Feed efficiency (FE) in livestock represents the efficiency of the production at the animal level. It is usually computed asthe ratio of outputs (body weight gain in growing animals) on inputs (feed intake), or its inverse, feed conversion ratio(FCR). It is an integrative trait quantifying gross FE. Combined management, nutrition and genetic strategies haveimproved FE in the past. Monogastric livestock are usually bred indoors and fed from composite feed: breedingenvironment (temperature and ventilation, pathogen loads) and energy and nutrient supply have been adjusted to maximiseanimal performances. Genetic selection essentially has improved FE by increasing lean growth rate in growing animals. Inprolific monogastric species, feed efficiency in breeding adults has been essentially studied in laying hens (eg, Bordas andMerat, 1981), as FE of these adults has a lower economic importance compared to FE of growing animals, and furthermore,is difficult to evaluate.
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