Physiological aspects of growth promotion in poultry

Growth promotants used in poultry production fall into two categories: those which increase the utilization of food (e.g. antibiotics) and those which have anabolic activity.The mode of action of antibiotics is not certain. Several interacting factors may be involved: altered gut microflora, reduced ammonia production, thinner intestinal walls, reduced epithelial cell turnover and improved nutrient absorption. It has been suggested that antibiotics have a systemic action but evidence of gross physiological change is difficult to find. Thyroid function was not affected after feeding aureomycin but penicillin-fed chickens had a greater plasma growth hormone concentration than control birds. The plasma corticosterone responses to a stressor, corticotrophin, were similar in control and penicillin-fed birds.Goitrogenic substances cause lipid deposition and thyroactive substances improve feather cover but neither consistenly improve growth.Oestrogenic substances tend to stimulate energy consumption and lipogenesis at the expense of protein deposition. Use is made of them to improve carcass quality of capons and turkeys. Androgenic substances have either no effect or depress growth.Growth hormone improves weight gain, increases protein and decreases fat in the carcass. It may be of use in the production of older table birds.