ABSTRACT
Broilers have been intensively selected for high feed efficiency and fast growth over the past 60 to 70 years. This selection has been extremely successful, increasing broiler growth rate between 1957 and 2005 by over 400%, with a concurrent 50% reduction in feed conversion ratio (Zuidhof et al., 2014). However, this selection for highly efficient, fast-growing broilers had negative effects on broiler welfare. Activity levels of fastgrowing broilers are low, especially during the second half of their life. Leg health can be problematic, with issues such as lameness, and high incidences of footpad dermatitis and hock burns playing a major role. In the past, also tibial dyschondroplasia caused many health and welfare problems, but the incidence of this metabolic disease has been strongly reduced by selective breeding. Fast-growing broilers are also more sensitive to heart and circulation problems compared with slower-growing genotypes, especially when placed in a suboptimal environment. When subjected to a mild cold stress, the incidence of mortality due to heart and circulation problems in fast-growing broilers was 15% compared to 2% in slower-growing genotypes (van Horne et al., 2004). The majority of mortality cases was caused by ascites, a disease causing circulation problems resulting in fluid accumulation in the peritoneal cavity. Regarding reduction of ascites incidence in commercial broilers, substantial progress has been made in recent years (Thiruvenkadan et al., 2011).
