A comparative study of different selection strategies to breed leaner meat-type poultry

Prolonged and intensive selection of broiler chickens for fast body weight gain has produced rapidly growing broiler chickens with a high feed efficiency. However, several unfavorable indirect selection responses have also occurred, amongst these an augmented fat deposition. In order to counteract this increased fat deposition as well as to study the underlying causal physiological mechanisms, in several Poultry Research Institutes genetic lean and fat broiler lines have been established, although created by different selection strategies. Selecting for a high 6-week body weight or for improved feed efficiency has resulted in fat and lean broilers, respectively (Denmark, The Netherlands). In France and Israel, divergent selection on abdominal fat content has also produced lean and fat broiler lines. In Scotland, lean and fat broiler lines were obtained by divergent selection on low and high plasma very low density levels, respectively. The present paper describes the effects of these (in)direct selection procedures on the broiler performance characteristics,: endocrine parameters, energy, nitrogen and lipid metabolism, interactions of genotypes with environmental temperature and diet composition, and of the reproductive performance of the dam lines. Besides the direct selection responses, several indirect selection responses were also observed, albeit sometimes different according to the selection criteria employed. Lean broiler chickens had a better feed and protein conversion efficiency, and were characterized by a lower hepatic lipogenic activity and higher percentage breast weight compared to their fat counterparts. Indirect selection for leanness enhanced the pulsatile growth hormone release, linked to a better protein conversion efficiency and increased lipolytic capacity, whereas direct selection for fat content had no effect on the somatotrophic axis. On the other hand, the latter selection procedure affected the thyrotrophic axis, resulting in a higher plasma T-3:T-4 ratio. This observation, together with the lower plasma insulin:glucagon ratio, at least for the French lines, may cause the leaner body composition of broiler chickens obtained by direct selection for a low abdominal fat content. With respect to energy metabolism, the most striking difference between lean and fat broilers irrespective of the selection strategy - was the differential partition of the retained energy between fat and protein energy, Finally, direct selection for low fat content resulted rather in differences in egg composition whereas indirect selection for leanness improved laying performance. It is clear that both direct and indirect selection for leanness produces lean broiler chickens but this leanness, as well as the altered reproductive performances, are mediated by different endocrine axes according to the selection strategy employed.