Nutrients bioavailability

Each feed in the diet contains different and different quantities of nutrients, molecules needed by the animal metabolism. Nutritionists balance these nutrients in the diet to fulfill the requirements of different animal species. During diet formulation, is important to consider the real nutrients bioavailability, defined as the amount of the ingested nutrients absorbed and used by the animal and its metabolism. Nutrients bioavailability is different for each ingredient and for each animal species. In monogastric, gastric enzymes modify the ingested feed so that nutrients become bioavailable at the intestinal level, with little differences between the ingested and the absorbed ones. In ruminants, the rumen microflora uses most of the dietary nutrients and the intake is always really different from the amount bioavailable for the animal. Thus for ruminants, we need to consider both nutrients bioavailability and their rumen-degradation.

The Cornell Net Carbohydrate and Protein System (CNCPS®) is widely used in ruminant nutrition and includes precise descriptions of each ingredient and of the requirements of different animal species. Metabolizable protein (MP) is composed of microbial protein (from rumen microflora) and the protein ingested with the diet, as well as a little amount of protein derived from the body tissue turnover of the animal. The CNCPS calculates the metabolizable protein (MP) of the feed on the basis of the ingredients and the target animal species. The software takes into account:

  • Diet;
  • Animal physiology;
  • Feed ingredients rumen degradation;
  • Their bioavailability after rumen fermentation;
  • Their intestinal digestibility.

In this way, the system indicates how many amino acids (both essential and not), volatile fatty, minerals, lipids, and non-structural carbohydrates by-pass the rumen and are available for the absorption or second fermentation in the hindgut. Rumen degradation is one of the most important data for the balance of ruminant diets because it is highly variable among additives. For example, amino acids and vitamins are preferably used in a rumen-protected form: their rumen degradability is extremely high and only a little amount of the supplemented free-form additives reach the intestine. Nutrients rumen degradation is related to different aspects: physical and chemical properties of the natural or synthetic additive, the general composition of the diet, animal characteristics, and physiological/productive status. The calculation is further complicated by the final use of the additives: some of them are immediately used by the metabolism or converted into other active principles so that they are not precisely detectable in the bloodstream. An example of this are choline or methionine, that are used for their mutual synthesis in case of a secondary deficiency of one of them.

Ruminant diets are normally supplemented with minerals and vitamins, while essential and limiting amino acids (i.e. Met and Lys) or other molecules such as choline, carnitine, or betaine are added only when there are specific problems or during specific productive phases (i.e. during the transition period to sustain the hepatic function).


There are heterogeneous indications about which vitamins have to be supplemented to ruminant diets and in what dose. Nowadays, both fat- and water-soluble vitamins are included in the diets.

There is a specific requirement for fat-soluble vitamins A, D3, and E so that they are normally supplemented to the diet, but the amount of this supplementation is still under discussion. As an example, vitamin A requirement for lactating dairy cows, according to the “Nutrient Requirements of Dairy Cattle” (NRC, 2001), is around 75,000 UI/day; nutritionists included a higher amount of these vitamins according to their practical experiences; finally, the CE 724/5015 regulatory indicated a limit of 9,000 UI/day in the complete feed (with moisture 12%), much higher than the NRC indications.

Vitamins bioavailability depends on different aspects. Vitamin A is a retinyl ester (acetate, propionate, and palmitate), a very unstable molecule, as well as the active form of vitamin E (tocopherol, administrated as the esterified tocopheryl-acetate) or vitamin D3, as cholecalciferol. All vitamins are highly sensitive to the environmental pH and, in general, are degraded by external agents such as high temperature, moisture, and the interaction with other nutrients in the diet. The solution to give to the animals a well-defined amount of bioavailable vitamins is to use their rumen by-pass forms. If not rumen-protected, the vast majority of the supplemented vitamins are degraded by the rumen microflora.


The mineral requirement is different for each breeding and productive phase of ruminant life. The first step is to define how much and which minerals are needed by the animal and what is already given through the diet. Both macro-minerals (calcium, phosphorus, sodium, chlorine, magnesium, and sulfur, calculated as grams) and micro-minerals/trace-elements (copper, iron, manganese, zinc, cobalt, selenium, and molybdenum, calculated as milligrams) can be added to ruminant diets. Analytic analysis of the diet can be useful to define the required supplementation. After that, blood and milk analysis can be indicators of the animal requirements fulfillment.

Different ingredients provide different types and quantities of minerals, with different bioavailability for the animals. Calcium is an essential element because of its several metabolic functions, among which (in combination with phosphorus and magnesium) milk coagulation during cheese production. Normally calcium is added to ruminant diets as calcium carbonate: this molecule has lower bioavailability than calcium chloride, mono-, and di-calcium phosphate, but it is also cheaper and easy to find. Calcium and other macro-minerals can be measured in the blood to determine if the intake and its bioavailability are adequate. This is not possible for trace-elements, because their bioavailability depends on several factors.  Nowadays the goal is to improve trace-elements bioavailability to avoid harmful and costly dispersions in the environment through feces. Each feed additive for ruminant nutrition must report the active ingredient concentration, rumen degradability, and intestinal bioavailability. The chelated forms of the trace-elements are composed of the metal ion that is linked to an amino acid, a peptide complex, a carbohydrate, or a polysaccharide. The chelation normally improves trace-element bioavailability because the animal uses the absorption pathway of the ligand instead of that of the trace-element itself. Some trace elements, such as selenium, are also available in the microencapsulated form to by-pass rumen microflora fermentations that produce unavailable forms of this element: microencapsulation allows the complete bioavailability of the trace-element at the intestinal level.

Amino acids and others

Amino acids, choline, betaine, and carnitine are highly degraded by rumen microflora and the dietary intake has low bioavailability. These nutrients are extremely important especially during the transition period and in fresh cows, when the animal experience a strong negative energy balance: they help hepatic metabolism to face the energy requirement and to improve fertility and milk quality. Rumen protection is fundamental to make this kind of nutrients really available for the animal.

During the first weeks of lactation in Holstein cows milk protein is lower than 2.9% and is a good biomarker for limiting and essential amino acid deficiency. Italian studies indicated that the period between 30 and 60 DIM is the most delicate period about amino acid balance in Holstein cows. The two most important and studied amino acids for ruminants are methionine and lysine: these are the most probably deficient or unbalanced in dairy cows’ nutrition.


Supplementation of ruminants diets with vitamins, amino acids, and other feed additives is more and more unavoidable, especially in high genetic value bovine. These additives are highly degradable by environmental conditions and rumen microflora: microencapsulation allows protection and rumen by-pass of the active ingredients with full availability at the intestinal level. This allows a more accurate balance of the dietary intake of each nutrient and avoids wasting money.For more information: marketing@vetagro.comOriginal article here.