Lesson 6, Topic 4
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Energy requirements for maintenance, growth, pregnancy, lactation, egg, wool, and meat production

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Energy Requirements for Maintenance

Maintenance: The maintenance component includes all the nutrients required for the animal to breath, move, digest food, keep warm, repair tissues, and maintain body weight.

Weight, age, breed, physiological status, activity, and environmental conditions are the primary variables impacting maintenance requirements. The larger the animal, the greater its maintenance requirement, especially energy and protein.

Extremely heavy muscled breeds will have greater maintenance requirements than light muscled breeds. Pregnancy and lactation increase basal metabolism, so maintenance requirements are altered accordingly.

Heavy milking breeds have an increased maintenance requirement. Increased activity or rough terrain will increase maintenance energy needs as will extremely cold, hot, wet, or muddy conditions.

Feeding for Maintenance

Primarily the nutrients in a ration are used for maintaining the life of the animal. Certain amount of energy, protein and other nutrients is required for life sustaining activities of the body such as for the heart to pump blood, for respiration, for the nervous system to maintain its own activity and muscle tone, for temperature regulation, for the general metabolism of most tissues, for active absorption and transport of chemical compounds, for repair of damaged or worn tissues, protein turnover and for the production of hormones and enzymes.

If an animal is not fed, the energy, protein and other nutrients required for the above functions will be drawn from the animal’s body reserves of fats, proteins etc. leading to negative energy or protein balance and the animal will lose body weight over a period of time. The purpose feeding animals is to prevent this drain on the animals’ body reserves

Maintenance Requirement

The maintenance requirement of a nutrient can be defined as the quantity which must be supplied in the diet so that the animal experiences neither net gain nor net loss of the nutrient.

Energy Required for Maintenance

  • The maintenance energy requirements include three components,

    • Basal metabolism

    • Energy to maintain the animal’s body temperature

    • Energy for voluntary activity, protein turnover etc.

  • An animal is in a state of maintenance when the amount of nutrients in the feed will maintain the animal in equilibrium i.e., its body composition remains constant and is not growing, not working or not giving any product as milk or mutton or egg. This minimum demand of feed is referred to as the maintenance requirement.

  • If this need is not met, the animals are forced to draw upon their body reserves to meet their nutrient requirements for maintenance, commonly revealed by a loss in weight and other undesirable consequences. The destruction of body tissue is referred to as fasting catabolism.

  • Livestock are fed for production and generally not for maintenance.

Energy Metabolism of Fasting Animal

  • The energy expended in the fasting animal is represented by the fasting animal heat production and this can be measured in the respiration calorimeter (Direct Calorimetry) or can be obtained by one of the methods of indirect calorimetry.

  • The quantity of heat arising in this way is known as the basal metabolism and measuring this heat produced gives a direct estimate of the net energy the animal should get from its feed to meet the maintenance energy requirement.

Methods to Determine the Energy Required for Maintenance of Animals

  • Measuring basal or fasting metabolic rate.

  • Metabolizable energy requirements can also be estimated by conduction short and long-term trials with mature, non-producing animals fed at the maintenance level (if the energy content of their food is known).

  • By conducting feeding trials with different levels of feed intakes and by extrapolation of intake of feed towards zero level of production.

  • Regression method. By conducting slaughter experiments.

Energy Requirements Calculation

RER Calculation

Formula Example

RER = (30 x bw in kg) + 70kcal

RER stands for Resting energy requirement and represents kcal. In order to determine the Resting Energy Requirement of an animal the body weight (in kilograms) is multiplied by 30. 70 is then added to the outcome of this calculation which then gives the RER for that animal in kcal.

MER Calculation

Formula Example

MER = RER x 2

MER stands for maintenance energy requirement. Before working out the MER of an animal, the RER must first be calculated. Once the RER is known, this outcome is then multiplied by two. Animals in different life stages however require more energy than others. In this case, a young cow who is growing and has only 50% of their adult weight requires the normal MER x 2.

Energy Requirements for Growth, Pregnancy & Lactation


Growth is measured as an increase in body weight. Most rapid early in life, declines gradually until puberty, then even slower rate until mature size is reached. Requirements for growth are determined by actual weight, average daily gain (growth rate), weight at maturity, and composition of gain.

Composition of gain simply means whether animals are putting on more muscle or more fat. For example, protein requirements will be higher for young animals because they are gaining more muscle than fat.

As animals reach maturity, gain then has a larger percentage of fat, and requires relatively more energy. Nutrient requirements per unit body weight are greater for younger animals. Efficiency is greatest when growth rates are very rapid.

Following a period of subnormal growth caused by energy restriction, young animals will gain weight at faster than normal rates (compensatory growth or gain). When mature animals need to gain weight to increase their body condition score, this is also considered growth.


Adjustments to requirements for reproduction are based on expected birth weight and stage of gestation. Requirements include development of maternal tissue as well as the fetus. Usually, pregnancy does not significantly affect requirements until the last one-third of pregnancy when the fetus is growing rapidly.

Nutrient deficiencies prior to breeding may result in low fertility or failure to maintain pregnancy. Underfeeding during growth can result in delayed sexual maturity. Fetal tissues have priority for nutrients over maternal tissues. Body reserves may be depleted.


Heavy lactation has greater nutrient demands than any other production state. Nutrient requirements for lactation are based on the amount of milk at peak lactation and the composition of the milk. Animals that produce more milk, and milk with more fat and protein, will have higher nutrient requirements.

Energy requirement for maintenance, growth, pregnancy, and lactation …

Energy requirements for wool growth

  • Energy makes up the largest portion of the diet and is usually the most limiting nutrient in sheep diets. Carbohydrates, fat, and excess protein in the diet all contribute towards fulfilling the energy requirements of sheep. Carbohydrates are the major sources of energy.

  • Concentrates (grain) contain starch, which is a rich source of energy. Forages contain fiber or cellulose, which is not as rich in energy as starch.

  • The major sources of energy in a sheep’s diet are pasture and browse, hay, silage, and grains.  Meeting energy requirements without over or underfeeding animals is one of the producer’s biggest challenges.

Nutrient Requirement of Sheep

36 g TDN and 5 g of DCP per kg metabolic weight (or 10 g of TDN and 1 g of DCP per kg live weight) has been recommended as the requirement for maintenance.

Consumption of TDN and DCP in the ratio of 8.5 at an intake level of 3 g DCP/kg live weight produced the highest growth in weaner lambs. Pregnant ewes during their advanced pregnancy required 7.8 g of DCP and 58.7 g of TDN per kg metabolic weight for producing lambs with 3.5-4.0 kg birth weight.

 AttributesTDN (g) Requirement/kgW0.75DE (kcal)ME (kcal)Digestible protein (g)
Growth (per g gain)1.797.876.460.2
Wool production35.00154.00126.284.2

Study on energy expenditure of sheep indicated that a housed sheep spent 4.16 MJ/D, while grazing animals spent 43% higher. The expenditure of energy was 4.85 MJ/D during winter, which increased to 6.70 MJ/d during rainy season. The animals exposed to heat stress required more energy to meet their enhanced requirements of thermolysis and maintenance.

Nutrient Requirement for Wool Production

  • Optimum nitrogen-sulphur ratio in the diet of wool producing sheep was found to be 5:1. Sulphur concentration of 0.24% in the diet produced maximum quantity of wool (i.e. 1392 g in 6 months clip with a scouring yield of 80%).

  • The optimum level of CP and TDN in the ration for wool production in Marwari sheep was 10 and 50% respectively. Sulphur supplementation at 0.3% (as Na2SO4) with urea (1% of diet) supported reasonable increase in wool yield and quality (good carpet wool type) in sheep on grazing plus concentrate ration.

  • Energy deficiency is the most common nutritional deficiency in sheep. An energy deficiency will manifest itself in many ways, as for example, in growing animals, an early sign of energy deficiency is reduced growth, then weight loss, and ultimately death.

  • In reproducing females, early signs of an energy deficiency are reduced conception rates, fewer multiple births, and reduced milk production. 
  • With restricted energy consumption, wool growth slows, fiber diameter is reduced, and weak spots (breaks) develop in the wool fiber.

  • An energy deficiency reduces the function of the immune system. Energy is quantified in the ration in many ways.

  • The simplest measure is TDN or total digestible nutrients. Metabolizable energy (ME) and net energy (NE) values are more accurate measures of energy in a sheep’s diet.
  • The weight of wool produced by sheep varies considerably from one breed to another, and an average value is useful only for guidance

  • For eg: a Merino weighing 50 kg produces annually 4 kg fleece. Such a fleece would contain about 3 kg of actual wool fibre, the remaining 1 kg being wool wax, suint, dirt and water.

  • Wool wax is produced by the sebaceous glands, and consists mainly of esters of cholesterol and other alcohols.
  • The wool fibre consists almost entirely protein and wool keratin. To grow in one year, a fleece containing 3 kg protein the sheep would need to deposit a daily average of about 8 g protein or 1.3 g nitrogen.

  • If this latter figure is compared with the 6.6 g nitrogen which a sheep of 50 kg might lose daily as endogenous nitrogen, it seems that in proportion to its requirement for maintenance, the sheep’s nitrogen requirement for wool growth is small.

  • These figure however do not tell the whole story, since the efficiency with which absorbed amino acids are used for wool synthesis is likely to be much less than that with which they are used for maintenance.

  • Keratin is characterised by its high content of the sulphur-containing amino acid, cystine, which although not an indispensable amino acid is synthesised from another indispensable amino acid, methionine.

  • The efficiency with which food protein can be converted into wool is therefore likely to depend on their respective proportions of cystine and methionine.

  • Keratin contains 100–200 g/kg of these acids, compared with the 20– 30 g/kg found in plant protein and in microbial proteins synthesised in the rumen and so the biological value of food protein for wool growth is likely to be not greater than 0.3.

  • Wool growth reflects the general level of nutrition of the sheep. At sub-maintenance levels, when the sheep is losing weight, its wool continuous to grow, although slowly.

  • As the plane of nutrition improves and the sheep gains in weight, so wool growth too increases.

  • There appears to be a maximum rate of growth for wool, varying from sheep to sheep with a range as great as 5 to 40 g/day.

  • The dependence of wool growth rate on the plane of nutrition (i.e. energy intake) of the sheep is due in part to the association between energy intake and the synthesis of microbial protein.

  • The real determinant of wool growth rate is likely to be the quality of protein digested and absorbed in the small intestine of the sheep and it has been shown, for example, that a Merino must absorb 120 – 150 g protein/day to achieve its maximum rate of wool growth.

  • In a ewe with a metabolisible energy intake of 12 MJ/day (i.e. twice its maintenance requirement), only 101 g microbial protein would be synthesised per day and only 101 x 0.8 x 0.85 = 69 g would be absorbed as amino acids.

  • To achieve maximum growth of wool the sheep is therefore dependent on a good source of under-graded food protein.

  • In practice this is likely to be supplied by the consumption of large quantities of protein in pasture herbage.

  • This may be relatively highly degradable but can still supply much undegradable protein. For example, a ewe might consume 250 g protein/day, of which 0.3 (75 g) would be undegraded and 0.85 x 75 = 64 g would be absorbed in the small intestine.

  • Nevertheless, wool growth in sheep is considerably increased by protein supplements protected from rumen degradation such as casein.

  • As would be anticipated, the most effective of such supplements are those rich in the sulphur-containing amino acids.

  • Wool quality is influenced by the nutrition of the sheep. High levels of nutrition increase the diameter of the fibres and it is significant that the finer wools come from the nutritionally less favourable areas of land.

  • Periods of starvation may cause an abrupt reduction in wool growth; this leaves a week point in each fibre and is responsible for the fault in fleeces with the self-explanatory name of ‘break’.

  • An early sign of copper deficiency in sheep is a loss of ‘crimp’ or waviness in wool; this is accompanied by a general deterioration in quality, the wool losing its elasticity and its affinity for dyes.

Energy requirements for egg and meat production

Egg laying chickens(Nutrient Requirement)

Modern layer hens

The intake of nutrients is defined by the nutrient levels in the feed and the amount of feed consumed. Nutrient requirements of egg laying chickens is outlined below.

Feed consumption

There are a number of factors that influence voluntary feed intake (discussed in the section on feed intake). Table 1 provides data on typical feed consumption for modern brown-egg laying hens in relation to target body weight. From Week 18, hens start to enter their laying period, reaching peak of lay around 32 weeks of age, and typically maintaining egg production until 65-68 weeks of age. Feed intake will increase to a steady level of 100-105 grams per day and hen body weight will reach a mature level of 1700-1800 grams.

Table 1. Body weights and associated feed consumption for a brown-egg laying breed during the growing period

Age (wk)Body weight (g)Feed consumption (g/bird/day)Age (wk)Body weight (g)Feed consumption (g/bird/day)

Growing period nutrition recommendations

Chicks require a diet that can provide the nutrients needed for rapid growth and feather development. Chicks are given relatively high levels of energy, protein and the vitamins and minerals for the starter period. Once the chicks are fully feathered their energy requirements are reduced.

Feeding management for layer pullets aims to maintain a growth rate that will lead to the pullet reaching sexual maturity at the desired age while avoiding obesity. The stage at which a pullet will start laying eggs is affected by age, body weight and day length. On a percentage basis, layer pullet diets have lower energy and protein levels than chick diets. Different breeders recommend different feeding strategies for their birds, including the number of different diets fed during the pullet growing stage.

Many breeders recommend a pre-lay diet that increases some of the nutrient levels, such as calcium, that will be required by the bird when it begins to lay eggs. Table 2 provides data on typical nutrient levels for layer diets for the growing period.

Table 2. Growing period nutrition recommendations

Nutrient Units  Starter
0 – 6 wks
6 – 12 wks
12 – 15 wks
15 wks – Prod.
Protein %Min20.017.5015.5016.50
Metabolisable Energy Mj/Kg11.5-12.411.5-12.611.3-12.411.4-12.4
Metabolisable Energy Kcal/Kg  2750-2970  2750-3025  2700-29702725-2980
Lysine %Min1.100.900.660.80
Methionine %Min0.480.410.320.38
Methionine + Cystine %Min0.820.710.580.65
Tryptophan %Min0.
Threonine %Min0.730.550.520.55
Calcium %Min1.*
Av Phosphorus %Min0.450.430.420.40
Sodium %Min0.
Chloride %Min0.

*At least 30-65% of the added limestone should have a minimum particle size of 2250 Microns.

Nutrient levels for layer diets

The aim of layer diets is to optimise egg production (in terms of egg numbers, egg size or egg mass), provide the nutrition required to safeguard health and maintain the desired body weight. As with layer pullets, different breeders recommend different feeding strategies for their birds, including the number of different diets fed during the laying stage. Calcium is increased for egg shell formation. Table 3 provides data on typical nutrient levels for layer diets.

Table 3. Examples of layer diets (at 100 grams per day intake level)

NutrientsUnits1-32 wks32-44 wks44-55 wks> 55 wks
Metabolisable EnergyMJ/kg11.60-11.9711.41-11.9711.20-11.9710.68-11.83
Crude protein %19.8017.5017.0016.00
Lysine %1.020.930.890.83
Methionine %0.510.460.410.38
Linoleic acid %1.101.601.601.60
Calcium %4.404.254.504.75
Av.phosphorous %0.480.400.360.35

Meat chickens/broilers (Nutrient Requirement)

Broiler chickens

The intake of nutrients is defined by the nutrient levels in the feed and the amount of feed consumed. Nutrient requirements of meat chickens (broilers) are outlined below.

Feed consumption and body weight

There are a number of factors that influence voluntary feed intake. These are discussed in the section on feed intake. Table 1 provides data on typical feed consumption and body weight for modern broiler chickens in relation to age and sex.

Table 1. Body weight and cumulative feed consumption for male and female broilers

Age  (weeks)Body weight (g)Cumulative
Feed Intake (g)
Body weight (g)Cumulative
Feed Intake (g)

Nutrient levels for broiler diets

Feeding strategies for broiler chickens will vary depending on the target market for the final product. Strategies for feeding broilers destined for the whole bird market will differ from strategies for broilers destined to be sold as pieces.

Furthermore, the nutrient intake of fast growing broilers must be carefully controlled to prevent metabolic diseases such as ascites and leg weakness. Table 2 provides data on typical levels of selected nutrients for broiler diets.

Table 2. Examples of broiler diets

0-10 days
11-24 days
>25 days
Metabolisable energyMj/Kg12.6013.3013.50
Total Arginine%1.481.311.11
Digestible Arginine%1.331.181.00
Total Lysine%1.441.251.05
Digestible Lysine%
Total Methionine%0.510.450.39
Digestible Methionine%0.470.420.36
Total Methionine +Cystine%1.090.970.83
Digestible Methionine +Cystine%0.940.840.72
Total Threonine%0.930.820.71
Digestible Threonine%0.800.700.61
Total Trypophan%
Digestible Tryptophan%
Total Valine%1.090.960.81
Digestible Valine%0.940.830.70