ANN 111: Principles of Animal Nutrition and Feed Technology (2+1)

METABOLISABLE PROTEIN

• ‘Metabolisable protein’ system is used in the USA .
• Metabolisable protein is that part of the dietary protein which is absorbed by the host animal and is available for use at tissue level.
• It consist partly of dietary true protein which has escaped degradation in the rumen but which has been broken down to amino acids which are subsequently absorbed from the small intestine.
• Microbial protein, synthesised in the rumen, similarly contributes to metabolisable protein.

Calculation of ‘Metabolisible Protein’ of diet:

• 1000g of dietary protein yields 750g of metabolisable protein, but this depends upon the validity of certain assumptions particularly the proportion of dietary crude protein present in non-protein form, the degradability of dietary true protein, and the efficiency of synthesis of microbial protein, which is determined by the supply of energy readily available to the rumen micro-organisms.
• In the system of protein allowances, in terms of rumen degradable protein (RDP) i.e. that available to the micro-organisms, and undegradable dietary protein (UDP) which escapes degradation in the rumen but which undergoes digestion and absorption in the lower gut, and utilisation at tissue level.
•  In calculating allowances, assumptions have to be made concerning microbial nitrogen requirements, efficiency of NPN capture by rumen micro-organisms, digestibility of protein in the small intestine and utilisation of absorbed nitrogen at tissue level.
• The proportion of protein escaping breakdown in the rumen may be estimated in vivo by measuring dietary nitrogen intake, and the non-ammonia nitrogen and microbial nitrogen passing the duodenum. Degradability of nitrogen is then expressed as:

• The method requires accurate measurement of duodenal flow and microbial nitrogen. The former, which requires the use of a dual phase marker system, has a large coefficient of variation (between animals) and many published values must be suspect owing to the small number of animals used in their determination.

• Microbial nitrogen in duodenal nitrogen is usually identified by means of marker substances such as diaminopimelic acid (DAPA), amino ethylphosphonic acid (AEPA), ribonucleic acid and ³⁵S, ³²P and ¹⁵N labeled amino acids.
•  The concentration of marker in the micro-organisms is measured in a sample of rumen fluid.
• Different markers may give results which vary widely, sometimes by as much as 100 per cent and frequently by 20-30 per cent.
• The assumption that the micro-organisms isolated from rumen fluid are representative of those in the duodenum is of doubtful validity since the latter include organisms normally adherent to food particles or the rumen epithelium.
• The formula for calculating degradability given above ignores the fact that duodenal nitrogen contains a significant fraction which is of endogenous origin.
•  It would be more accurate if degradability was calculated as follows:

• The endogenous fraction constitutes about 50 to 200 g/kg of duodenal nitrogen but is difficult to quantify,. A value of 150g/kg is frequently assumed.
•  Measurements of degradability are thus subject to possible error owing to uncertainties in measuring duodenal flow, microbial nitrogen and the endogenous nitrogen and, in addition, are affected by dietary considerations such as level of feeding and the size and frequency of meals.
• It has been calculated that estimates of degradability may vary over a range of 0.3 to 0.35 owing to errors of determination alone.
• Despite its inadequacies this technique remains the only method, currently available, for providing an absolute measure of protein degradability and a standard against which other methods have to be assessed.
• A method of estimating protein degradation in the rumen by incubation of the food in synthetic fibre bags suspended in the rumen is often used. The degradability figure is calculated as the difference between the nitrogen initially present in the bag and that present after incubation, stated as a proportion of the initial nitrogen.