Basis of feed value calculations - Ruminants

For the different animal species, official criteria for calculating feed values have been defined. For ruminants, ME, MP and PB values are calculated.  The compositional data needed to calculate the feed values are obtained from the results of the feed analysis or from the feed table and the coefficients needed for the formulae are obtained from the feed tables. 

The energy values for ruminants are based on metabolizable energy (ME).
The concentration of metabolizable energy is calculated according to a British method (MAFF 1975, 1981, 1984). For further information on comparison of energy evaluation systems, see: Kaustell, K., Tuori, M. & Huhtanen, P. 1997. Comparison of the energy evaluation systems of feeds for dairy cows. Livestock production science 51, 3: 255-266.

Concentrate feeds

The concentration of metabolizable energy in concentrates is calculated based on the amount of digestible nutrients in the feed using the following equation (Schiemann ym. 1972, MAFF 1975, 1984):

ME (MJ/kg DM) = (15.2 × dCP + 34.2 × dCFat + 12.8 × dCF + 15.9 × dNFE)/1000,

where:
dCP = digestible crude protein, g/kg DM
dCFat = digestible crude fat, g/kg DM
dCF = digestible crude fibre, g/kg DM
dNFE = digestible nitrogen free extract, g/kg DM.
 

The concentration of a digestible nutrient (e.g. dCP) is calculated by multiplying the concentration of a nutrient (e.g. CP) by its own digestibility coefficient (e.g. CPd). The digestibility coefficients for the different nutrients are given in the Feed Tables.

Forages

The concentration of metabolizable energy in forages is calculated based on the digestible organic matter concentration in DM, i.e. D-value using the following equations. D-value is expressed as g/kg DM.

1) DM of haylage is over 500 g/kg

The protein value of ruminant feeds is expressed by using two values, which are MP (metabolizable protein) and PB (protein balance in the rumen).

MP presents the amount of amino acid absorbed from the small intestine, which originates from rumen undegradable feed protein and microbial protein synthesized in the rumen. The amount of microbial protein is calculated based on the amount of digestible organic matter minus undegradable feed protein. The effective protein degradability value in the rumen (EPD) is used to calculate the feed protein degradability. The PB value describes the adequacy of nitrogen to the microbes in relation to the energy supplied to the microbes from the feed.

The PB value for feeds describes the sufficiency of the degradable protein in the feed to meet the nitrogen demand of the microbes in the rumen. If the PB value of the feed is negative, the calculated MP content will not be met due to the nitrogen deficiency limiting microbial growth. 

If rumen degradability value of the feed is manipulated by feed processing, the beneficial effects of improved amino acid supply to the animal must be demonstrated in feeding trials before the altered EPD values are included in the Feed Tables.

(Tuori, M., Kaustell, K.V. & Huhtanen, P. 1998. Comparison of the protein evaluation systems of feeds for dairy cows. Livestock Production Science 55: 33-46.)

The feed protein values for ruminants are calculated as follows:

MP = MP_MiP + MP_BP
PB = RDP – MiP
MP_MiP = AA_MiP × MiPD × MiP
MP_BP = BPD × BP
MiP = 152 × (D-value – BP)/1000
RDP = EPD × CP,
BP = CP – RDP = (1 – EPD) × CP,

where:
MP = metabolizable protein (g/kg feed DM)
PB = protein balance in the rumen (g/kg feed DM)
MiP = Microbial protein (g/kg feed DM)
RDP = Rumen degradable protein (g/kg feed DM)
BP = By-pass protein (g/kg feed DM)
MP_MiP = Microbial protein absorbed from small intestine (g/kg feed DM)
MP_BP = By-pass protein absorbed from small intestine (g/kg DM)
D-value = Digestible organic matter, dOM (g/kg feed DM)
EPD = Effective protein degradability
CP = crude protein concentration (g/kg feed DM)
AA_MiP = Proportion of amino acids in microbial protein (constant value 0.75)
MiPD = Microbial protein digestibility (constant value 0.85)
BPD = By-pass protein digestibility (constant value 0.82)

In the Feed Tables the MP value of dairy by-products has been calculated separately for calves that do not yet ruminate. The proportion of amino-N in total N has been estimated to be 0.9. The  digestibility of CP of dry  products is 0.95 and that of liquid ones 1.0.