Protein systems

Question 1

Crude protein & N requirement used for…?

Answer 1

protein synthesis

CP = N x 6.25

Question 2

What does crude protein assume?

Answer 2

All food N part of protein (not nitrates or nitrites)

all food protein contains 16% N

Question 3

Digestible crude protein…?

Answer 3

determined by digestibility trials
‘apparent digestible crude protein’
- not all faecal protein of dietary origin

Question 4

Sources of endogenous N…?

Answer 4

saliva, bile, gastric secretion, pancreatic secretion, sloughed gut mucous cells, bacterial N

Question 5

Determining endogenous N…? Most common technique?

Answer 5

various techniques, but most common is 15N fed in food

amount also affected by fibre in diet & protein status of the animal

Question 6

Protein quality measures for monogastrics…?

Answer 6

• 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)

Question 7

What is biological value limited?

Answer 7

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

Question 8

Interpreting amino acid assays…?

Answer 8

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

Question 9

What is the measure for food protein in pigs & poultry?

Answer 9

digestible crude protein (DCP)

Question 10

Content of essential AAs most likely what?

Answer 10

limiting (eg. lysine)

Question 11

What does the term ‘ideal protein’ mean?

Answer 11

reference limiting AA to amount of food in tissue protein (will differ depending on fate - weight gain, milk)

Question 12

Pig & poultry diets formulated for…?

Answer 12

pig - lysine

chooks - lysine, methionine, tryptophan

Question 13

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

Answer 13

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

Question 14

Protein quality for ruminants…? What provides most energy requirement? Fibrous/soluble CHO digestion…?

Answer 14

• 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

Question 15

Current ruminant protein quality systems require what…?

Answer 15

require determination of:

• protein degradability
• microbial protein yield
• SI digestibility of microbial & dietary protein
• efficiency of use of absorbed AAs (biological value)

Question 16

Protein degradability (ruminants)… Highly degradable vs undegradable…? What does degradibility depend on…? Extent of degradation?

Answer 16

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

Question 17

Measuring degradability…?

Answer 17

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

Question 18

Rate of passage…? What does it affect? What factors influence rate of passage?

Answer 18

• 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)

Question 19

Efficiency of N capture…? Dependent on…?

Answer 19

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

Question 20

What does microbial crude protein depend on? Affected by…?

Answer 20

fermentable energy supply
affected by - level of fibre (increases microbial energy requirements)
- rumen pH (low reduces)
- variation in form of N required by microbes

Question 21

Factors affecting digestibility of microbial protein reaching SI…?

Answer 21

microbial protein mix of bacteria & protozoa
consists of protein, nucleic acid & cell wall proteins
‘true’ digestibility 70%
digestibility of undegradable protein varies with source

Question 22

Efficiency of use of absorbed AAs (k factors)…?

Answer 22

depends on:

• whether origin is microbial or dietary (ie. undegradable)
• what it will be used for (eg. Kwool = 0.3)
• consisten AA composition