Understanding nutrient interactions in poultry feed
Understanding the nutrient interactions in poultry feeds helps in determining the requirements for a specific nutritional element and the effects of their source and level on the utilisation of other dietary elements. This should then be considered when formulating diets to meet the demand for performing various biological functions.
In feed formulation, every change has an effect on the total feed. If the metabolisable energy (ME) level of the diet is increased, so too should the supply of dietary protein to avoid excessive fat formation in the carcass (Table 1). A proportion of the body fat formed with ‘high energy, low protein’ diets is deposited around the viscera in the abdominal area, resulting in a reduction of at least 3-4% in the dressing-out percentage.
A further study was also conducted to examine 2 energy sources (starch vs. tallow) at 2 dietary protein levels (15% and 20%). For all treatments, chickens of the same weight also received the same metabolisable energy allowances from the experimental diets (Table 2). It was proven here that fat energy per se may not always be more likely than carbohydrate energy to find its way into the energy of the fatty tissues. The effect of the fat source is mediated largely by the level of the dietary protein.
Protein interaction with minerals and vitamins
An adequate supply of protein in the diet helps in increasing the absorption of iron. This is particularly important if chickens are reared in areas at a high altitude where the oxygen-carrying capacity of blood is reduced. In this case, iron increases the oxygen-carrying capacity of blood and facilitates its utilisation by the cells. Besides, the protein as such is, of course, essential for health, body weight gain, and survivability in high altitudes.
Protein also interacts with dietary vitamin A which is important for immunological competence, growth, development and reproduction, among other activities. A deficiency decreases intestinal absorption of vitamin A and also depletes the liver stores of this vitamin, particularly when using lysine-deficient proteins. Protein sources such as soybeans cause a lower rate of vitamin A depletion than other proteins, even when they provide more protein, probably because of the higher lysine digestibility in soybean protein compared to other protein sources.
Mineral-mineral interaction
The optimal ratio of calcium to phosphorus in broiler chicken starter and grower diets is about 2:1. Broilers have a relatively high utilisation of calcium from different sources in the diet, but phosphorus is usually present in the form of phytate phosphorus, resulting in a low utilisation rate. Therefore, non-phytate phosphorus sources are usually considered for incorporation into diets. This will increase plasma phosphorus concentrations and also increase ash and phosphorus content in the tibia.
Mineral-vitamin interaction
Vitamin C deficiency causes iron accumulation as hemosiderin (a brown iron-containing pigment usually derived from the disintegration of exudative red blood cells). The application of vitamin C as a therapeutic feed additive would favour the absorption of iron by binding and solubilising it at the physiological intestinal pH and also facilitate iron mobilisation by inhibiting ferritin breakdown at the lysosome. Although chickens can synthesise vitamin C from natural feedstuffs through endogenous biosynthesis in the kidneys, the rate of endogenous biosynthesis may not be sufficient under stress conditions.
Fibre interactions
In long-term feeding of diets that have moderate levels of fibre, there might be improved utilisation of minerals. This effect, however, might vary with the source of fibre used. For example, it was found that retention of sodium and potassium was increased by oat hulls, but not affected by fibre sources such as alfalfa meals or soybean hulls. On the other hand, retention of copper was found to increase with soybean hulls but not with the 2 other sources of fibre. The 3 sources of fibre equally increased the retention of iron, suggesting that the iron contained in either source has a high relative bioavailability. Selection of the fibre source to be incorporated into the diet could, therefore, be an effective means of satisfying requirements for a specific mineral and correcting deficiency.
Excess feeding of fibre sources of high molecular weight, such as citrus, apple, and tomato pomaces, should be avoided. Such fibre sources may lead to enlargement of the intestinal villi arising from physical stimulation of villous growth, coupled with about a 2-fold increase in goblet cell numbers which adversely affects absorption and utilisation of feed nutrients due to the production of large amounts of goblet mucin barrier, with vitamin A and dietary fats being most affected. It is, therefore, recommended to use such sources of fibre in limited amounts if better performance is to be achieved.
Drug-nutrient interactions
Therapeutic agents may modify nutrient status in several ways:
- Reducing nutrient availability in the intestinal lumen. For example, the antibacterial agents of the tetracycline group inhibit the absorption of several minerals due to their chelating action.
- Reducing the availability of vitamins by eliminating the local bacterial flora that synthesises them.
- Inhibiting nutrient transport across the intestinal wall. This is a mechanism common to drugs such as chloramphenicol that inhibit protein synthesis.
- Increasing nutrient catabolism. Anticonvulsant drugs promote the oxidation of vitamin D3 in the liver in the liver, and thus can negatively affect bone calcium status.
- Inhibiting intestinal nutrient absorption, thus increasing faecal loss of the affected nutrient.
Drug-nutrient interactions may also occur in the opposite direction. That is, nutrients affecting drug action are also possible. High-protein diets, for example, can generally alter the rate of excretion, and consequently the half-life of several drugs.
Animal protein ingredients such as fishmeal or meat and bone meal are often associated with an increased risk of necrotic enteritis, especially when fed in excessive amounts. This is probably due to the high levels of methionine and glycine in these products, which stimulate the growth and establishment of C. perfringens in the gut, thus reducing the efficacy of the drugs used to fight against necrotic enteritis. The increased fat content of the diet above the recommended levels also affects the efficacy of drugs by competing for albumin binding, which may modify their uptake by target tissues.
