Metabolisable Protein (MP) and Lipid Digestion in Ruminant Nutrition

Fundamental Concepts of Metabolizable Protein (MP) and Nitrogen

• Metabolizable Protein (MP) is a central concept in ruminant diet formulation and protein metabolism.

• Crude Protein (CP) calculation is based on the standard assumption that proteins contain roughly $16\%$ Nitrogen ($N$).

• The formula for Crude Protein based on Total Nitrogen is:          $CP = \text{Total N} \times \left(\frac{100}{16}\right) = \text{Total N} \times 6.25$

• Non-Protein Nitrogen (NPN) sources frequently found in animal feeds include:

  • Free amino acids (AA).

  • Urea.

  • Ammonia.

  • Amines.

  • Nitrates and nitrites.

  • Small peptides.

• Protein and NPN content varies significantly across different feed types:

  • Protein meal: Contains greater than $88\%$ true CP.

  • Forages: Generally contain between $40\%$ and $45\%$ NPN.

  • Lucerne silage: Can contain as much as $88\%$ NPN.

Amino Acid Requirements and Microbial Protein

• Protein synthesis requires 20 different amino acids.

• Ruminants require 10 essential amino acids (EAA) that they cannot produce on their own:

  • Histidine.

  • Isoleucine.

  • Leucine.

  • Lysine.

  • Methionine.

  • Phenylalanine.

  • Threonine.

  • Tryptophan.

  • Valine.

  • Arginine.

• Nonessential amino acids (NEAA) are also necessary for the synthesis of proteins.

• Rumen bacteria are capable of producing both essential (EAA) and nonessential (NEAA) amino acids.

• Amino acid flow to the small intestine originates from three distinct sources:

  1. Microbial Crude Protein (MCP) leaving the rumen.

  2. Rumen Undegradable Protein (RUP).

  3. Endogenous protein secreted by the gut tissue of the animal.

• The primary goal when formulating ruminant diets is to achieve optimum amounts of Microbial Crude Protein (MCP), as MCP provides the majority of the metabolizable protein (MP), including both EAA and NEAA.

• MCP possesses an excellent amino acid profile; diets with a high supply of MCP usually satisfy amino acid requirements if total MP requirements are met.

• Specific software tools are used to estimate amino acid supply for ruminants on various diets, including:

  • NRC Dairy.

  • CNCPS.

  • CPM-Dairy.

  • BCNRM 2016.

• While the first limiting amino acids vary based on the specific diet, the most extensively studied are lysine, methionine, histidine, and leucine.

Dynamics of Protein Degradation in the Rumen

• The proportion of protein degraded in the rumen is determined by two main factors:

  • The inherent properties of the protein itself.

  • The rate of ingesta passage through the rumen.

• The recapture of Nitrogen from Rumen Degraded Protein (RDP) into Microbial Crude Protein (MCP) is influenced by the microbial growth rate. This growth rate is dependent on the supply of rumen-available energy.

• Nitrogen that is not successfully recaptured as microbial protein follows a specific metabolic path:

  • It is absorbed from the rumen as ammonia.

  • It is then converted to urea by the liver.

  • While some urea is recycled back to the rumen, a large portion is eventually excreted in the urine.

Measurement and Classification of Protein Degradability

• Degradability of the nitrogen fraction can be measured using several techniques:

  • In vivo measurement (within the living animal).

  • Determination in sacco (or in situ).

  • Laboratory procedures: solubility in buffer solutions, solubility in enzyme solutions, chemical analysis, Near-infrared reflectance spectroscopy (NIRS), and in vitro rumen fermentation techniques.

• Determination in sacco (or in situ) involves categorizing protein into fractions:

  • $c$: Soluble fraction.

  • Slowly degradable fraction.

  • Undegradable fraction.

  • $r$: Represents the rate of passage through the rumen to the abomasum.

• Protein fractions identified in academic literature (McDonald et al., 2002) include:

  • A: Soluble fraction.

  • B1, B2, B3: Slowly degradable fractions (ordered from more to less degradable).

  • C: Undegradable fraction.

Chemical Partitioning of Dietary Protein

• Chemical analysis utilizes Borate buffer solution, Neutral-detergent (ND) solution, and Acid-detergent (AD) solution to partition protein into specific fractions:

  • Fraction A: Soluble in buffer solution.

  • Fraction B1: Soluble in buffer and precipitated by trichloracetic acid (TCA).

  • Fraction B2: Insoluble in buffer but soluble in both neutral-detergent and acid-detergent solutions.

  • Fraction B3: Insoluble in buffer and insoluble in neutral-detergent solution, but remains soluble in acid-detergent solution.

  • Fraction C: Insoluble in buffer, insoluble in neutral-detergent solution, and insoluble in acid-detergent solution.

Lipid Digestion and Metabolism in the Rumen

• Microorganisms in the rumen can break down dietary fats, modify them, or synthesize entirely new lipids from simple raw materials.

• Difference between species:

  • In non-ruminants, the fatty acid composition of body fats can be changed by altering the composition of dietary fat.

  • In ruminants, the predominant fatty acid found in depot fat is stearic acid. This is the result of hydrogenation, where unsaturated acids are converted to saturated acids within the rumen.

• Dietary fats can be treated so they bypass the rumen and are absorbed directly in the small intestine (SI). This allows for the modification of body and milk fat compositions.

• Characteristics of fat in the diet:

  • High energy source.

  • Highly palatable.

  • Relatively inexpensive.

• Constraints on fat inclusion:

  • If dietary fat exceeds approximately $5\%$ to $6\%$ of the total diet, the animal's ability to utilize dietary fiber decreases.