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Flashcards in L22: Protein Systems Deck (18)

Crude protein:
most of N requirement of animals is used for protein synthesis, and most N in plants present as protein.
CP = N x 6.25
This assumes:?

  • All food N part of protein
  • All food contains 16% N


How is digestible crude protein determined?
Not all faecal protein is of dietary origin 
-where does the rest come from?

  • Determined by digestibility trials
  • Endogenous N sources other protein origin:
    -gastric secretion
    -gut MM


How do you determine endogenous N?

  • Most common is using labelled 15N in feed
    -non-radioactive stable isotope
    -known conctration of 15N in diet
    -proportion of 15N in faeces

    -difference = endogenous N
  • Amount is also affected by fibre in diet and protein status of animal
    -high fibre = high endogenous N losses in faeces


Is protein digestibility a good indicator of protein value?

No. Efficiency (k value) of absorbed protein varies from source to source


List the protein quality measures applicable to monogastrics

  • Protein efficiency ratio
    - gain in weight/protein eaten
  • Net protein retention
    - weight gain eating protein - weight lost eating no protein) / weight of protein eaten
  • Gross protein value
    - weight gain per g protein eaten / weight gain g casein (protected protein)
  • N Balance (more commonly used)
    - N in diet consumed - N excreted (faece, milk, urine etc)


What is biological value? (simple explanation)

  • proportion of absorbed N retained by body


Describe factors affecting biological value

  • Depends on the number and composition of AA in the molecule

- Closer the composition of the food protein to the body protein = Higher BV

  • Unable to store AA in free state
  • AA either converted to non-essential AA or used as an energy source

- Low BV value

  • Diets with high variety of AA = High BV
  • Diets with low variety of AA = Low BV
  • Theoretically could compare AA profile in feed to that required for different animals and functions – Amino Acid Assays

- Practically difficult due to analysis losses of some AA’s


Describe some food protein measures for pigs and poultry

  • Digestible crude protein
  • Content of essential aa most likely to be limiting (eg lysine)


Describe some protein quality measures for horses

  • Dietary CP commonly used
    -inappropriate as horses mostly digest protein in SI
    -little MCP produced in hindgut is absorbed = high endogenous N excreted
  • More accurate method is calculating available protein (AP)
    -CP less NPN (x6.25) - acid detergent insuluble n (ADIN) (x6.25)


Microbes provide most of energy requirement of ruminant. To do this effectively they must synthesise protein, what do they use to synthesise?

Dietary N


Soluble CHO digesting microbes derive about ..........% of their N from aa and peptides



Current protein quality systems for ruminants require determination of:

• Protein degradability

• Microbial protein yield
• Small intestinal digestibility of microbial and dietary protein • Efficiency of use of absorbed amino acids (biological value)


Protein degradability depends on:

• Depends on
- Surface area available for microbial attack 
- Presence of protective layers
- Physical and chemical nature of protein

• Extent of degradation in rumen depends on this and on rate of passage


What are the 3 ways of measuring protein degradability?

• In vivo – Gold Standard
- N intake, endogenous N, non-ammonia N, microbial N of dietary origin passing the duodenum
- Laborious, error prone, but benchmark

• In sacco
-  Also error prone

• Lab methods
- solubility in buffers, enzymes, chemicals • NIR


Efficiency of N capture is dependant on?

Not only dependent on level of protein degradation but also the provision of energy for MCP synthesis

• Insufficient ME = Absorption of excess ammonia by host
- Excretion of urea

Urea recycling via rumen and saliva high (70%) on low protein diets, low (10%) on high protein diets
- Efficiency of using degraded protein about 0.8


What does microbial crude protein yield depend on and what can it be affected by?

Depends on fermentable energy supply

• Can be affected by
Level of fibre (increases microbial energy requirements)
- Rumen pH (low reduces)
Variation in form of N required by microbes

• Some microbes use amino N, other ammonia, some peptide N
• Simple relationships predicting yield from ME intake can be erroneous

• More complex models relate microbial yield to - Rate of CHO fermentation and passage
- Theoretical microbial growth rate
- Bacterial maintenance energy cost
- Form of N available to microbes


True digestibility of proteins reaching small intestine is assumed to be ....%



Efficiency of use of absorbed amino acids (k factors) depends on?

• Whether origin is microbial or dietary
- i.e. degragable or undegradable protein • What it will be used for e.g. km = 1.0, kwool=0.3

• Often assumed biological value of microbial protein is consistent
- i.e. consistent amino acid composition
- actually inconsistent, BV varies