INTRODUCTION

Minerals are essential nutrients that need to be supplemented in poultry feed. They play a crucial role in the birds’ growth and performance. Different minerals are required for the formation of the skeleton, as components of various compounds with particular functions within the body, as cofactors of enzymes, and for the maintenance of osmotic balance within the body of the bird. In fact, plant and animal tissues contain more than 50 minerals, with around 25 of these minerals listed as ‘essential’ for animals (Underwood & Suttle, 1999).

MINERAL CLASSIFICATION

Minerals are classified into major or macronutrient minerals and minor, trace, or micronutrient minerals based on their amount in the body and the diet. The requirements for major minerals are usually stated as a percentage of the total diet; whereas trace mineral requirements are stated as milligrams per kilogram or parts per million of feed. Of the 25 essential minerals, seven are major minerals, as they are present in relatively larger quantities in the body than the trace minerals and need to be supplemented in higher quantities in the feed. These major minerals are Calcium, Phosphorus, Magnesium, Sodium, Chloride, Potassium, and Sulphur. The trace minerals are present in small quantities in the body and their requirement in the diet is less. The 18 trace minerals are Iron, Copper, Zinc, Manganese, Iodine, Selenium, Cobalt, Molybdenum, Chromium, Tin, Vanadium, Fluorine, Silicon, Nickel, Arsenic, Aluminium, Lead and Rubidium. Among these, the first nine trace minerals have been shown to play a significant role in birds.

Minerals are classified as inorganic or organic based on their source. This will be discussed in detail in the coming sections.

ZINC

Zinc is the most abundant trace mineral in the body along with iron. Zinc plays an important role in poultry, particularly for layers, as a component of a number of metalloenzymes. Zinc is involved in virtually all of life's processes and has major catalytic roles in protein, lipid, carbohydrate, and nucleic acid metabolism. It is therefore required for growth, reproduction, appetite, vision, wound healing, blood synthesis, immunocompetency, and hormone activity.

As a component of carbonic anhydrase, zinc is essential for eggshell formation and bone calcification and acid-base equilibrium
As a component of carboxypeptidases and DNA polymerases, zinc plays important roles in the immune response, in skin and wound healing, and in hormone production (testosterone and corticosteroids).

Classic zinc deficiency symptoms in poultry include a suppressed immune system, poor feathering, dermatitis, shortening and thickening of leg bones and enlargement of hock joint, infertility, and poor shell quality.

MANGANESE

Manganese is the least abundant trace mineral in the body tissues.

• Manganese plays a critical role in the formation of chondroitin sulfate. This mucopolysaccharide is an important component of bone cartilage.
• Manganese has an important function in blood clotting and lipid and carbohydrate metabolism.
• Through the manganese dependent superoxide dismutase enzyme, it is important for protection of cells against damage by free radicals.
• Manganese is also required for eggshell formation.

COPPER

Copper also plays an important role in a number of enzyme functions in the bird.

• Copper is closely associated with iron metabolism and thus involved in the red blood cell formation.
• Another important enzyme dependent on copper is lysyl oxidase. This is an integral enzyme in elastin and collagen formation, both of which are important for proper bone formation.
• Through its role in disulfide bond formation, copper plays an important role in feather development.
• A copper deficiency can cause anemia, tibial dyschondroplasia, and loss of feather pigmentation.
  

IRON

Iron has a very specific function in all animals as a component of the protein heme found in the red blood cell’s protein hemoglobin and in the muscle cell’s protein myoglobin. As iron plays a key role in many biochemical reactions, it is important for proper metabolism of nutrients. Iron has a rapid turnover rate in poultry (10 times per day), so it must be provided in a highly available form in the diet on a daily basis. (Sheila, 2006)

Any internal infection such as coccidiosis can also interfere with iron absorption and availability. Iron deficiency can result in anemia in poultry.

SELENIUM

Selenium is a very unique trace mineral and is required in very small amounts. Selenium was recognized as a potentially toxic mineral long before it was identified as an essential nutrient.

• Selenium has powerful antioxidant role, by being a part of the enzyme glutathione peroxidase. It’s mechanism of action as an antioxidant, overlaps with the mechanism of Vitamin E. Selenium also spares Vitamin E in three ways,

  1. by helping it’s absoption
  2. as a part of glutathione peroxidase
  3. by helping it’s retention in blood

• Other selenoproteins in poultry play an important role in prevention of exudative diathesis, normal pancreatic function, and fertility.
• Selenium works together with Vitamin E to boost immunity.
• Selenium also plays role in iodine metabolism

IODINE

Iodine is an essential element in all animal species and is present in very small amounts in the body.

• The thyroid hormones, which contain iodine, are known to have a role in thermoregulation, intermediary metabolism, reproduction, growth and development, hematopoiesis and circulation, and neuromuscular functioning.
• Iodine deficiency results in a condition called goitre wherein thyroxine output is decreased and the thyroid gland subsequently becomes enlarged.
• Lack of thyroid activity can stop egg production and results in long lacy feather growth.

OTHER TRACE MINERALS

Molybdenum, which was earlier considered a toxic element at higher levels, was discovered to have a beneficial role as an essential nutrient. Molybdenum is a component of the metalloenzyme xanthine oxidase, which is important for purine metabolism. It is also a component of aldehyde oxidase, which is involved in the electron transport chain in cells.

Chromium, whose activity is linked to that of insulin, is known to help in the utilization of glucose.

There are other essential trace elements like fluorine, vanadium, silicon, nickel, tin, and arsenic. However, there is either no evidence of their importance in poultry or the very low requirement levels are readily met by concentrations in practical diets. So most of these need not be supplemented in the diet.

BIOAVAILABILITY OF TRACE MINERALS

There are several factors that affect the bioavailability of minerals:

1. Factors related to the animal (species, sex, age, physiological state, health, differences between individual animals).

2. Factors related to the composition of the diet (amount of mineral intake, ratio between various individual minerals, vitamin levels, protein, fat, fiber and phytate levels).

3. Factors related to the mineral source (fineness, concentration of other minerals, crystallinity, production process, chemical-physical techniques applied, the source of the raw material, the presence as anion or cation, or in some cases in an organic complex).

4. Factors related to the technological treatment that has been applied to the final diet, e.g. mash feed or pelleted diet.
This article will briefly elaborate on factors related to mineral interactions and mineral sources.

MINERAL INTERACTIONS

Interactions, particularly negative interactions, between minerals affect absorption and bioavailablity. An excess of one trace mineral can affect or interfere with another trace mineral’s availability.

The most common antagonism occurs between zinc and copper, as both compete for the same binding site during absorption. High levels of dietary zinc will inhibit copper absorption, hepatic accumulation and deposition in the egg. Ratios greater than 4:1 of zinc : copper can be considered antagonistic (Sheila, 2006)

High levels of calcium, copper and iron can interfere with zinc. Excess dietary phosphorus will interfere with manganese availability in poultry. High manganese increases iodine excretion and reduces iron absorption. Excess calcium and phosphorus inhibit manganese absorption. Iron and cobalt slightly reduce manganese availability.

These interactions can be reduced by supplementing a portion of the mineral requirement through chelated minerals, which are protected from interactions and have better bioavailablity.

DIETARY MINERAL SOURCES

For poultry, inorganic compounds are usually used as sources of trace minerals in the diet. Inorganic sources include salts such as sulfates, chlorides, carbonates, and oxides. The sulfate and chloride mineral forms are more available than the carbonates and oxides.

During recent years, organic chelates of trace minerals have become available for supplementation in poultry diets. A trace mineral chelate is defined as a trace mineral with ligands to amino acids or proteins. A number of companies have patented organic trace mineral products. These have been classified by the American Feed Control Officials (AAFCO, 1997) into different classes of chelated minerals.

Research has indicated that these organic sources are more bioavailable than their inorganic counterparts due to the following reasons:

1. Minerals are protected from the unwanted chemical reactions in the gut.
2. Chelates are absorbed more efficiently in the gut.
3. Fewer interactions occur between competing minerals for absorption.

Numerous studies have reported beneficial effects of chelated organic trace mineral supplementation on bird health and production parameters as well as product quality.

TRACE MINERAL REQUIREMENTS

Given below are the trace mineals requirements taken from Provimi Poltry Guidelines (2004). Do not interpret this requirement as a fixed law, these can only be used as a guidelines when formulating diets.

Broiler Breeder Minerals Guidelines

contents per kg complete feed in mg/kg
Source : Provimi Poultry Guidelines (2004)

Broiler / Layer Minerals Guidelines

contents per kg complete feed in mg/kg
Source : Provimi Poultry Guidelines (2004)

Other references can be provided on request.

Relative Biological Value (RBV) of different sources of minerals

Source : Ammerman et al. (1998)

TOXIC LEVELS OF MINERALS

All minerals, both essential and non-essential, can negatively affect the birds at excessively high levels in the feed.

Maximum Tolerable Levels of dietary trace minerals vary from species to species. Given below are the Maximum Tolerable Levels in feed for Poultry.

Source : Mineral Tolerances of Domestic Animals, NRC, 2004

FUNCTIONS OF TRACE MINERALS

Trace minerals have a wide range of functions. It is important to understand these functions in order to better understand how these minerals affect animal growth and performance.

As described by Underwood and Suttle (1999), the functions of trace minerals can be broadly classified into four categories – structural, physiological, catalytic, and regulatory functions.

• Structural Functions – Trace minerals can form structural components of organs and tissues, such as zinc, which is a structural part of membranes and molecules.
• Physiological Functions - Minerals are involved in maintaining osmotic balance, acid base balance, membrane permeability and tissue irritability.
• Catalytic Functions - Of all the functions, these are the largest. Trace minerals act as catalysts in enzyme and hormone systems and also serve as integral structural components of numerous metalloenzymes. These metalloenzymes are required for a broad variety of metabolic activities such as energy production, protein digestion, cell replication, antioxidant activity, and wound healing.
• Regulatory Functions - These are illustrated by the role of trace minerals in regulating cell replication and differentiation. While zinc influences transcription, iodine as a constituent of thyroxine is associated with thyroid function and energy metabolism.

Compiled by Dr. D. Krishnamurthy, Product Manager, Poultry Nutrition Division, Vetcare Bangalore