Genetic parameters of body weight of broiler chickens measured at 22°C or 32°C.

1. Male broilers (n = 1521) from 247 families were reared from 4 to 6 weeks of age at 22° or 32°C. 2. Genetic correlations between measurements recorded at 22°C and 32°C were 0.73 ± 0.12 for weight gain between 4 and 6 weeks and 0.74 ± 0.08 for food conversion ratio (FCR). Genes controlling weight gain at both temperatures differed to some extent. 3. Heritability of weight gain from 4 to 6 weeks was much lower at 32°C than at 22°C (0.13 ± 0.03 vs 0.24 ± 0.04): selection for increased body weight will thus be much less efficient at 32°C than at 22°C. 4. Conversely, heritabilities of the FCR were very similar at the 2 temperatures (0.28 ± 0.04 at 22°C and 0.27 ± 0.04 at 32°C). Selecting for FCR would thus be efficient at 32°C too. 5. These results justify, at least under our experimental conditions, selecting broiler lines for improved growth performance at 22°C. However, it could be more efficient if broilers are to be reared in hot climates to select for improved FCR rather than for increased body weight.