Essential oils in broiler nutrition

Abstract

Dietary antibiotics at low, subtherapeutic levels have been shown to improve growth performance in farm animals. However, there is a trend to look for alternatives to dietary antibiotics, due to occurrence of antibiotic-resistance bacteria. The present thesis explored the essential oils as the possible alternative to dietary antibiotics. An essential oil is a mixture of fragrant, volatile compounds derived from the aromatic plant. Dietary feed additives based on essential oil components are marketed to enhance growth performance and health. It is claimed that under practical conditions dietary essential oil components improve digestion and stimulate feed intake, and consequently raise growth performance. However, the effect of essential oils on animal performance has not been proved in a controlled study. The aim of this study was to evaluate the role of essential oils as the possible alternatives to antibiotics in poultry nutrition. Female broiler chickens were used as an animal model to study the effect of essential oils on growth performance, activities of digestive enzymes, macronutrient digestion and lipid metabolism. Furthermore, experiments were carried out to test whether dietary essential oil components could either counteract anti-nutrient effect of carboxymethylcellulose (CMC) or rye. Finally, the possible synergistic effects of individual essential oil components were tested in relation to growth performance when chickens were fed on a diet containing CMC. The literature review describes the general biological activities of essential oils. Dietary essential oils can have several useful biological properties, e.g., acting as antimicrobial substance and antioxidants, and being hypocholesterolemic, affecting flavor, and stimulating the digestion process. Their metabolic pathways have been well elucidated, and based on that observation, it was suggested that their side effects are unlikely. Due to a limited number of controlled studies on the effect of essential oil components on growth performance in broiler chickens, an experiment was conducted to describe the effect of thymol, cinnamaldehyde and a commercial preparation of essential oil components (CRINA Poultry) on growth performance, digestive enzyme activities and macronutrient digestibilities. Corn-soybean meal based diet was used to prepare the experimental diets and chicks were housed in wire-bottomed cages. It was shown that no effect of essential oil components on any of parameters monitored in female broiler chickens was observed, but it can not be excluded that positive effects would have been observed under less hygienic environmental conditions or when using a less digestible diet. However, in this study dietary cinnamaldehyde significantly lowered water intake without affecting growth performance. Consequently, dry matter contents of excreta were higher in birds fed diet containing cinnamaldehyde. The observed cinnamaldehyde-induced reduction of spontaneous water intake could be commercially interesting. In subsequent studies, we further screened dietary carvacrol, and -ionone as potential feed additives in female broiler chickens. Furthermore, we tested the alleged hypocholesterolemic effect of dietary essential oil components when the chickens were fed either cholesterol-free or cholesterol-rich diets. It was found that dietary carvacrol lowered feed intake and weight gain, but also lowered the feed:gain ratio. The carvacrol effect on feed:gain ratio could relate to increased efficiency of feed utilization and/or altered carcass composition. Unfortunately, carcass composition of the chickens was not analyzed. Carvacrol lowered plasma triglyceride concentrations, but did not affect plasma cholesterol. Dietary -ionone failed to affect growth performance and the plasma lipid profile, irrespective of whether the diet was cholesterol free or rich in cholesterol. The question then addressed was whether dietary essential oils (thymol, cinnamaldehyde, CRINA Poultry) could antagonize the negative effect of CMC on growth performance in broilers. Intestinal viscosity was increased by CMC inclusion, but was not lowered by dietary essential oils. However, addition of cinnamaldehyde and CRINA Poultry to the CMC diet tended to counteract the negative effect on growth performance. Fat digestibility was reduced by CMC, but cinnamaldehyde and CRINA Poultry partially relieved the CMC-induced inhibition of fat digestion. This study indicates that cinnamaldehyde and CRINA Poultry, but not thymol, may antagonize the negative effect of CMC on growth performance which may relate to improving fat digestibility. In following study, we observed whether dietary essential oil components, i.e., thymol, cinnamaldehyde and CRINA Poultry could have effects on growth performance, fat digestibility, and intestinal viscosity in female broiler chickens fed on rye. Feeding rye to chicken lowered weight gain, elevated intestinal viscosity and reduced fat digestibility. Addition of dietary cinnamaldehyde to rye diet partially counteracted rye-suppressed weight gain in female broiler chickens. However, intestinal viscosity and fat digestion were not modified by dietary cinnamaldehyde. The partial counteraction of dietary cinnamaldehyde on rye-induced growth depression seemed to be mediated by concomitant increase in feed intake, however underlying mechanisms remain unknown. It would expect that a mixture of essential oil components could act synergistically on growth performance. Carvacrol and cinnamaldehyde were selected based on earlier studies. In addition, CMC was included to enhance treatment contrasts. Individual components when supplemented to the CMC diet had no apparent effects on growth performance. On the other hand, a mixture of carvacrol and cinnamaldehyde added to CMC diet significantly impaired growth performance of the chickens. This latter study suggests that a certain combination of two or more essential oil components can impair growth performance of chickens when birds are fed a diet with the soluble fiber, CMC. It is generally concluded that dietary essential oil components have potential to improve performance of broiler chickens. However, the consistently alleged hypocholesterolemic effects of dietary essential oil components were not observed. Based on the present work, future studies with dietary essential oil components should focus on identifying synergistic effects of different essential oil components and their application to animal production.