Enhancing meat production by genetic selection for growth has already produced 5-week-old broilers weighing more than 2 kg. As growth performance characteristics continue to improve and the time it takes to achieve market size decreases, the period of embryonic development becomes a greater proportion of the bird's life. Therefore, in parallel to genetic selection, other approaches, such as environmental manipulations in the embryo or in the early days posthatch, are becoming more relevant for increasing muscle growth and meat production.
Recently, we have shown that nutritional treatments, i.e., providing feed immediately posthatch, or environmental treatments, such as heat conditioning or monochromatic green-light illumination during the first days posthatch, increase muscle growth and breast muscle weight at marketing day. In all cases, the increase in muscle growth was due to changes at the cellular and molecular levels leading to increased satellite cell proliferation and differentiation.
The significant effects on muscle growth resulting from the treatments in the first days posthatch raised the hypothesis that muscle growth could be affected during the embryonic development. In experiments in which eggs were illuminated under monochromatic green light from embryonic day 5 (E5), there was a positive effect on embryo development and posthatch muscle growth. Further studies revealed that this enhanced muscle weight was due to increased satellite cell number and fiber synchronization during early days posthatch. Thermal manipulation at 38.5°C from E16 to E18 for 3 h/day had a delayed effect on satellite-cell proliferation and differentiation, resulting in enhanced hypertrophy of myofibers at market age.