Nutrients and their Metabolism with Reference to Milk Production and its Composition | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Nutrients and Ruminant Digestion

• Ruminant Digestion Overview
• Feed consumed by cattle is partially chewed and mixed with saliva before moving to the rumen and reticulum in the ruminant stomach.
• The rumen and reticulum can hold a significant amount of liquid, typically ranging from 40 to 60 gallons in mature animals.
• Ruminants engage in a process called "Chewing the Cud" where they regurgitate and rechew their food.
• Rumination involves regurgitation of ingesta, remastication of solids, and reswallowing of the bolus.
• During rumination, the reticulum contracts, raising the contents above the cardia level and relaxing the cardia to allow the passage of digesta into the esophagus.

• The movement of food from the lower part of the esophagus towards the mouth is facilitated by reverse peristaltic contractions in cows.
• To aid in this process, cows also contract their diaphragm and close the glottis, decreasing thoracic pressure below atmospheric levels, creating a gradient in the esophagus.
• Reverse peristalsis carries the food bolus back to the mouth for further chewing.
• Rumination, the process of regurgitating and re-chewing food, is triggered by the presence of coarse material in the stomach.
• Typically, cows spend around eight hours ruminating, which is the same amount of time they spend grazing.
• During rumination, the bolus is re-chewed, and saliva, especially rich in bicarbonates from the parotid glands, is added to aid in the digestive process.
• The re-chewed bolus is then swallowed, entering the rumen where it mixes with other digestive contents.
• After the entry of a re-chewed bolus into the rumen, another bolus is regurgitated for further rumination, synchronizing with the mixing cycle.
• Cows produce a significant amount of saliva, which helps prevent froth formation in the rumen and aids in the breakdown of food by rumen microorganisms.
• Food remains in the rumen and reticulum for several days, where it is digested by a multitude of organisms, converting carbohydrates into volatile fatty acids, methane, and carbon dioxide.
• The primary volatile fatty acids produced are acetic, propionic, and butyric acid, with smaller amounts of valeric acid and other acids.
• These fatty acids are absorbed, metabolized by the rumen wall, and picked up by the blood, playing various roles in the cow's metabolism.
• Acetic acid is utilized by the mammary gland for milk fat production and energy, and it can also be oxidized by muscle tissue.
• Propionic acid is converted into glucose by the liver, serving as an energy source and a precursor for milk lactose production.
• Butyric acid is transformed into ß-hydroxybutyric acid by the rumen or liver and is used for milk fat synthesis by the mammary gland.

Ruminant Digestive System Overview

• Rumen gases produced during digestion consist mainly of carbon dioxide and methane, which are expelled through belching and absorption into the blood.
• The rumen serves as a storage site for feed, where coarse particles are broken down into finer ones by microorganisms, facilitating digestion in the abomasum and intestines.

• Micro-organisms in the rumen aid in carbohydrate digestion and protein synthesis by utilizing nitrogen from non-protein sources to create amino acids.
• These micro-organisms also contribute to the production of vitamins, particularly B vitamins, which are essential for the ruminant's health.

• The omasum, the third compartment of the stomach, absorbs water from digested material and acts as a filter before passing it to the abomasum.
• The abomasum, the true stomach, secretes gastric juice with hydrochloric acid, pepsin, and renin to break down proteins into simpler compounds and aid digestion.

• The small intestine, with villi lining its walls, is responsible for enzyme secretion and nutrient absorption from ingested material.
• Pancreatic juice, bile, and intestinal juice assist in the breakdown of chyme, preparing feed ingredients for absorption.
• Enzymes like trypsin, chymotrypsin, and carboxypeptidase work together to break down proteins into amino acids and aid in digestion.
• Lipase, another enzyme, facilitates fat digestion by separating fats into fatty acids and glycerol in the intestines.

Nutrient Digestion and Absorption in Animals

Bile Function in Digestion:

• Bile is produced by the liver and stored in the gall bladder, where it is released to aid digestion. It neutralizes acidic chyme from the stomach and helps in the absorption of fatty acids by forming micelles.

Enzymes in Digestive Juices:

• Various enzymes in pancreatic and intestinal juices play crucial roles in breaking down food components. Amylase converts starch to maltose, enterokinase activates trypsinogen, and other enzymes aid in the breakdown of proteins and carbohydrates.

Absorption of Nutrients:

• Once digested, nutrients like amino acids, sugars, vitamins, and minerals are absorbed by the intestinal wall and transported to the bloodstream. These nutrients are essential for the body's physiological functions.

Nutrients and Their Importance

• Nutrient Classification

  Nutrients are classified into proteins, carbohydrates, fats, minerals, and vitamins. Essential nutrients must be obtained from the diet, while non-essential nutrients can be synthesized in the body.

• Role of Microflora in Nutrient Synthesis

  The microflora in the rumen of ruminant animals can synthesize B vitamins and amino acids. Young animals rely more on essential nutrients until their microflora develop sufficiently.

• Utilization of Nutrients

  After digestion, nutrients are absorbed and travel to the liver, where they are modified before being distributed throughout the body. Different nutrients play specific roles in maintaining physiological functions.

Nutrient Requirements for Dairy Cattle

• Energy Requirements

  Lactating cows have varying energy needs based on milk production, reproductive requirements, and growth. Energy from carbohydrates is crucial for supporting these functions.

• Protein and Fat Needs

  Protein requirements are linked to milk yield, while essential fatty acids are synthesized in the rumen. Fat content in feed can impact milk production but should be balanced to avoid excess costs.

• Mineral Supplementation

  Dairy rations typically provide sufficient minerals, but calcium, phosphorus, sodium, cobalt, and iodine may need supplementation to meet cow's nutritional needs.

Dairy Cattle Nutrition Overview

• Calcium and Phosphorus Ratio: The ratio of calcium to phosphorus in a cow's diet during the dry period is crucial. A wide calcium-to-phosphorus ratio can lead to an increased risk of milk fever. It is recommended that the calcium-to-phosphorus ratio in the dry period ration should be around 2:1.
• Sodium Requirements: Cows require 0.9g of sodium chloride per 100 pounds of body weight for daily maintenance. The sodium chloride needed for milk production is approximately 0.7 to 0.8 grams per pound of milk. It is suggested to add 1% salt to concentrate feed due to the universal lack of salt in farm rations.
• Iodine Importance: Iodine is essential for cows, especially during pregnancy and lactation. Iodine deficiency can lead to goitre in cows. Feeding cows iodized salt is recommended in areas prone to goitre.
• Cobalt and Vitamin Requirements: Cobalt is crucial for ruminants as it becomes part of vitamin B12, synthesized by rumen bacteria. The optimal quantity of cobalt required is 0.03 to 0.05 milligrams per pound of dry matter of feed. Vitamins, especially B vitamins and Vitamin K, are synthesized by microorganisms in the rumen.
• Vitamin A, D, E, and Fat Content: Vitamin A deficiency can occur in cattle fed poor-quality forage. Vitamin D is essential for various functions including calcium absorption. Vitamin E helps reduce oxidized flavor in milk. Fat content in feed affects milk production primarily through its energy value.
• Proteins and Non-Fat Solids: The protein and non-fat solids composition of milk can be slightly altered. Changes in protein content affect non-fat solids composition significantly. Mineral and lactose contents in milk are relatively stable.
• Mineral and Vitamin Content in Milk: The mineral content of milk remains stable, with major elements like calcium, phosphorus, chloride, and sodium unaffected by changes in feed. Vitamin levels in milk can be influenced by feed composition.
• Hormones and Milk Production: Thyroxine and growth hormones have been used to increase milk production, but they come with challenges and costs. Thyroxine can boost milk production temporarily, while growth hormones can enhance yield but may not be economically viable for commercial use.