Growth curve responses of two ECC-approved genotypes to locomotor, thermal, and nutritional challenges

Growing public concern for animal welfare is shifting European market demand towards alternative poultry production. A key challenge remains choosing a suitable genotype that balances improved welfare with productive performance and contributes to greater stability of production under challenging conditions. In this regard, three trials were performed to test the responses of medium-growing (MG; Ranger Gold) and slow-growing (SG; Kabir) genotypes to walking activity (TRIAL 1; Sw: spontaneous walking activity, Iw: induced walking activity for 30 min/day starting from 15 d of age), temperature (TRIAL 2; TN: thermoneutral at 22.8 ± 2.8°C; HS: heat stress at 27.2 ± 2.8°C), and dietary protein variation (TRIAL 3; CON: conventional basal diet; LOW: diet with a 10% reduction in the crude protein-to-metabolizable energy ratio), with respect to growth curve dynamics. The Gompertz model and generalized linear mixed models were employed to analyze the growth curve of the animals. The parameters evaluated were A, adult body weight; B, the relative distance between initial body weight and projected asymptotic weight; k, the instantaneous relative growth rate; MGR, maximum growth rate; T50, T70 and T99 the estimated times required to reach 50%, 70%, and 99% of projected asymptotic body weight, respectively. The effects analysed were genotype, challenge, and their interaction. In each trial, genotype had a significant effect on all evaluated parameters, with MG outperforming SG across all model estimates. Regarding the genotype × treatment interaction, in TRIAL 1, the MG genotype exhibited higher values for B and MGR. In TRIAL 2, the increase in temperature decreased A and B only in MG. Significant effects of temperature and/or its interaction with genotype were also observed for T50 and MGR, with SG showing smaller changes in growth-curve parameters. In TRIAL 3, the LOW diet tended to reduce MGR, with a numerically larger reduction in MG birds, whereas SG birds showed only a limited change. In conclusion, new sustainable rearing strategies should account for the lower expected performance of SG strains while seeking to maximise the stability and overall value of the production system.