There’s a lot of myths about horse’s mineral feeds out there. Increasingly, horse owners are expressing the following statement “They (internet sources or stable mates) say that horses can’t metabolise synthetic mineral feeds at all, which is why I want an organic mineral feed for my horse.” Unfortunately, this statement is complete nonsense. Why? To understand this, let’s take a step back.
In the wild, feral horses obtain their minerals from various sources. One of the most important sources is their primary feed. The plants they consume are typically rich in calcium, as plants absorb it effectively from the soil and use it for various purposes, including skeletal stability.
Plants also absorb other minerals from the soil quite well. Most of these minerals are necessary for their own metabolism, such as iron or copper. Some minerals, however, seem to have little or no impact on plant metabolism, as plants do not exhibit deficiency symptoms even when these minerals are absent from the soil. Selenium is an example of such a mineral..
Within plants, minerals exist in different chemical forms. For instance, sulfur is often bound to amino acids like methionine or cysteine. Other minerals are present as ions, such as Zn2+, and serve as cofactors that activate enzymes regulating plant metabolism.
Only a small proportion of minerals in plants are organically bound, while the majority exist in inorganic form. However, it’s important to clarify the meaning of these terms.
Mineral feed: organic vs. inorganic
Many people mistakenly believe that organic is synonymous with “biological” and inorganic with “synthetic.” Unfortunately, that is incorrect. hese terms originate from chemistry. Organic chemistry encompasses all molecules composed of carbon (organic molecules), while inorganic chemistry covers everything else.
Therefore, organic minerals are bound to molecules that consist of carbon atoms, usually amino acids. In contrast, inorganic minerals are bonded to other atoms, with compounds like oxides or sulfates commonly encountered. It’s impossible to determine whether a mineral comes from a synthetic or natural source based solely on its name.
When we examine organic minerals in feed, we find that they typically take forms that do not occur naturally.
These minerals are synthetically bound to organic molecules, creating unnatural compounds. In contrast, many inorganic compounds exist naturally. For example, lime (calcium carbonate) is an inorganic compound obtained through mining. Oxides and sulfates are also frequently found in nature.
To date, 300 different inorganic zinc compounds are known to occur in nature. Inorganic forms are also produced synthetically, mostly from naturally degraded starting compounds.
The crucial difference does not lie in the production process. Generally, organic compounds are more complex and therefore more costly to produce than inorganic compounds. The discrepancy arises in how the horse’s intestines utilise these minerals. It’s often claimed that organic compounds are more “biologically available” than inorganic ones.
Mineral feed: understanding “biological availability”
To grasp this concept, we must first comprehend how biological availability is determined in horses. Test horses are divided into subgroups and fed the different mineral forms at increasing doses.
The aim is to measure at what amount an increase in the mineral can be detected in their blood. At first glance, this seems reasonable, as the objective is to ensure efficient absorption of the fed mineral, allowing it to reach the bloodstream. However, this overlooks the “sorting function” of the intestinal wall.
Nutrients released during digestion or contained in the feed don’t simply pass through the intestinal wall without control. The intestinal lining is equipped with specialised cells that possess transporter molecules on their surface. Each transporter is responsible for a specific molecule or ion, serving as its substrate. Transporter molecules exist for sugar, fatty acids, vitamins, and minerals.
Mineral transportert
Mineral transporters are specific to certain minerals. For example, calcium is only absorbed by calcium transporters, while phosphorus is exclusively absorbed by phosphorus transporters. Mineral absorption generally occurs in the form of ions. Inorganic minerals dissolve (“dissociate”) within the food bolus, transforming substances like zinc sulphate (ZnSO4) into zinc (Zn2+) and sulphate (SO42-). These components are then absorbed through different transporters: zinc via the zinc transporter and sulphur via the sulphur transporter.
The activity of these transporters depends primarily on two factors: the presence of their target substrate in the food and the body’s storage status.
For instance, if a horse consistently has low levels of copper in its primary feed and its copper stores are depleted, the transporters for copper become more active. In fact, the intestinal wall cells may even produce more transporter molecules to extract copper from the food bolus as efficiently as possible. When copper levels in the feed increase, the horse can replenish its stores.
Once the stores are full, feedback mechanisms prompt the intestinal wall cells to reduce or halt copper absorption. Continuing to absorb copper excessively would necessitate filtering it out of the bloodstream through the kidneys and excreting it in urine.
This precise regulation ensures that minerals are absorbed (and excreted) within narrow limits, preventing elevated levels in the body (blood). However, the situation is entirely different when organically bound minerals are fed.
Mineral feed: Organic minerals overrule the natural selection mechanisms
These minerals are typically bound to amino acids and do not detach during the digestion process. Instead, the amino acid, which serves as a building block of protein, is recognised and absorbed by amino acid transporters.
It is only at this point that the body realises it cannot utilise this amino acid since it has bound a mineral in a place where it does not belong. Consequently, the amino acid is broken down and excreted.
This process releases not only urea, which must be excreted through the kidneys, but also the bound mineral. The mineral enters the bloodstream, is subsequently filtered by the kidneys, and eliminated through urine. This likely explains the increase observed in blood levels after feeding organic minerals.
However, this observation does not indicate the fullness of the body’s mineral stores or the cells’ ability to process the respective minerals.
Therefore, the term “bioavailability” is misleading in this context, as what is considered bioavailable is actually what is immediately absorbed and excreted. Moreover, all organic minerals taste unpleasant, requiring the use of sugar or similar tricks to administer them to horses.
Mineral feed: what and when to feed?
The choice between organic and inorganic minerals depends on whether you aim to provide a basic supply or address a confirmed deficiency.
For basic supply, it is advisable to opt for inorganic minerals. These minerals can be efficiently regulated by the body upon absorption, ensuring that any excess remains in the food bolus and is excreted with faeces, rather than overburdening the kidneys.
If the horse has a confirmed deficiency, such as zinc deficiency, it makes sense to administer organic zinc (zinc chelate) as a remedy. This allows for faster absorption of zinc, replenishing the stores more rapidly. Subsequently, the horse’s normal zinc requirement can be met by feeding inorganic mineral supplements once again.
Watch out: organic selenium!
Organic selenium, also known as “selenium yeast,” presents a unique case. It is bound to an amino acid, but in this case, sulphur is replaced with selenium in the amino acid structure. This results in the formation of selenocysteine or selenomethionine, which are the only sulphur-containing amino acids found in horses.
Sulphur is crucial as it imparts stability to proteins through disulphide bridges. Consequently, these amino acids are typically incorporated into proteins requiring a stable structure, such as in hoof horn, skin, hair, and various other areas of the body. Unfortunately, the body does not promptly recognise when selenium replaces sulphur in the amino acids.
As a consequence, the body incorporates these incorrect amino acids into proteins. However, they cannot form stable disulphide bridges with other amino acids. The outcome is the production of unstable proteins, which, at best, are broken down and eliminated. However, in the worst-case scenario, these proteins can damage vital structures.
During degradation, a significant amount of selenium is released, which is difficult to excrete. Consequently, it can contribute to substantial selenium surpluses in tissues that are seldom detectable in blood tests. Therefore, supplementing with organic selenium is highly inadvisable. It is preferable to gradually replenish selenium requirements using inorganic selenium found in mineral feed.
