Progress in broiler selection: benefits, limitations as assessed by the digestive function, and consequence on dietary lysine concentration
La sélection du poulet de chair : les bénéfices, les limites avec l'exemple de la fonction digestive, et les conséquences sur la concentration alimentaire en lysine.
Résumé
The results observed previously in the broiler lines genetically selected for high (D+ line) and low (D-) digestibilities were considered together with those observed in modern broiler strains. It was deduced from these comparisons that the selection for high meat yield in modern broilers was a major driving force explaining the evolution of their physiological functions. In modern broilers, the reduction of relative organ sizes resulting from the increased meat yield was at the opposite of what it should be expected from an increased feed intake. This paradox was made possible in modern broilers either by an increase in the efficiency of organs or by the disappearance of some functions, as assessed by the observations regarding intestine or gizzard, respectively. The relationships between daily feed intake (DFI), body weight (BW), daily growth, age and feed composition were precisely calculated using a computation model based on the analysis of 42 publications. For a 2.5 kg slaughter weight, it was calculated from this model that one day reduction in the rearing period produces linearly a 0.034 reduction in the value of feed conversion ratio. Thus, selection for increased growth rate is by far the most efficient way for improving feed efficiency in broilers. According to the model, the requirement for relative organ size, as assessed by the DFI/BW ratio, increases slowly compared with the daily growth change. This means that the consequences of an organ weight deficiency can be difficult or long to be perceived. According to the model, future progress in growth rate is associated with no change in protein efficiency, and with higher increase in feed lysine concentration than in the past for each day gained in the rearing period.