(a) Describe the measurement of energy retention by carbon-nitrogen balance technique. (15 marks) (b) What is "growth curve"?…

(a) Energy Retention via Carbon-Nitrogen Balance

The measurement of energy retention (ER) using the carbon-nitrogen (C-N) balance technique relies on the assumption that energy is retained in the body only as protein and fat, since carbohydrate (glycogen) storage is small and nearly constant. This method estimates ER by quantifying the net accumulation of carbon and nitrogen in the animal’s body, assuming that the chemical composition of retained tissue is consistent.

The experimental procedure involves a rigorous balance study. First, the total carbon and nitrogen intake is determined by analysing the feed for carbon (dry combustion/elemental analysis) and for nitrogen (Kjeldahl method). The animal is kept in a respiration chamber, and the carbon lost in faeces, urine, respired carbon dioxide and methane is measured, together with the nitrogen in faeces and urine (gaseous nitrogen losses are negligible). The net retention is calculated as: Cᵣₑₜ = Cᵢₙₜₐₖₑ - (C_feces + Cᵤᵣᵢₙₑ + C_CO₂ + C_CH₄) Nᵣₑₜ = Nᵢₙₜₐₖₑ - (N_feces + Nᵤᵣᵢₙₑ)

Protein retained = Nᵣₑₜ × 6.25. The carbon in this protein is subtracted from Cᵣₑₜ, and the remaining carbon is assigned to fat. The energy value of retained carbon and nitrogen is derived from the energy content of body tissues. Based on standard physiological constants, where fat contains about 76.7% carbon and 9.5 kcal/g, and protein contains about 52% carbon, 16% nitrogen and 5.7 kcal/g, the energy retention (in kcal) is calculated using the formula: ER = 12.38 × Cᵣₑₜ (g) − 4.63 × Nᵣₑₜ (g) Here, Cᵣₑₜ and Nᵣₑₜ must be expressed in grams; in SI units, ER (kJ) = 51.8 C − 19.4 N. The positive coefficient for carbon reflects the high energy density of fat retention, while the negative coefficient for nitrogen corrects for the carbon held in retained protein, which carries less energy per gram of carbon than fat. A key limitation of this technique is the assumption of constant tissue composition; variations in body fat percentage or protein quality can introduce errors. Because the animal must stay in a respiration chamber, the method is costly and mainly used in research. Once ER is known, heat production follows by difference (HP = ME intake − ER), so the C-N balance complements direct calorimetry and the comparative slaughter technique.

(b) Growth Curve and Its Stages

A growth curve is a graphical representation of the relationship between live weight and age, typically exhibiting a sigmoid (S-shaped) pattern. It describes the rate of growth over time, reflecting the biological processes of cell division and cell enlargement. The curve is mathematically modeled using functions such as the Gompertz, Logistic, or von Bertalanffy models, which describe the asymptotic approach to mature weight.

The stages of the growth curve are:

1. Prenatal (lag) phase: growth in the uterus or egg by rapid cell division (hyperplasia) and organogenesis; absolute gain is small although relative growth is very high. In the chick it ends at hatching (21 days).
2. Self-accelerating phase: from birth or hatching, daily gain rises steadily as the body grows; growth is mainly by cell enlargement (hypertrophy), feed conversion is most efficient, and bone and muscle develop fast. In broilers the whole production cycle of about 5–6 weeks lies in this phase.
3. Inflection point: the point of maximum daily gain, where acceleration turns to deceleration; in farm animals it falls near puberty (at 37% of mature weight in the Gompertz model and 50% in the logistic model).
4. Self-inhibiting (decelerating) phase: after puberty daily gain falls, a larger share of energy goes to maintenance and fat deposition, and feed efficiency declines.
5. Maturity (plateau): weight approaches the asymptote (mature weight); gain is near zero, and weight may fall in old age.

Hammond described differential growth along the curve: tissues mature in the order nervous tissue, bone, muscle and then fat, and body parts grow at different rates (allometry, y = a·xᵇ). This is why meat animals are marketed around or soon after the inflection point, before fat deposition dominates.

The shape of the curve is influenced by species, breed, sex, and nutrition. For instance, Indian indigenous breeds like Kadaknath or Aseel grow slowly, reaching only about 1–1.5 kg at 20 weeks, whereas commercial broilers reach about 2–2.5 kg in 5–6 weeks.

(c) Nutrient Requirements of Chicken

Chicken nutrient requirements vary significantly across production stages: starter, grower, finisher, layer, and breeder. Standards include the NRC's Nutrient Requirements of Poultry (1994), the Bureau of Indian Standards specification for poultry feeds (IS 1374:2007) and ICAR's nutrient requirement tables (2013).

Energy: Measured as Metabolizable Energy (ME), it is the primary driver of growth and egg production. Broilers require high ME (3,100–3,200 kcal/kg) in starter diets to support rapid growth, while layers require lower ME (2,600–2,800 kcal/kg) to prevent obesity. In hot weather birds eat less, so nutrient density (especially amino acids, minerals and vitamins) is raised and part of the energy is supplied as fat, which produces less heat increment than carbohydrate or protein.

Protein and Amino Acids: Protein is essential for tissue synthesis. In maize-soybean diets methionine and lysine are the first and second limiting amino acids, followed by threonine; chickens also need dietary arginine (they lack a functional urea cycle) and glycine in early life. Broiler starters need about 1.1–1.3% lysine and 0.5% methionine. Starter diets for broilers contain 20–22% crude protein, decreasing to about 18–20% in finisher diets. Layers require about 16–18% crude protein for egg albumen and yolk.

Minerals: Calcium (Ca) and Phosphorus (P) are critical for skeletal development and eggshell quality. Growing chicks need about 1% Ca and 0.45% available P. Layers need 3.5–4.0% Ca, partly as coarse limestone or shell grit fed in the afternoon because the shell forms overnight, and about 0.35–0.40% available (non-phytate) P; added phytase improves the use of plant phosphorus. Sodium (Na) and Chloride (Cl) maintain electrolyte balance. Trace elements like Zinc, Manganese, and Selenium are vital for enzyme function and immunity; manganese or choline deficiency causes perosis (slipped tendon).

Vitamins: Fat-soluble vitamins (A, D, E, K) are stored in the body, while B-complex vitamins must be supplied regularly; chickens synthesise vitamin C, which is added only under heat stress. Vitamin D3 is crucial for calcium absorption, especially in confined birds with limited sunlight, and poultry use D3 far better than D2. Classic deficiencies are rickets and thin-shelled eggs (vitamin D3), curled-toe paralysis (riboflavin), crazy chick disease or encephalomalacia (vitamin E) and haemorrhages (vitamin K).

Water: the most critical nutrient. Birds drink about 1.8–2 times their feed intake by weight, and up to twice as much in hot weather; clean, cool water must always be available, since even a short shortage cuts feed intake, growth and egg production.