Animal Nutrition and Feed Rations

[Kvet Forum]

Inadequate nutrition is a major cause of low live-weight gains, infertility and low milk yields and other health issues in animals.

The following will explain the principles of animal nutrition and some examples of how to make home feed rations based on the types of feed available in major agro ecological zones.

Introduction​

Livestock keeping in all its ventures is a major source of incomes all over Kenya, from the most productive to nearly desert areas, and for all livestock keepers livestock feeding and nutrition is a major concern.

Inadequate nutrition is a major cause of low live-weight gains, infertility and low milk yields and other health issues in cattle.

Also pig, chicken, dairy goat and many other livestock keepers have expressed challenges in feeding their animals optimally. The following will explain the principles of animal nutrition and some examples of how to make home feed rations based on the types of feed available in major agro ecological zones.

The Feed Nutrients​

Animal feed needs to meet the requirements of the animal. For cattle it must contain a certain structure to keep the stomach healthy, and for all animals it must contain dry matter, various groups of nutrients, minerals and trace-elements and should not be mouldy or mixed with dirt and soil nor contain poisonous ingredients (for example pesticides or herbicides on crop residues). The composition of feeds in a ration depends on the type of animals being fed and their stage of production. Cattle and goats, being ruminants, are better equipped to digest crude fibre than pigs or chickens. Young animals need more protein than adult animals, while animals in milk need a higher protein content in the ration than non lactating animals.

Generally animals must have a ration containing:

• Energy (from carbohydrates and fats) to maintain the body and produce (milk, meat, work). The carbohydrates and fats not needed for production are converted to fat and stored in the body.
• Protein is needed for body building (growth) and maintenance as well as milk production. Without protein there would be no body weight gain nor milk production. Excess protein is converted to urea and fat
• Minerals help in body building as well as in biological regulation of growth and reproduction. They are also a major source of nutrients in milk.
• Vitamins help regulate the biological processes in the body and become a source of nutrients in milk
• Water helps all over in body building, heat regulation, biological processes as well as a large constituent of milk production as well as eggs.

Partition of Feed Energy​

Only parts of the nutrients in feeds are available for the animal. All feeds contain energy: the gross energy (GE) that is the energy that is available when the feed is burned. For instance fresh sugarcane forage has a gross energy content of 18.2 MJ per kg of dry matter (DM). This GE is a value of the feeds itself and is not influenced by animals.

• Cattle can digest sugarcane forage for 68% (the rest is lost in the faeces): the digestible energy (DE) for cattle is 11.3 MJ/kg DM.
• Pigs are able to digest only 37% of all energy in sugarcane forage, so the digestible energy for pigs is only 6.7 MJ/kg DM, much lower than that for cattle.

From the digestible energy part cannot be used by the animal but is lost as methane gas or in urine. Especially ruminants loose part of the energy in methane gas while in non-ruminants this is a minor loss. The remaining energy is called metabolizable energy (ME).

• The ME of sugarcane forage for cattle is 9.3 and for pigs it is 5.5 MJ/kg DM.
• From the ME some heat is lost and that energy cannot be used by the animal itself. The energy that can be used by the animal is the net energy (NE). The efficiency this net energy is used for the various functions differs: for maintenance 1 kg of dry matter of sugarcane delivers 8.1 MJ, for growth the same kg of sugarcane forage delivers 8.6 MJ and for lactation it contains 6.7 MJ/kg DM.
• So the net energy of the sugarcane forage for cattle varies from 6.7 - 8.6 and for pigs it varies from 4 - 5.
• For high crude fibre forages and compared to cattle, goats use feeds with the same efficiency, camels and donkeys are less efficient, pigs are far less efficient, most poultry is far less efficient than cattle and rabbits are about as efficient as cattle.
• For low crude fiber feeds pigs are more efficient than cattle. In the scheme "Fate of feed energy within the animal" the different parts of the systems are given.

When calculating rations, the value of the feeds and the requirements of the animals must have the same system. If energy of feeds is given in metabolizable energy, the requirements of animals should be stated in ME to be able to calculate rations. In some countries the net energy (NE) system is used, in other countries the digestible energy (DE) or metabolizable energy (ME) system is used. The feed system used is mainly determined by the available information from feedstuffs and animals and the wish to be more or less precise. It is a compromise between the costs of research and analysis and the benefits of using a very sophisticated feed system. With limited information about feeds it is best to use a system based on Metabolizable Energy or on a DE system, e.g. Total Digestible Nutrient (TDN) system.

When calculating feed needs of different animals a system called Metabolisable Energy as a basis for formulating rations on the farm is used. Metabolisable energy basically means that part of the feed which the animal is able to utilize.

The unit of energy is the joule of which one million units (1 000 000 joule) is referred as Megajoule (MJ). The energy value can be given in the dry matter or in the product (as fed). For ration calculations the energy in the product is used. When comparing feeds (for example to compare prices of energy) it is more convenient to use the energy per kg of dry matter.

Basically, feed organic nutrients are required by the animals for three things: These are:

1. Use as materials for the construction of body tissues (growth and maintenance)
2. Synthesis of products such as milk and eggs
3. Use as sources of energy for work done. The work done include both metabolic (heat increment and maintenance) and physical e.g. walking and feeding.

Fate of feed energy within the animal

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Gross energy (GE) The feed is comprised of chemical ingredients which are broadly classified as carbohydrates, proteins, lipids and vitamins. Heat is released when organic material in such feed is burnt. For this reason, methods have been developed to measure the quantity of chemical energy present in a feed by determining the amount of heat generated from complete burning a known quantity. This is referred to as gross energy. Most of the common feeds have energy content of about 18.5 MJ/kg DM.

Digestible energy (DE) Not all the gross energy in consumed feed is available and useful to the animal. Some energy is lost from the animal though excretions: it is fixed in the feed in a way the animal cannot reach it. The digestible energy is calculated by subtraction of faecal energy from gross energy. The DE represents the energy content of the digested nutrients. From these digestible nutrients the Total Digestible Nutrients (TDN) can be calculated.

Metabolizable energy (ME) The animal further loses energy containing-substances through excretion of urine and production of gases during metabolic processes. Metabolizable energy is what remains after subtraction of energy lost from urine and combustible gases resulting from the digestible energy of a feed. Loss of energy through methane (a combustible global warming gas) can be substantial, particularly from ruminants; hence can be of serious nutritive and environmental consequence.

Heat increment (HI) The ingestion of feed by an animal is also followed by losses of energy not only as the chemical energy excreta and gases produced but also as heat. Animals are continuously producing heat and losing it to their surroundings, either directly through radiation, conduction and convection or indirectly through water evaporation from the body. The heat is generated through processes of digestion and metabolism of nutrients derived from the feed. For instance, the act of eating, which includes chewing, swallowing and secretion of saliva, requires muscular activity and this generates heat. Unless the animal is in a particularly cold environment, this heat energy is of no value to it, and must be considered, like the energy of the excreta, as a tax on the energy of the feed. Energy lost in this manner is referred to as Heat increment.

Net energy (NE) The deduction of the HI of a feed from its ME gives the Net energy, which is the energy available to the animal for useful purposes such as body maintenance and various forms of production (milk, meat, eggs, wool and labour).