Metabolisable protein

The demand for amino acids at tissue level is quantified in terms of truly digestible protein required to be absorbed from the small intestine and designated metabolisable protein (MP), as described in Chapters 14-16. 

The metabolisable protein content of a food is of no use as a guide to the food s ability to satisfy the residual demand for metabolisable protein, since it includes a contribution from ERDP, which has already been taken into account in the form of DMP. The protein content of foods is thus stated in terms of ERDP and DUP. 

The UK metabolisable protein system divides the requirement of an animal into that which is required for supplying the needs of the rumen microbes and that which is required at tissue level. After estimating the contribution of microbial protein to satisfying this demand, the requirement for undegraded dietary protein is calculated. 

As indicated above, most of the protein systems for ruminants (see Chapter 13) used around the world base their estimates of protein requirements for maintenance on endogenous losses of nitrogen but use different factors to translate endogenous losses into dietary requirements. For example, using the UK metabolisable protein... 

Using the AFRC (1993) system, calculate the metabohsable energy (ME) and metabolisable protein (MP) requirements of a 300 kg bull of large breed gaining at 1.1 kg/day. Assume that the animal s diet has an M/D value of 11.0 MJ/kg DM. 

In order to calculate the quantities of metabolisable protein required to satisfy these net requirements, factors for the efficiency of utilisation of metabolisable protein for maintenance, dermal losses, lactation and growth and the conversion of mobilised body protein to milk protein are required. The following are recommended by AFRC (1993) ... 

Amino acids arising from the mobilisation of body protein are utilised with the same efficiency as absorbed amino acids, and liveweight loss has a sparing action on metabolisable protein requirement equal to its protein content, i.e. 138 g/kg. An example of the calculation of the metabolisable protein requirement of a lactating cow is given in Box 16.4. 

Net protein requirement (g/day) Efficiency factor Metabolisable protein requirement (g/day)... 

Microbial protein contributes to satisfying the demand for metabolisable protein, but in the majority of cases, particularly at high levels of production, it cannot completely satisfy the demand. The deficit has to be made good by the truly digestible undegradable true protein (DUP) of the diet. 

It is clear from the above that unlike in AFRC (1993), calculation of the protein requirement for maintenance is not possible unless the DM intake and predicted microbial protein synthesis in the rumen are known. In practice, most commercial dairy cow diets will result in a DMTP ranging from 1000 g/day to 1200 g/day, and a reasonable mid-point of 1100 g/day may be used if a computer rationing program is not available. The MP requirement for pregnancy, milk and body weight change in FiM are the same as that proposed by AFRC (1993). An example of a metabolisable protein requirement calculated according to FiM is provided in Box 16.7. 

The net requirement for nitrogen for maintenance is that required to replace the endogenous urinary loss (EUN) plus part of the metabolic faecal nitrogen loss (MFN) plus loss of nitrogen in hair and scurf (D).The total is termed the basal endogenous nitrogen (BEN) and is of the order of 0.35 g/kg W° /day, which translates to a metabolisable protein demand of 2.19 g/kg Metabolisable protein is assmned... 

The metabolisable protein requirement, where Y is milk yield in kilograms, is then ... 

The daily requirement for degradable protein (ERDP) is given by (FME X 11) g, where FME (MJ/day) is the fermentable metabolisable energy intake. The contribution of dietary ERDP to satisfying metabolisable protein demand may be calculated, as for the cow, as 0.6375 X ERDP. 

The requirement for metabolisable protein is made up of that for maintenance, calculated as 2.19 g/kg plus that for milk production calculated as 49/0.68 = 72 g/kg milk, plus that for wool growth taken as 20.4 g/day, plus or minus an adjustment for liveweight change (AW), calculated as —119 g/kg AW when AW is negative, and +140 g/kg AW when AW is positive. 

Protein requirements for ruminant animals are stated in terms of metabolisable protein, and protein supply to rumen microbes are expressed in terms of effective rumen-degrad-able (ERDP) protein or effective rumen-degradable nitrogen (EDN). 

Calculate the daily metabolisable protein requirements of a dairy cow weighing 625 kg, yielding 33 kg milk with 39 g/kg fat and 33 g/kg protein, and losing 0.5 kg/day using AFRC (1993) and Feed into Milk (2004) (predicted digestible microbial true protein production = 1100 g/day and eating 20 kg DM per day). 

Calculate the daily metabolisable energy and metabolisable protein requirements of a 55 kg hill ewe in the third week of lactation, suckling one lamb and receiving a diet of of 3.55 and losing 100 g W/day. 

MPB Metabolisable protein derived from digestible undegradable protein... 

MPE Metabolisable protein derived from MPB energy supply to the rumen plus... 

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