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Adjusting mineral supply to the roughage is a widely known concept and sounds logical at first, but biological systems operate in a more complex manner. It starts with the fact that determining the exact amount of minerals horses obtain from roughage is not feasible. Of course, you could have samples of each hay bale or batch analysed.

Assuming that the hay is sourced from diverse meadows rather than monoculture fields, it becomes apparent that the distribution of plants across the meadows is not uniform.

Herbs and grasses tend to form clusters or “nests” within specific areas. The larger the meadow, the more varied the vegetation in different sections. Even a gentle slope can foster different plant species on its south side compared to its north side. Similarly, a small ground depression can retain moisture for longer durations after rainfall, resulting in distinct vegetation patterns.

© Adobe Stock / Achim Banck

During harvesting, plants from a given area remain relatively close together rather than being homogeneously mixed with plants from other sections. Consequently, hay bales from such areas exhibit accumulations of plants in identifiable nests. You can see it for yourself: unroll a round bale in the stable aisle and walk along it. Depending on the sampled area of the bale, you can end up with substantial variations in mineral values.

One might argue that multiple samples should be taken from different parts of a bale (outer and inner sections) as well as from several bales to create a composite sample that approximates the average mineral content of the meadow. However, this does not reflect the actual way horses consume hay. When feeding hay, different bale layers and separate bales are typically not mixed together. They are fed one after the other, only starting the next bale once the previous one is fully gone.

As a result, the hay’s mineral content can vary significantly from one feeding to another.

Furthermore, if hay from different meadows and cutting times is involved, it is necessary to take a composite sample from each batch to determine their respective mineral contents. A batch refers to a single cut from the same meadow, and subsequent cuts constitute subsequent batches. This is because mineral content can vary greatly not only between meadows but also between cuts or years. The first cut of a meadow in one year may have vastly different mineral contents compared to the same meadow’s cut at the same time in a previous year.

Stabled horses

Determining the amount of hay consumed by each horse presents challenges. For horses kept in stables, it is quite easy to determine how much hay was eaten. By weighing the amount of hay offered and subsequently measuring the amount left in the stables that needs to be mucked out, one can gauge the horse’s intake accurately.

It is important to ensure that any moist hay removed (e.g., due to urination) is dried to a residual moisture content of 10-15% before weighing to avoid distorting measurement results. This method provides a reasonably precise estimation of the horse’s actual intake, assuming that the bedding material is non-edible.

If the horse is on straw bedding, it becomes difficult to determine the exact amount of straw consumed. Additionally, it is not possible to accurately measure the availability of chemically measurable minerals contained within the straw. Unlike hay, straw’s woody nature makes it less efficiently broken down by horses in the large intestine. Analysing mineral content that was more accessible to the horses from the phloem and xylem, the plant’s conducting tissues, would be necessary, taking into account the proportion trapped within the cells. Unfortunately, such analysis is not currently feasible, making it challenging to determine the precise mineral intake of the horse.

Three horses looking over the doors of their stables
© Adobe Stock/castenoid

Horses in open yard groups

Determining hay consumption in an open yard is also challenging. Although a certain amount of hay per horse can be weighed and placed in the hay rack, it is impossible to know precisely if the horse has actually consumed that amount. Horses’ eating speeds vary greatly and can increase up to four times. Therefore, if a particularly greedy horse is present in a group, it is likely to have consumed a more significant portion of the hay than its fair share, leaving less for others.

The same principle applies to grazing on pastures. Horses do not graze uniformly like lawnmowers; they are selective eaters. Thus, even if samples from a pasture are submitted and analysed, it does not guarantee that the horses have consumed the plants represented in those samples. Furthermore, determining the exact amount of grass consumed during grazing periods can be challenging, as intake amounts can vary considerably. Once again, a horse’s eating rate can increase by a factor of four. Analytical data obviously significantly differs depending on whether a horse has consumed 10 kg or 40 kg of fresh grass. Without precise measurements of roughage intake, it is impossible to determine the exact amounts of minerals ingested by the horse, rendering even the most accurate analysis data useless.

© Adobe Stock/leszekglasner

From this perspective, it becomes apparent that the theory of precisely matching mineral feed to the mineral content of the roughage does not work effectively in practice in a species-appropriate horse keeping context.

Mineral stores

The good news is that horses have evolved over approximately 50 million years to adapt to fluctuating mineral supplies from roughage.

Their bodies have storage systems for various minerals. For example, calcium is stored in bones, excess phosphorus in muscles, and temporary reserves of zinc, copper, and iron can be found in the liver. When feed contains sufficient amounts of a specific mineral, it is absorbed and stored. If the herd moves to an area with vegetation that provides fewer amounts of that mineral, the body draws from its stores to ensure smooth metabolic function. Once the forage contains more of that mineral again, the stores are replenished. Additionally, the horse’s intestinal wall possesses a mineral sorting mechanism. Minerals are not simply absorbed from the intestine into the body – that would be fatal. Instead, the process is highly regulated, involving mineral receptors and transporter molecules in the intestinal wall cells. This intricate process is controlled by the nervous and hormonal systems.

For instance, calcium is typically present in the horse’s primary feed with an immense surplus compared to consumption, as well as in relation to phosphorus (which are regulated in interdependence). Consequently, not all calcium is absorbed from the food bolus, with a significant portion remaining in the intestine and excreted with the faeces. Moreover, any excess calcium that is absorbed but not consumed can also be excreted.

The kidneys play a vital role in regulating the mineral balance through the excretion of minerals. As a result, horses’ urine is usually milky due to the excess excretion of calcium carbonate. This phenomenon does not occur in carnivores, as they tend to have lower calcium levels relative to phosphorus. Thus, phosphorus is more likely to be excreted, and the urine appears clear. The relationship between the intake, storage, and excretion of calcium and phosphorus is regulated by various factors, including the parathyroid hormone and calcitonin, which interact with vitamin D3.

In horses, vitamin D3 is either produced in the skin under sunlight exposure or derived from the precursor vitamin D2 found in the primary feed when sunlight is insufficient. The intricate details of this extensively studied physiological process can be found in any textbook on physiology. Similar regulatory mechanisms exist for all minerals. Their uptake is already controlled, they are stored for times of deficiency, and, apart from selenocysteine, they can be excreted without issues.

However, in cases of extreme oversupply or the use of organic minerals, most of which are not naturally occurring, these natural regulatory mechanisms can be overridden. This can lead to oversupply or poisoning. Examples for these cases are iodine from seaweed, when it is the sole source of mineral feed, or selenium in the Great Plains of the USA, where selenium indicator plants excessively accumulate this trace element.

Due to these concerns, feed regulations include maximum allowable amounts for many minerals that must not be exceeded.

Feeding minerals based solely on the requirement values provided in textbooks on feeding is largely futile.

On one hand, one must subtract the portion obtained through the primary feed (which presents various problems, as mentioned above) from the requirement value. On the other hand, the requirement values found in books are typically derived from Thoroughbred racehorses or sport warmbloods. This selection is primarily based on the need for a statistically significant evaluation, which requires a large number of horses from the same breed that ideally share the same feeding and training conditions. Thoroughbreds and sport horses fulfill these criteria. However, a mixed open yard herd with different individual feeds does not offer a sufficiently standardised basis for meaningful statistical evaluations.

Meanwhile, studies have revealed that Thoroughbreds and warmblood horses have metabolic differences compared to ponies and robust horses. Additionally, it can be assumed that the feeding practices for Thoroughbreds and warmblood horses are not exactly in line with species-appropriate methods (often involving insufficient hay, long breaks between meals, and high amounts of concentrated feed). Their higher workload and resulting higher mineral consumption is significantly above those of leisure horses. Consequently, professionals have been discussing for some time that the daily requirement values stated in literature are too high for most horses. Nevertheless, at this point in time, the specific requirement values for horses such as the ambitiously ridden Icelandic horse, the leisurely strolling Gypsy Vanner, or the Haflinger at risk of laminitis remain uncertain.

© Adobe Stock / cynoclub

Determining such values is also complicated because the body possesses storage mechanisms that buffer deficiencies over long periods of time. This means that a horse is more likely to experience bone problems due to demineralisation than the owner detecting a calcium deficiency through a blood test.

Furthermore, there is no possibility to feed horses completely mineral-free. Hay and grass typically contain so much iron that you cannot induce an iron deficiency (which is also highly recycled by the liver from hemoglobin), unless you extract several liters of blood from the horse or infest it massively with worms. But these are not natural circumstances.

Additionally, there are minerals where it has long been recognised in professional circles that blood values have no correlation with the values within the tissue, such as selenium. Other minerals, like sulfur, cannot be detected in the blood because they occur exclusively in a bound form. Therefore, even with the best blood test, we cannot timely detect a deficiency for most minerals in order to supplement accordingly. Only copper and zinc provide some meaningful information in the blood test. If they are deficient, it could be due to either inadequate supply or excessive consumption. This could for example be through disrupted detoxification processes. Hence, theoretically, targeted mineral supplementation according to the horse’s needs may sound like a good idea. But in practice, it simply does not work within biological systems.

Horses eating mineral feed
© Adobe Stock/Annalene

In terms of feeding mineral supplements, this pragmatically means that it doesn’t matter exactly how many milligrams of each mineral are present or whether the calcium-phosphorus ratio is 2:1 or 4:1. The horse’s organism can regulate these details naturally based on the mineral content already obtained through the primary feed.

What is important is that mineral supplements are offered regularly and intermittently, allowing the horse to replenish its mineral stores whenever they are depleted. Minerals that are still plentiful in storage remain in the intestines and are excreted through faeces or can be eliminated through urine. This helps to compensate for variations in the primary feed that may not always be evident in hay and grass.

For example, many hay samples have insufficient copper content because the plant’s copper uptake from the soil depends not only on the vegetation but also on the level of molybdenum in the soil. When copper stores become depleted, they need to be replenished, and this can be achieved through a mineral supplement that includes copper along with other minerals.

Consequently, offering minerals through mineral licks (as long as they are not based on candy-like substances) is just as effective as using regular (non-flavored) mineral feeds.