In ruminant nutrition, the balance of Rumen Undegradable Protein (RUP) and Rumen Degradable Protein (RDP) is essential for optimizing protein utilization. Two principal methodologies are utilized for assessing RUP and RDP in dairy cow diets:
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NRC 2001 System: Introduced in 2001, this method employs in situ techniques within the rumen of live animals via rumen fistulation or through simulated rumen models, identifying three distinct protein fractions (Schwab et al., 2003):
- A: Rapidly Degradable
- B: Potentially Degradable
- C: Undegradable
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CNCPS System (Cornell Net Carbohydrate and Protein System 4.0): Also known as the beef NRC, this system uses in vitro methods with enzymatic or chemical assays in a laboratory environment to classify five protein fractions: A, B1, B2, B3, and C.
CNCPS Protein Fraction Classification:
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Fraction A: Contains non-protein nitrogen (NPN), including ammonia, peptides, and amino acids, which are rapidly converted to ammonia in the rumen and subsequently utilized by ruminal microbes. A significant portion of the degradable crude protein (CP) from dry forage is categorized here.
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Fraction B1: Consists of true proteins that are swiftly converted to ammonia. The majority of degradable proteins from pasture fall into this category.
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Fraction B2: Encompasses true proteins with potential degradability. This fraction substantially influences the level of RUP. The rate of digestion (Kd) and the rate of passage (Kp) are critical in determining the degradability of these proteins in the rumen.
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Fraction B3: True proteins that are predominantly RUP, making them particularly important for ruminant nutrition.
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Fraction C: Comprises proteins that are both undegradable and indigestible. Although these proteins are part of the RUP, they cannot be digested by mammalian digestive enzymes or microbial enzymes and are excreted. This fraction includes tannin-bound proteins, lignin-associated proteins, and Maillard reaction products.
Critical Considerations:
- Protein sources such as poultry slaughter by-products and blood meal, which largely consist of waste materials like feathers, nails, and hooves, exhibit high RUP levels. However, most of this RUP falls into the C fraction of the CNCPS system, which contains indigestible proteins, thereby failing to meet the animal's nutritional requirements.
- The B3 and B2 fractions are vital for effective RUP utilization. During soybean processing, the B2 fraction typically increases, which in turn determines the overall RUP content.
The CNCPS system provides a more precise and comprehensive classification compared to the NRC system, making it a preferred choice for accurately assessing protein fractions in ruminant diets.