Trace minerals are an important nutritional requirement in animal production that vary throughout the animal’s lifecycle. They are involved in several important biochemical processes; functioning as essential co-factors for enzymes involved in metabolic processes and the normal functioning of vitamins and hormones. For animals to perform optimally, it becomes essential to understand the different forms of minerals that are available on the market and how to supplement animals according to their nutritional requirements.
Forages and other feedstuffs consumed by the animals are an important source of trace minerals, however, the bioavailability of trace minerals naturally present in feeds is not well known. Bioavailability may be defined as “the proportion of an ingested mineral that is absorbed, transported to its site of action and converted to the physiologically active species” (Byrne & Murphy, 2022). Many factors can affect the bioavailability of a mineral such as animal factors, chemical aspects, dietary factors, environmental factors, evaluation of minerals.
If these factors are not considered, mineral deficiencies can arise and are typically classed into two groups: clinical or marginal. Clinical deficiencies are highly unlikely to occur, whereas marginal mineral deficiencies are more frequent. Marginal deficiencies can reduce growth, milk production, reproduction or the health status of an animal while showing little to no signs of a deficiency.
Inorganic mineral sources
Inorganic trace minerals are often considered as the cheaper source of minerals in animal nutrition. These are mineral salts that include sulphate-, oxide- and carbonate sources of trace minerals. Sulphate trace minerals are water soluble in comparison to oxide- and carbonate minerals but can be partially soluble under acidic conditions.
The bioavailability of inorganic trace minerals is low and as a result of that, high amounts need to be supplemented in the diet to fulfil the mineral requirements of the animal. The source of the inorganic mineral is important as they differ greatly in purity and other factors such as the type of inorganic mineral used, presence of antagonistic compounds in the diet and the factors mentioned in Table 1 can further influence its bioavailability.
Organic mineral sources
Organic trace minerals are generally believed to be more bioavailable than inorganic minerals. Organic sources are based on the type of ligand, either an amino acid, peptide, polysaccharide or organic acid, that is bound to the mineral. The functionality and pH stability differ between different organic sources. The main idea behind complexing minerals and organic components is that it may increase passive absorption of minerals in the intestine by decreasing the incidence of interaction between the mineral and possible antagonistic substances present in the diet. This prevents the formation of insoluble complexes forming with substances such as carbonates, phosphates, oxalates, etc., that would lead to the mineral becoming unavailable for absorption.
These organic compounds increase water and lipid solubility of the mineral, leading to increased passive absorption and favor mineral absorption over a wider range of pH values. It is important to remember that the absorption of an organic trace mineral will also be affected by different molecular weights, geometry, charge density and size of the chelate or complex formed. Grouping all organic trace mineral products together when there are large differences in chemical characteristics between different organic ligands is not suggested as it is unlikely that all organic minerals will have a better bioavailability compared to inorganic minerals (Byrne & Murphy, 2022).
The type of bond formed between the mineral and the organic group is very important. With an optimal bond or chelation, the strength of the bond could prevent dissociation in the digestive tract and allow the mineral to reach the intestinal brush border more efficiently, which leads to a higher bioavailability of organic minerals (Byrne & Murphy, 2022). However, if the bond is too strong the complex may not release the mineral in time and render it unavailable for use by the animal. If the organic mineral compound bond is too weak, dissociation is likely to occur, similar to the inorganic mineral forms.
Hydroxy mineral sources
Minerals can be covalently bonded to multiple hydroxy groups, forming hydroxychloride trace minerals (Spears, 2023). These are mostly insoluble at a neutral pH but become more soluble under acidic conditions, e.g. in the abomasum of the ruminant animal. This source of trace mineral is generally considered more bioavailable than inorganic sources, possibly due to less dissociation between the mineral and the hydroxy group. Spears et al. (2004) demonstrated the higher bioavailability of Cu hydroxy chloride versus Cu sulphate when fed in a diet high in Cu antagonists (Mo and S). The authors theorised that due to the lower solubility of hydroxy Cu in neutral and slightly acid environments the Cu was insoluble in the rumen which prevented the formation of stable complexes and allowed a greater supply of the Cu hydroxy chloride to enter the small intestine and be absorbed. However, when diets fed to the growing cattle were not high in dietary antagonists for Cu, both Cu sulphate and Cu hydroxy chloride showed a similar bioavailability.
Absorption pathways in the gut
Most trace mineral absorption occurs in the small intestine, although absorption can occur anywhere along the gastrointestinal tract. Studies using organic trace minerals have reported that they can be absorbed via the amino acid or peptide transport pathways more effectively than through the general mineral uptake pathway, which could be another explanation for their increased bioavailability. Organic trace minerals have two pathways of being absorbed, they can either be absorbed intact via the specific pathway of the ligand (amino acid, peptide, etc.) or the mineral can dissociate from the complex and be absorbed alone. When minerals are absorbed alone the mineral is bound by a chaperone protein following dissociation to prevent sub-cellular damage occurring which aids in maintaining homeostasis within the cells. When minerals are fed in high enough concentrations, many minerals can use the paracellular absorption mechanism, where the mineral is able to diffuse across the tight junction. This occurs when the minerals are ionized and in a higher concentration in the fluid on the luminal side of the tight junction compared to the extracellular fluids in the interstitial space. This diffusional force can be great enough to allow the ionized mineral to push through the tight junction and into the interstitial space from where it passes into the capillary endothelium and into the blood (Goff, 2018). Another method of absorption is solvent drag where the mineral moves with the bulk flow of water between intestinal epithelial cells to enter the blood. Complexed organic minerals are also able to be absorbed using the solvent drag if they are soluble and small in size. At lower concentrations the body relies on transcellular absorption which requires transport proteins to move the mineral across the membrane.
Studies have shown varying benefits with the use of organic minerals versus inorganic minerals, some studies show little to no effect of organic mineral addition while others have shown vast improvement in the animal’s welfare and production. In addition to the benefits in animal production there is also an environmental benefit to using organic minerals. Less minerals are excreted in the urine and faeces, leading to less impact on the environment. However, excess minerals will still be excreted. Firstly, lower levels of organic minerals are required in animal diets and secondly, due to the higher bioavailability of the minerals, they are more readily used by the animal leading to lower levels being excreted. A review by Byrne & Murphy (2022) highlighted a few studies and their findings on the bioavailability of trace minerals and are summarised in this article. A study in beef animals showed an improvement in growth performance, health and antioxidant status and a strong impact on the animals’ metabolism and immune functions when inorganic trace minerals were replaced by organic trace minerals. In dairy cows the use of organic minerals showed higher milk yields, protein synthesis and milk lactose content. Additionally, the pregnancy rates improved and mastitis rates were also lower. Poultry studies have also shown advantages to the use of organic minerals, broilers receiving organic minerals at a level of 25% of the commercial level for inorganic minerals showed no signs of deficiency. The lower levels of organic minerals have little effect on feed efficiency or body weight gain in the broilers. In swine the total replacement of minerals with organic trace minerals improved the average daily gain and lead to an increased slaughter weight of 2kg and improved slaughter efficiency in one study which could help in reducing costs. Replacing the inorganic trace minerals with lower concentrations of organic trace minerals did not adversely affect the growth performance of piglets. The hydroxychloride trace minerals also appear to have a higher absorption rate in comparison to inorganic sulphate trace minerals and are considered more bioavailable, however, the hydroxychloride minerals are a newer product on the market and not much information is available on these minerals. In dairy cows hydroxychloride minerals have been shown to improve neutral detergent fibre (NDF) digestibility and lower the ruminal butyric acid concentration in the rumen compared with cows fed sulphate trace minerals. In broilers the hydroxychloride trace mineral source resulted in higher mineral concentration in the liver and plasma and showed a greater apparent ileal absorption of copper and manganese ions compared to the sulphate forms.
In conclusion
While inorganic minerals remain a viable option for supplementation due to their cost-effectiveness, their low bioavailability requires careful management to avoid deficiencies. Organic minerals, with their higher bioavailability, present notable advantages for animal health and productivity, despite their higher cost. Including organic minerals in animal diets during critical periods, such as the start of lactation or times of increased stress, can enhance overall animal performance and health. Therefore, strategic use of both inorganic and organic minerals can optimize nutritional supplementation, ensuring animals receive the necessary trace minerals for optimal growth, production, and health.
