Evaluation of Feeding Experiments, Digestibility and Balance Studies | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Feeding Experiments

• Feeding of animals initially relied on trial and error methods.
• Later, feeding trials began in Agricultural Experimental stations with a scientific approach.
• These trials were conducted in laboratory animals, providing essential data for human and farm animal nutrition.

Comparative Feeding Trials:

• Feeding trials record the results of a specific feed or ration on growth, milk production, and other functions.
• Comparisons between different rations help determine the effectiveness of each.
• For example, comparing fish meal and linseed meal for hogs showed differences in daily weight gain.

Feeding Trials with Laboratory Animals:

• Small laboratory animals like rats are chosen for these trials due to lower costs and quicker results.
• Results from these trials cannot be directly applied to larger livestock but can offer valuable preliminary information.
• These trials can provide insights into the value of feeds in terms of growth, reproduction, and lactation.

Purified Method:

• In experiments with laboratory animals, nutrients are provided in pure forms rather than as part of a feed mixture.
• This method allows for the study of individual nutrients without affecting other nutrient interactions.
• Carbohydrates, proteins, fats, minerals, and vitamins are given as pure substances for precise evaluation.

Purified Diet Method Overview:

• The purified diet method led to the discovery of various minerals and vitamins.
• It involves using purified diets to assess feed for vitamin content and protein quality, known as the biological method.

Disadvantages of the Purified Diet Method:

• Ingredients in purified diets cannot be completely devoid of minerals without altering their structure.
• Trace elements required in minute quantities may go unnoticed due to the inability to completely eliminate them from experimental diets.
• Proteins can undergo changes from their natural form during the purification process.
• The type of carbohydrates used can impact the accuracy of results, especially concerning certain vitamins affected by different carbohydrates' influence on vitamin synthesis in the body.

Characteristics of Purified Diets:

• Purified diets should possess suitable physical properties and be palatable to ensure they can be consumed in adequate amounts to serve their intended research purposes.
• Preparation of purified diets is relatively straightforward for smaller animals like rats and poultry.
• Recent experiments involving purified diets have extended to larger farm animals such as calves, lambs, and pigs.

Types of Feeding Experiments

Feeding Methods: Individual vs. Group Feeding

• Group feeding is convenient and saves labor, but it can be challenging when there is significant variability in individual behavior within the group. This variability complicates determining feed consumption and production.
• Individual feeding, on the other hand, eliminates these challenges. It allows for a direct correlation between food intake and performance.
• Individual performance becomes particularly valuable when small differences are expected, and when quantitative data is necessary. Individual records are more beneficial statistically compared to group records.
• Comparing the composition of animals before and after the feeding experiment provides crucial results.

Determination of Digestibility:

• The nutrient value of feed is crucial in assessing the significance of a particular food. The actual value of ingested nutrients depends on how effectively the body can utilize them, known as digestibility. Undigested nutrients do not contribute to the body's nourishment.
• Variations in digestibility exist among different foods in various species, with the most prominent differences observed in roughages. Ruminants efficiently digest and utilize large quantities of roughages, facilitating rapid growth, unlike non-ruminants. However, the variance in digestibility of concentrates is not as significant between ruminants and non-ruminants.
• Digestibility of feeds can be determined by conducting a digestion trial specific to the species under consideration. This trial involves monitoring the intake of nutrients and their excretion in feces and urine.

Digestion Trial:

• A digestion trial involves tracking the nutrients consumed and excreted in feces and urine. Various methods, including using markers in omnivorous and carnivorous animals, help in determining digestibility accurately.
• In herbivorous animals, due to their complex digestive system, the ration to be tested is fed consistently over an extended period. The collection of feces begins after ensuring the digestive tract is free from undigested material.
• Longer experimental periods yield more precise results, particularly crucial for herbivorous animals like ruminants.

Controlled and Ad libitum Feeding:

• Controlled feeding involves regulating the amount of feed provided, while ad libitum feeding allows animals to consume as much as they desire. Ad libitum feeding is common in farming and offers unbiased results.
• Equalized pair feeding compares two rations under controlled conditions, using similar animals to eliminate variable food intake effects.

Slaughter Experiments:

• In specific cases where data on protein tissues or bone effects is required, slaughter experiments are conducted. This involves slaughtering a portion of animals before the feeding experiment begins.
• Special care is taken in digestion trials to ensure uncontaminated feces collection, often facilitated by the use of metabolic cages or stalls for accurate urine and feces collection.

Calculation of Nutrients in Animals:

• Animals' ingested nutrients are determined through chemical analysis and multiplying by feed consumption.
• Excreted nutrients are calculated by multiplying excreta amount with nutrient percentages found in it.
• Digested nutrients are found by subtracting excreted nutrients from consumed nutrients.
• The percentage of digested nutrients is termed the digestion coefficient.

Minerals Excretion:

• Minerals are excreted through feces and gut, making it challenging to separate excreted minerals.

Digestibility Determination:

• Roughages' digestibility can be easily determined through feeding trials.
• Concentrates' digestibility is found by difference as they cannot be fed separately.
• Indicator methods use inert substances to estimate digestibility, like lignin or chromogens.

Factors Affecting Digestibility

Nature and Relationship of Nutrients: 

• The breakdown of nutrients during digestion varies based on their nature and how they interact with other nutrients. For instance, the presence of easily digestible carbohydrates like starch or sucrose can reduce the digestion of crude fiber in cattle rations. The type of roughage affects bacterial flora activity in the rumen, irrespective of its fiber content. For example, replacing poor-quality hay with alfalfa hay can enhance microbial activity, possibly due to specific vitamins or factors present in alfalfa hay. Additionally, a wider nutritive ratio can lower the digestibility of nutrients, especially proteins.

Source of Feed: 

• The digestibility of nutrients is influenced by the source of the feed. This is particularly evident in the case of ether extract in seeds and their by-products, which mainly consist of easily digestible fatty acids. In contrast, the ether extract in roughages contains a significant amount of non-saponifiable portions that are not easily digestible.

Time Constraints for Digestion: 

• The digestibility of feed can be hindered by insufficient time for complete digestion, especially for less easily digestible substances or due to inadequate absorption. Some feeds may pass through the digestive tract too quickly, reducing the digestion time. Conversely, slow movement of certain feeds through the intestine can lead to wastage through fermentation. This also explains why increasing food intake beyond a certain point can decrease nutrient digestibility.

Feed Mixtures: 

• The digestibility of feed mixtures is not merely an average of the digestibility values of their individual constituents. Each feed component can influence the digestibility of other constituents present in the mixture.

Feed Preparation Effects: 

• Grinding grains does not necessarily increase digestibility in ruminants like cattle, which chew food thoroughly. However, grinding can enhance digestibility in animals like sheep that chew less efficiently. Grinding is beneficial for very young animals without teeth or for older animals with poor dental health. Chopping roughages does not offer significant benefits, as most animals can chew roughages effectively to facilitate digestion, especially with high-quality hay. 

Impact of Cooking: 

• Cooking generally does not enhance the digestibility of feed, except in the case of swine feeding.These factors collectively influence the digestibility of feeds and play a crucial role in the efficiency of nutrient utilization by animals during digestion.

Value of Data Obtained in Digestion Trial:

• Factors Affecting Digestibility: Several factors influence how well feed can be digested. It is crucial to consider these factors when analyzing data from digestion trials.
• Optimal Ration Levels: The most valuable data is obtained when animals are fed at levels that support satisfactory production. This balance ensures adequate intake and protein content.
• Nutrient Supply Importance: All nutrients must be adequately supplied, as a deficiency in one nutrient can impact the digestibility of others.
• Nutrient Loss in Digestion: Digestion data helps determine the loss of nutrients as they pass through the digestive system. For ruminants, higher carbohydrates are often lost as gases, leading to potential inaccuracies in absorption measurements.
• Digestibility Challenges: Certain nutrients like crude fiber may undergo transformations during digestion, affecting the accuracy of digestibility coefficients.
• Challenges in Fat Digestion: Fat digestion coefficients in herbivorous animals may face errors due to limitations in the analysis method. Despite these challenges, the impact is generally minimal due to the small amounts of fat present.
• Significance of Digestion Trials: Despite the complexities and limitations, digestion trials play a crucial role in determining the digestibility of feed constituents.

Balance Studies - Nutritional Balances

Introduction to Balance Studies:

• Physiologists in the 16th century identified losses of nutrients not only through urine and feces but also through invisible perspiration, now known as respiration.
• Sanctorius conducted the first balance experiments, quantitatively tracking nutrient intake and loss to determine changes in body weight.
• These experiments help measure nutritive value and the body's nutritional status.

Types of Balance Measures:

• Balance of Matter Experiments: Focus on measurable substances that can be weighed.
• Balance of Energy Experiments: Include considerations of heat losses.

Balance Experiments - Nitrogen and Minerals:

• Quantitative estimation of minerals and nitrogen in feed versus body's output to determine balance.
• Positive balance indicates an excess input, equilibrium shows a balance, and negative balance signifies a deficit.
• Nitrogen balance reflects protein development, while calcium and phosphorus balances indicate sufficiency for growth, lactation, and other needs.

Balance Experiments and Kellner's Starch Values

• The measurement of gaseous exchange is crucial for determining the gain or loss of fat in the body and for evaluating energy production ratios.
• Gaseous exchange studies involve using a respiration apparatus, either by placing the subject in a controlled chamber or utilizing a face piece for analyzing inhaled and exhaled air.

• The relationship between oxygen consumed and carbon dioxide exhaled is termed as the respiratory quotient (RQ).
  (i) The RQ varies based on the substance being oxidized. For carbohydrates, the RQ is 1, as oxygen is solely used for carbon oxidation. In contrast, fats have an RQ of less than 1 (0.7 for body fat) due to incomplete oxidation.
  (ii) Proteins have an uncertain RQ (average of 0.8) as they are incompletely oxidized in catabolism.
• Respiratory quotient analysis helps determine the type of feed consumed by an animal.

Nitrogen-Carbon Balance:

• Combining carbon and nitrogen balance assists in assessing fat gain or loss, especially when carbohydrate content is minimal.
• By examining carbon and nitrogen levels in food, feces, urine, and gaseous output, the impact of feed on animal consumption can be determined.

Kellner's Starch Values or Starch Equivalents:

• Kellner introduced starch values by determining the relative amounts of digestible nutrients required to produce a unit of body fat when added to a basal maintenance ratio of pure carbohydrate, protein, and fat.
• The fat-producing power of feed was found to be lower than that of pure nutrients due to factors like fiber content affecting digestion efficiency.
• Starch values represent the amount of starch needed to produce the same fat amount as 100 kg of a specific feed.
• Kellner provided starch values for approximately 300 feeds for evaluation purposes.

Balance of Energy and Nutrition

• Food primarily serves to generate energy for various bodily functions.
• Measuring energy gain or loss helps gauge nutritional intake and assess the value of feed.
• It accounts for heat loss not explained by other means.

Energy Measurement:

• Energy changes in the body are quantified in calories.
• A small calorie raises 1 gram of water by 1°C. A large calorie raises 1 kg of water by 1°C (also known as kilocalorie).
• A therm equals 1000 kcal.

Types of Energy

• Gross Energy: Heat released when burning energy-rich substances completely.
• Carbohydrates: 4.16 kcal per gram
• Proteins: 5.65 kcal per gram
• Fats: 9.40 kcal per gram
• Net Energy: Portion of gross energy usable by the body.

Physiological Fuel Values:

• Primarily applicable to humans, not universally to animals.

Net Energy Values:

• Developed by Armsby using a respiration calorimeter.
• Considers energy losses in digestion and assimilation of feed.

Utility of Energy and Balance Studies:

• Energy metabolism influences all bodily functions.
• Understanding energy needs aids in efficient production.
• Energy studies assist in determining food values, feeding methods, and animal feed efficiency.
• Balance studies provide insights into metabolic processes without animal sacrifice.