Feeding earth (clays) to animals is not new or abnormal. Wild animals always consume a healthy portion of earth alongside their feed, especially those that look for food below ground, such as pigs, but fine earth is found everywhere in the form of dust. In fact, young animals are expected to consume some amounts of earth (which is rich in iron, by the way) that their mother’s milk is very deficient in — perhaps nature’s way to conserve efforts where something is provided already in abundance.
Clay is composed of very fine grains of earth or rock (silica) combined with one or more minerals (calcium, sodium, potassium, etc.), giving form to different classes of clay. The very basic properties of all clays beyond their fine granular nature are a high affinity to water and plasticity — hence, mud.
Wild animals always consume a healthy portion of earth alongside their feed, especially those that look for food below ground.
In brief, earth is a natural product, and animals are well accustomed to consuming it — but this does not imply we should start feeding mud to our farm livestock! In contrast, in today’s modern farming world, it is imperative to look at natural earth compounds to find suitable additives that will optimize the quality of feeds.
Unique properties of bentonite
Such a product is bentonite, a mineral clay deriving its name in 1898 from a natural deposit called Benton Shale near Fort Benton in Wyoming, U.S. Of course, bentonite is not unique to the U.S., as its counterparts can be found all over the world. In fact, in Europe, a similar if not identical mineral clay is called montmorillonite, after the French town Montmorillon, where it was first discovered in 1847 (the clay, not the town). But we will call it bentonite, if only because it is easier to write and pronounce. It is interesting to note that scientists still debate nomenclature of all these clays, something that escapes the scope of this work.
Learn more: Can clays fight Salmonella in broilers?
Bentonite, thus, is a mineral clay, and more specifically an aluminum phyllosilicate clay. In other words, it contains silicon dioxide (earth) and aluminum, bound with two major minerals (calcium or sodium) that give bentonite unique industrial properties and possible applications in feeds for monogastric animals.
The major characteristic of sodium bentonite is that it expands and absorbs water several times its mass. This mineral clay is used as a gut “sponge” that can absorb excess moisture from digesta, something that can happen due to pathological disturbances (bacterial diarrhea, for example) or nutritional imbalances (osmotic diarrhea). In either case, it helps control digesta moisture and, along with other additives, can prevent or alleviate the symptoms, but not the cause, of wet droppings (poultry) or watery fecal matter (pigs).
The usual inclusion rate for this purpose is up to 20 kg per metric ton, or 2 percent, at which level it can affect the plasticity of feeds being thermally processed. Feeds that are being pelleted (most often) or expanded (rarely) containing high levels of sodium bentonite will have different handling characteristics and final product traits compared to non-fortified formulas. Bentonite will absorb water usually added as steam during thermal processing and make the conditioned mass more plastic, but it will also harden the pellets or extrudate upon drying. We need to keep in mind that clay is a fragile element after being baked, and if this is the only pelleting aid, it can result in more fines than anticipated.
In contrast with sodium bentonite, the main characteristic of calcium bentonite is not that of a water absorbent material. Instead, its main trait is the adsorption (binding) of ionic particles — such as trace minerals, but also some mycotoxins. Thus, using calcium bentonite is a delicate exercise. It is one of the most widely-used additives in controlling aflatoxin contamination (used at levels of 0.5 to 1.0 percent), but has no effects on any other mycotoxin.
When used as an aflatoxin binder, care should be taken to ensure diets are well fortified with all possible ionic compounds that can be bound by calcium bentonite. Here, it is required mentioning that in the presence of sodium in solution (such as in the conditions found in the gut of animals), calcium bentonite can be converted to sodium bentonite. Thus, given enough time, calcium bentonite will lose some of its strong binding properties and start exhibiting the strong swelling properties of sodium bentonite. In rapid-growing animals consuming large amounts of feed (rapid transit time), this is not much of an issue, but it still merits considering when using a bentonite product.
So far, it is evident that bentonites are used either to absorb water or bind aflatoxins. They do have, however, some other side effects. The first is that of being a mild laxative. This might be of interest in diets for adult animals consuming diets poor in fiber but is also a concern in diets for very young animals. Thus, a moderate inclusion rate might be better than a higher one. On the other hand, feeds containing high levels of bentonite will withstand better the harsh conditions of humid climates because bentonite acts a natural desiccant. Finally, the use of bentonites in liquid feeding requires some attention because its colloidal nature can provide benefits but also cause disturbances.
I have used up to 2 percent bentonite in many commercial feeds without any troubles, but I am always aware of the possible side-effects. When I start new, I prefer to use only 1 percent as my upper limit. Other clays can substitute bentonite, and I have done so by pairing the main purpose of using bentonite with the characteristics of the replacement clay. In fact, for me, bentonite has been the first clay to use, hence my familiarity with the product. Other clays are at least as efficient or as problematic as bentonite.