(a) Define the term 'total digestible nutrients' and describe the various factors which affect the total digestible nutrients…

(a) Total Digestible Nutrients and Influencing Factors

Total Digestible Nutrients (TDN) is a standard measure of the apparent digestible energy content of feedstuffs, expressed on a percentage basis or weight. It is defined as the sum of digestible crude protein (DCP), digestible crude fibre (DCF), digestible nitrogen-free extract (DNFE), and 2.25 times the digestible ether extract (DEE), reflecting the higher gross energetic value of fats: TDN (%) = DCP + DCF + DNFE + (DEE × 2.25).

Digestibility coefficients represent the fraction of a given nutrient absorbed from the gastrointestinal tract. They vary widely based on chemical and physical factors. Feed composition heavily dictates TDN; increased crude fibre, extensive lignification, and silica reduce the enzymatic access to structural carbohydrates, depressing digestion. Anti-nutritional factors like tannins, phytates, and trypsin inhibitors bind dietary nutrients and endogenous enzymes, reducing overall digestibility.

Processing methods—including grinding, pelleting, steam flaking, and alkali treatment of crop residues—enhance digestion by breaking physical seed coats and disrupting lignocellulose matrices. Animal-specific factors such as species (ruminants digest structural carbohydrates far better than non-ruminants due to microbial fermentation), age, and physiological status (high intake levels accelerate passage rate, moderately lowering digestibility) also determine TDN. A key limitation of TDN is that it does not account for gaseous and heat increment energy losses, leading to an overestimation of the nutritive value of fibrous roughages relative to concentrates in high-producing animals.

(b) Protein Quality Evaluation for Monogastrics and the Biological Value Method

Measures used to evaluate protein quality in monogastric nutrition include Protein Efficiency Ratio (PER), Net Protein Ratio (NPR), Net Protein Utilisation (NPU), Biological Value (BV), Chemical Score (CS), Protein Digestibility-Corrected Amino Acid Score (PDCAAS), and Digestible Indispensable Amino Acid Score (DIAAS).

The Biological Value (BV) method, originally developed by Thomas and Mitchell, evaluates protein quality by measuring the percentage of absorbed nitrogen that is retained in the animal's body for maintenance and growth. It reflects how well the amino acid profile of the absorbed protein matches the animal’s tissue requirements.

BV = ((Nitrogen Retained)/(Nitrogen Absorbed)) × 100

The experimental procedure employs nitrogen balance studies using growing rats or poultry. Animals are housed in metabolism cages to collect precise daily outputs of urine and faeces. To differentiate metabolic and endogenous losses from dietary losses, animals are fed a test protein diet followed by an isocaloric protein-free diet.

True BV accounts for metabolic faecal nitrogen (Fₘ) and endogenous urinary nitrogen (Uₑ):

True BV = (I - (F - Fₘ) - (U - Uₑ))/(I - (F - Fₘ)) × 100

where I is nitrogen intake, F is faecal nitrogen, and U is urinary nitrogen. In contrast, Apparent BV is calculated without endogenous corrections as:

Apparent BV = (I - F - U)/(I - F) × 100

Apparent BV consistently underestimates true protein retention because it ignores non-dietary nitrogen shed from intestinal mucosal sloughing and basal metabolic tissue turnover.

(c) Balanced Ration for Breeding Bulls

A balanced ration is the total allowance of feed and fodder supplied to an animal over 24 hours that provides all essential nutrients—energy, crude protein, minerals, vitamins, and water—in adequate amounts and exact proportions required for maintenance and optimum production without adverse health effects.

For breeding bulls, the ration must strictly adhere to ICAR (Indian Council of Agricultural Research) feeding standards to maintain high libido, optimal semen volume, and viable sperm counts. Excess energy intake must be avoided; over-conditioning and excessive fat deposition in the scrotal neck insulate the testes, disrupt thermoregulation, and reduce semen quality. Bulls must be maintained at a Body Condition Score (BCS) of 3.0 to 3.5 on a 5-point scale.

The ration should possess moderate energy density, with 10–12% crude protein comprising balanced amino acids to support sustained spermatogenesis. The calcium-to-phosphorus ratio must be maintained between 1.5:1 and 2:1 to avoid urinary calculi and metabolic dysfunctions. Micronutrient fortification is critical: adequate zinc supports testosterone synthesis and sperm motility; selenium works synergistically with Vitamin E as an antioxidant protecting lipid-rich sperm membranes against peroxidation; and Vitamin A maintains the structural integrity of the germinal epithelium in seminiferous tubules. In Indian artificial insemination and frozen semen stations, precision balancing using quality green fodder alongside customized concentrate mixes forms the bedrock of herd sire longevity and fertility.