on health and productivity of the animals (Ensley, 2020).
Within this, trace elements are an essential part of an animal’s ration, and a fine balance exists in providing the small amount that is actually required and avoiding excess feeding, which for some elements can result in signs of toxicity.
The deficiency of trace minerals in the diet can reduce animal production by 20% to 30%, and this has been a large driver of trace element supplementation, often in the absence of the diagnosis of a true deficiency (Overton and Yasui, 2014).
With the environmental impacts of agricultural practices increasingly under scrutiny, and producers looking for more sustainable practices, a place for more targeted and evidence-based supplementation certainly exists. Veterinary surgeons are uniquely positioned to advise both on the implementation and monitoring of supplementation programmes.
Trace minerals that have been identified as important for normal immune function and disease resistance include zinc, iron, copper, manganese and selenium.
A deficiency in one or more of these elements can compromise immunocompetence of an animal (Beisel, 1982; Suttle and Jones, 1989).
Alongside the potential productivity impacts, the potential medication requirements mean that in light of the industry’s desire to reduce disease incidence, and minimise the use of medications such as antimicrobials, the link between immune function and trace element status has driven increasing interest in this area, and growing numbers of treatments are being promoted as ways of reducing antimicrobial usage on farm.
The role of trace mineral source on reproductive performance is complex –especially given the large number of processes each element participates in, and the essentiality of each in all aspects of development (Hostetler et al, 2003).
While reproductive efficiency is critical to the success of any livestock production system, it is the last priority for nutrient partitioning within the animal.
Olson et al (1999) proposed that nutrient priority first addresses that of basal metabolism and then reproduction function secondly.
Copper deficiency has been shown to be responsible for early embryonic death and resorption of the embryo, increased chances of retained placenta and necrosis of placenta, as well as reduced fertility associated with delayed or depressed oestrus. Copper is equally important for male fertility, with deficiencies being associated with poor semen production.
Abnormal levels of zinc are associated with decreased conception rate, abnormal oestrus and abortion. Zinc acts as a cofactor and coenzyme of many enzymes and various reproductive hormones, and is involved in the formation of prostaglandins from arachidonic acid.
After calving, zinc also plays an essential role in the maintenance and repair of uterine lining.
Manganese deficiency in adult cows results in suppression of conception rates and delayed return to oestrus in postpartum females. In young prepuberal heifers, insufficient manganese has been associated with infertility, abortion, immature ovaries and dystocia.
Iodine affects reproduction due to its action on the thyroid gland, with both conception rate and ovarian activity being reduced with the impaired thyroid functions. Iodine is also regarded as essential for the developing foetus.
Deficiency of iodine increases the abortion rate, the incidence of retained placenta and postpartum uterine infections. Subclinical iodine deficiency is characterised by increased stillbirths, suppressed oestrus, increased chances of retained placentas and prolonged gestation periods.
Forages provide minerals that are vital for nearly all processes within the body. However, forages often do not provide enough minerals to meet the requirements of livestock.
Many factors affect the mineral content of forages, including: soil, plant species, stage of maturity, yield, climate, and management practices.
The key reason for different trace element deficiencies is variable geology and soils; consequently, they are more likely to show up where the ration is mainly grazed grass or conserved forage.
Soil is one of the most important resources on a farm, providing the basis for forage and crop production. Farm management practices can change the chemical, biological and physical balance over time, and it is important that soils are properly managed to maximise forage and crop yields.
Regular, comprehensive soil analysis will identify chemical and mineral imbalances as well as biological activity, enabling soil improvement programmes to be put in place to help enhance crop and forage output, and, in turn, improve animal health.
Trace mineral deficiencies in livestock are often divided into two distinct categories:
Given the large number of different bodily processes trace elements are involved in, signs of deficiency can be very varied and non-specific.
Clinically, the most common reason to investigate the trace mineral status of ruminants is general poor performance (for example, reduced growth rates or lower than expected fertility).
This can represent a diagnostic challenge because a significant time lag can exist between when the deficiency occurred/started, when the problem is identified and the investigations started.
Given this, and that marginal deficiencies are a larger problem than acute mineral deficiencies, proactive monitoring and regular reviews can provide real benefit to our clients. Ultimately, determining the mineral status of production animals should form a key part of any animal health programme.
Blood measures are frequently used in the assessment of trace element status because they are easy to collect and are significantly correlated to nutritional status of some trace elements.
Blood analyses are not without their limitations. The long lifespan (160 days) of red blood cells and the homeostatic control mechanisms, which can limit changes in blood/serum concentrations of some trace minerals until endogenous reserves are substantially depleted, can make interpretation difficult, and mean that marginal deficiencies are difficult to detect.
Whole blood concentrations of selenium and iodine are useful clinical measures; however, for other elements, such as copper, iron and cobalt, liver biopsy samples provide a more sensitive measure of status (a more detailed overview of the procedure for liver biopsies can be found in the article by Aitken, 2015).
Careful sample handling is essential to get the most accurate results from blood samples. Serum should be removed from the clot within two hours of sample collection, and minimisation of haemolysis is critical for a representative serum sample.
Forage and diet analyses provide useful supporting data if representative samples of all feeds can be obtained. Actual chemical analyses need to be performed and should include those elements with important interactions (for example, molybdenum, sulphur and iron).
While knowledge of the mineral content of the diet is important, it is essential this is considered within the context of the quantities being consumed and the bioavailability.
If the trace minerals are found to be adequate in the diet, but the animals are found to be deficient, dietary or drinking water mineral antagonism may be occurring.
High sulphur or iron levels are examples of minerals that can cause deficiencies in copper and selenium, even though adequate concentrations of the latter are present in the diet.
Inorganic mineral sources are usually the main route of supplementing any calculated shortfalls of trace elements, due to their availability and relatively low cost. These are typically in the form of sulphates, phosphates, chlorides, carbonates and oxides.
Bioavailability differs between the different compounds (sulphates are generally more available than oxides). Copper and iron oxides should not be included in ruminant diets and/or supplements due to their low bioavailability.
Organic mineral sources, known as chelates, are generally bound to either amino acids or proteins (the exception is selenium, which is sourced as a yeast). They have a greater bioavailability and enter target tissues at higher levels compared to their inorganic counterpart.
Chelates are significantly more expensive than the inorganic forms, so a greater improvement in health or productivity for farmers must be observed to see a return on the investment.
Free choice minerals (such as licks or blocks) are widely used, but intake can be variable. The widespread belief that animals are aware of any mineral deficiency, and will only take what they need, is unproven and should not be relied on.
Mineral mixes, licks and block intakes, both between animals in the same group, and across breeds and farms, are significant with studies reporting ranges of 0g/d to 974g/d for cows and 0g/d to 181g/d for calves. As well as the variation in intake, shared licks pose the risk of disease transmission between cattle, but they also attract wildlife, in particular badgers, which poses a TB threat to the herd.
Providing in-feed mineral supplementation can be an easy way to address the mineral needs of cattle. Animals are more likely to get the required amounts than from licks or blocks, but it is worthwhile remembering that when feed intakes drop, either because of disease or around events such as calving, intakes of essential minerals will also fall.
In-feed administration is also not appropriate all the time – for example, while animals are at pasture – and can be challenging when animals are transitioning between different diets – for example, at weaning.
Direct administration of the required minerals to animals – via injections, oral drenches or ruminal boluses – has the advantage of ensuring each animal receives the prescribed dose of minerals and trace elements, and administration can be targeted to coincide with specific risk periods.
This is often viewed as more labour intensive, requiring handlings and so forth, but the benefits of improved performance and reduced disease can mean the payback is worth it.
Top dressing pasture provides a longer-term solution to raise the level of trace elements in grassland.
This can be effective in the case of simple deficiencies, but not where availability is restricted by other factors, such as alkaline soils.
However, it requires a disciplined approach with accurate records of application rates, timing and needs to be undertaken carefully as part of holistic soil and environmental management on farm.
Trace minerals greatly impact cattle health and performance. However, several factors may impact the effectiveness of supplementation.
It is essential the mineral status and requirement of the animal is known prior to supplementation, as several minerals can have no added benefit or even be detrimental if supplemented in excess.
Mineral needs are not constant throughout an animal’s lifetime, but rather fluctuate depending on an animal’s physiological state (such as pregnancy status, lactation status, age and so forth). This impacts the timing of when specific minerals should be supplemented – whether it be during gestation, following parturition or during periods of growth.
The knowledge of status and requirements enables us to design and implement programmes of mineral supplementation that are both biologically and economically beneficial.
