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Energy and protein utilization in growing cattle.

Limited data are available to describe the different phases of dietary protein and energy utilization in growing cattle as compared with those in adult cattle or in growing nonruminants. The European data on this topic are summarized to indicate application in appropriate feeding standards. Net protein requirements are widely variable with breed and sex. They are lower in steers than in bulls and lower in early maturing than in late maturing breeds. They are clearly defined for growing and fattening bulls where they are influenced by breed, live weight and live weight gain. New systems have been proposed to express the protein allowances. They provide a great step towards a concept explaining N supply to ruminants. However, protein degradability in the rumen, efficiency of microbial protein synthesis, intestinal digestibility and metabolic efficiency of amino acid absorption in the intestine need to be described more accurately. Even if body energy retention measured by the slaughter technique is systematically lower than when measured by calorimetric balance, both techniques can correctly describe the effect of breed sex, weight, or daily gain on energy retained, in relative value, and its distribution between protein and fat deposition. But further research is needed to confirm the distribution of metabolizable energy between maintenance and growth and the efficiency of metabolizable energy utilization for growth. Thus, different authors have preferred to calculate the energy allowances, not by a factorial method, but by regression between energy intake and the corresponding weight and daily gain of animals measured during feeding trials.