Adding plant extracts in broiler diets has been proposed as a strategy in the effort to replace antibiotics. It has been long surmised that plant extracts have additional effects, on a metabolic level, apart from any bacteriostatic or bacteriocidic effects.

In a recent experiment, a mixture of plant extracts was added in a low-energy diet to study broiler growth and energy metabolism. The two included a maize-based control diet, and the control diet with 100 g/t of supplementary plant extracts. Feeding the mixture of plant extracts increased weight gain by 14.5 percent (P = 0.009), improved feed efficiency by 9.8 percent (P = 0.055), and tended to increase (P = 0.062) carcass energy retention and reduce (P = 0.062) total heat loss compared with feeding the control diet. There was a 16.1 percent increase (P = 0.015) in carcass protein retention but no difference in carcass fat retention. Supplementation of plant extracts resulted in a 12.5 percent increase (P = 0.021) in dietary net energy for production, while no changes in dietary metabolizable energy were observed.

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It appears that certain plant extracts may improve the nutritional value of a low metabolizable energy maize-based diet through improving the metabolic efficiency of converting absorbed dietary energy into tissue. And, although these plant extracts do not affect dietary metabolizable energy, they may improve the utilization of energy for growth, most probably affecting metabolic utilization of absorbed nutrients. Unfortunately, being a mixture of plant extracts, it was not possible to discern which one(s) of its components had this desirable effect.