Crude protein & N requirement used for…?
protein synthesis
CP = N x 6.25
What does crude protein assume?
All food N part of protein (not nitrates or nitrites)
all food protein contains 16% N
Digestible crude protein…?
determined by digestibility trials
‘apparent digestible crude protein’
- not all faecal protein of dietary origin
Sources of endogenous N…?
saliva, bile, gastric secretion, pancreatic secretion, sloughed gut mucous cells, bacterial N
Determining endogenous N…? Most common technique?
various techniques, but most common is 15N fed in food
amount also affected by fibre in diet & protein status of the animal
Protein quality measures for monogastrics…?
- protein efficiency ratio = gain in weight/protein eaten
- net protein retention, gross protein value
- N balance - measure N intake & losses
- biological value - proportion of absorbed N retained by body, accounting for endogenous N (animal proteins usually higher)
What is biological value limited?
limited by need for different BVs for different animals & functions
theoretically could compare AA profile of feedstuffs to that required for different animals & functions but practically difficult due to analysis losses of some AAs
Interpreting amino acid assays…?
protein value based on animal experiments & on AA content not always the same
- small changes in AA can alter requirements of others
- antagonism between AAs
- presence of anti-nutritive factors
- animals grow better if N supplied as essential + non-essential AAs, rather than essential only
What is the measure for food protein in pigs & poultry?
digestible crude protein (DCP)
Content of essential AAs most likely what?
limiting (eg. lysine)
What does the term ‘ideal protein’ mean?
reference limiting AA to amount of food in tissue protein (will differ depending on fate - weight gain, milk)
Pig & poultry diets formulated for…?
pig - lysine
chooks - lysine, methionine, tryptophan
Protein quality for horses…? Why is dietary crude protein (CP) an inappropriate method for expressing the protein value of horse feeds? What is a more appropriate method? PAST EXAM QUESTION
dunno why it’s inappropriate (maybe as dietary CP does not take into account fermentation in caecum - LI)
Horse dig CP (‘MADC’ - French) a more appropriate method as it measures AA content of feed & whether it’s digested in SI or LI
Protein quality for ruminants…? What provides most energy requirement? Fibrous/soluble CHO digestion…?
- MICROBES provide most energy requirement of ruminant - to do this effectively they must synthesise protein by using dietary N
Fibrous CHO digesting microbes -> use ammonia
Soluble CHO digesting microbes -> derive 65% of their N from AAs & peptides
Current ruminant protein quality systems require what…?
require determination of:
- protein degradability
- microbial protein yield
- SI digestibility of microbial & dietary protein
- efficiency of use of absorbed AAs (biological value)
Protein degradability (ruminants)… Highly degradable vs undegradable…? What does degradibility depend on…? Extent of degradation?
N varies from highly degradable (1) to undegradable (0)
Depends on:
- SA available for microbial attack
- presence of protective layers
- physical & chemical nature of protein
Extent of degradation in rumen depends on this & rate of passage
Measuring degradability…?
In vivo - measure:
- N intake, endo. N, non-ammonia N, microbial N of dietary origin thru duodenum
In sacco:
- also error prone
Lab methods:
- solubility in buffers, enzymes, chemicals
- NIR
Rate of passage…? What does it affect? What factors influence rate of passage?
- Affects degradability
- Faster for particles that are smaller, denser, more hydrated or more digested, thus increase as digestion & rumination proceed
- Also faster when DMI increases (preg. limits rumen fill; lactation increases intake; high condition reduces intake; temp. effects)
Efficiency of N capture…? Dependent on…?
Dependant on degradation & provision of energy for microbial protein synthesis
- insufficient ME (absorption of excess ammonia - excretion of urea)
- urea recycling via rumen & saliva high (70%) on low protein diets, and low (10%) on high protein diets
- 0.8 commonly used
What does microbial crude protein depend on? Affected by…?
fermentable energy supply
affected by - level of fibre (increases microbial energy requirements)
- rumen pH (low reduces)
- variation in form of N required by microbes
Factors affecting digestibility of microbial protein reaching SI…?
microbial protein mix of bacteria & protozoa
consists of protein, nucleic acid & cell wall proteins
‘true’ digestibility 70%
digestibility of undegradable protein varies with source
Efficiency of use of absorbed AAs (k factors)…?
depends on:
- whether origin is microbial or dietary (ie. undegradable)
- what it will be used for (eg. Kwool = 0.3)
- consisten AA composition
