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 efficie ncy. However, several unfavorable indirect selection responses have also oc curred, amongst these an augmented fat deposition. In order to counteract t his increased fat deposition as well as to study the underlying causal phys iological mechanisms, in several Poultry Research Institutes genetic lean a nd fat broiler lines have been established, although created by different s election strategies. Selecting for a high 6-week body weight or for improve d feed efficiency has resulted in fat and lean broilers, respectively (Denm ark, The Netherlands). In France and Israel, divergent selection on abdomin al fat content has also produced lean and fat broiler lines. In Scotland, l ean and fat broiler lines were obtained by divergent selection on low and h igh plasma very low density levels, respectively. The present paper describ es the effects of these (in)direct selection procedures on the broiler perf ormance characteristics,: endocrine parameters, energy, nitrogen and lipid metabolism, interactions of genotypes with environmental temperature and di et composition, and of the reproductive performance of the dam lines. Besid es the direct selection responses, several indirect selection responses wer e also observed, albeit sometimes different according to the selection crit eria employed. Lean broiler chickens had a better feed and protein conversi on efficiency, and were characterized by a lower hepatic lipogenic activity and higher percentage breast weight compared to their fat counterparts. In direct 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 som atotrophic axis. On the other hand, the latter selection procedure affected the thyrotrophic axis, resulting in a higher plasma T-3:T-4 ratio. This ob servation, together with the lower plasma insulin:glucagon ratio, at least for the French lines, may cause the leaner body composition of broiler chic kens obtained by direct selection for a low abdominal fat content. With res pect to energy metabolism, the most striking difference between lean and fa t broilers irrespective of the selection strategy - was the differential pa rtition of the retained energy between fat and protein energy, Finally, dir ect selection for low fat content resulted rather in differences in egg com position whereas indirect selection for leanness improved laying performanc e. It is clear that both direct and indirect selection for leanness produce s lean broiler chickens but this leanness, as well as the altered reproduct ive performances, are mediated by different endocrine axes according to the selection strategy employed.