Metabolism of Absorbed Energy and Amino Acids
Absorbed carbohydrates, volatile fatty acids, and amino acids are used for meeting the animal’s requirements for various physiological functions (maintenance, pregnancy, growth, lactation, repletion of body reserves). The efficiency of use of these nutrients is variable, depending on productive states and many other variables. Therefore, in ration formulation programs, all absorbed energy available to the animal is called metabolizable energy (ME) and all absorbed amino acids are called metabolizable protein (MP). The energy actually deposited in body tissue and milk is called net energy (NE) and the protein deposited in body tissue and milk is called net protein (NP).
Simple equations and efficiency factors are used to compute the efficiency of use of ME and MP for synthesizing NE and NP in tissue and milk, because of the limitations in accuracy in predicting (1) end products of ruminal fermentation and amount of specific absorbed carbohydrates and amino acids, (2) the infinite metabolic routes connecting the numerous tissue and metabolic compartments, (3) the multiple nutrient interactions, and (4) the sophisticated metabolic regulations which drive the partitioning of absorbed nutrients in various productive states. The equations used to predict ME from energy digested in the rumen and intestines (digestible energy, or DE) reflect the variation in methane produced across a wide range in diets. The equations used for lactating dairy cows to predict NE for lactation from ME reflect the energetic efficiency associated with the typical mix of metabolites in the ME, based on respiration chamber data. The equations used for growing cattle to predict NE for maintenance and NE for gain reflect the wide variation in metabolites used in growing cattle and dry cows, and validated with little bias across a wide range of ME contents (NRC, 2000).