22 Sept 2021
Tim Potter
Job Title
Image: © Chris Brignell / Adobe Stock
The role of minerals in animal production is a key area of interest for farmers, nutritionists and veterinary surgeons.
Adequate trace mineral intake and absorption is essential for a variety of physiological functions including immune response, reproduction and growth. The minerals are usually classified into macro elements (calcium, phosphorus, potassium, sodium chloride and magnesium) and trace elements (copper, cobalt, selenium, manganese, iodine, zinc, iron, molybdenum and chromium) depending on the quantities (National Research Council, 2001). This list is not exhaustive, with Underwood (1981) identifying 22 elements that are essential for animal life.
The “trace elements” are those elements existing in natural and perturbed environments in small amounts, with excess bioavailability having a toxic effect on the living organism (Bhalakiya et al, 2019). The functions performed by minerals in animals can be divided into four broad types (Underwood and Suttle, 1999).
While only required in small amounts, the functions performed by trace elements can only be fulfilled if they are ingested, absorbed and retained in sufficient quantities to keep up with growth, development and reproduction, and to replace that “lost” either through production (such as milk) or simply as part of the process of living where often unavoidable loss exists via the faecal route in the form of sloughed mucosal cells, microbial residues or as unabsorbed digestive secretions.
The exact amounts required by an animal will be determined by a number of factors such as age, reproductive status and growth rate, and these demands need to be factored in correctly when considering diet formulation or assessing the mineral status of an individual or group of animals.
Under normal circumstances, livestock obtain most of their minerals from the feeds and forages they consume, and, therefore, their mineral intakes are directly influenced by the mineral contents of the constituent parts of their diets. The mineral composition of plants depends largely on four factors:
The importance of the factor will depend on the mineral being considered, but is also influenced by interactions between the factors as well as with the management of the crop, for example, by soil management, use of fertilisers, irrigation and crop rotation. While knowledge of the mineral content of the diet is important, it is essential to also consider the bioavailability – that is, the proportion of the minerals fed to the animal that it can actually utilise. Bioavailability has four components:
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 actually required and avoiding excess feeding, which, for some elements, can result in signs of toxicity.
The deficiency of trace minerals in the diet alone can reduce animal production by 20% to 30%. Therefore, supplementation of trace elements in animal diets has long been practised to ensure their rapid growth, boost reproductive performance and improve immune response (Overton and Yasui, 2014).
Mineral supplementation strategies quickly become complex because of the differences in trace mineral status and requirements of livestock dependent on species and stage of production, and it is critical to tailor the provision to the actual requirements to obtain optimum production in modern animal production systems.
Alongside the direct impact on animal production, the environmental impact of mineral supplementation practices is now coming to the fore, with today’s producers being faced with many challenging issues in reference to sustainable agriculture. In the near future, regulations may possibly limit the level of trace minerals fed to reduce the amount found in animal wastes, and more targeted and evidence-based supplementation certainly has a place.
Given the large number of different bodily processes trace elements are involved in, the signs of deficiency can be very varied and non-specific. It is worth remembering that subclinical or marginal deficiencies may be a larger problem than acute mineral deficiency, because in these instances specific clinical symptoms are not evident to allow the producer to recognise the deficiency; however, animals continue to grow and reproduce, but at a reduced rate. It is in these situations where proactive monitoring and regular reviews can provide real benefit to clients, and provide opportunity of vets to engage with their clients.
Ultimately, determining the mineral status of production animals is important when developing an optimum health programme. As animal trace mineral status declines, immunity and enzyme functions are compromised first, followed by a reduction in maximum growth and fertility, and finally normal growth and fertility decrease prior to evidence of clinical deficiency (Wikse, 1992).
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 now being promoted as ways of reducing antimicrobial usage on farm.
The impacts of trace elements on reproductive function are wide ranging, and it is beyond the scope this article to go into the specific impacts of each element. The reproductive performance of cattle has been shown to be compromised if zinc, copper, or manganese status is in the marginal-to-deficient range. Symptoms of copper deficiency include delayed or suppressed oestrus, decreased conception, infertility and embryonic death.
Deficiency in zinc has been associated with decreased fertility, manifesting as abnormal oestrus, abortion and altered myometrial contractility with prolonged labour. Manganese deficiency in adult cows results in suppression of conception rates, delaying return to oestrus in post-partum females. In young prepuberal heifers, insufficient manganese has been associated with infertility, abortion, immature ovaries and dystocia.
Clinically, the most common reason to investigate the trace mineral status of ruminants is because performance is below expectation. Accordingly, the assessment is done to determine the presence or prevalence of nutrient deficiencies (or toxicities) within a herd or flock.
When approaching this type of investigation, it is important to determine the most appropriate measurement criteria to maximise the diagnostic value of any samples taken, and avoid wasting time and money. As previously discussed, economically important effects on performance and health of animals can be affected by trace element deficiencies – even before clinical signs are evident. Physiological functions are progressively affected by deficiencies, so it is important to avoid simply ruling out deficiencies based on presenting clinical signs, which may only present in the most extreme stages of clinical deficiency.
Blood measures are frequently used in assessment of trace element status because they are significantly correlated to nutritional status of some trace elements and blood is less invasive to sample than liver. However, blood analyses has several limitations. Most of these surround the representative nature of the samples, due in part to the relatively 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. Whole blood concentrations of selenium and iodine are useful.
Whole-blood concentrations of copper, iron and zinc are not adequate to determine an accurate concentration. Copper, iron and cobalt liver biopsy samples are more sensitive measures of status than are serum/whole blood concentrations. When utilising blood samples, care should be taken to use the proper sampling method. The removal of the serum from the clot within two hours of sample collection and minimisation of haemolysis is critical for an accurate serum sample.
Liver is the organ that often represents the status of several trace elements in animals (McDowell, 1992). Copper, iron and cobalt liver biopsy samples are more sensitive measures of status than serum/whole blood concentrations are. A more detailed overview of the procedure for liver biopsies is provided in Panel 1.
Forage and diet analyses provide useful supporting data if representative samples of all feeds can be obtained, and it is essential to consider all of the animals’ intakes, as well as any other supplements provided. Actual chemical analyses need to be performed, and should include those elements with important interactions (for example, molybdenum, sulphur and iron). Determining the mineral concentration of the diet does not tell you what the mineral status of the animal is because we are not able to determine bioavailability of the minerals in the diet easily, and in most circumstances, we are unable to quantify exactly the amount the animal is consuming. However, it is an important part of being able to manage trace element status.
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 in the diet.
Trace elements are required for numerous metabolic functions in livestock, and optimal production and performance require intakes to be properly balanced with an animal’s requirements. As trace mineral status of the animal declines from adequate to marginal, immunity and enzyme function is compromised. This is then followed by the loss of performance and reduced reproductive performance.
Animals in a subclinical or marginal deficiency status are often difficult to identify; however, proactive monitoring and targeted interventions can result in improved production, immunity, growth and reproductive performance.
