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Recent trends and developments in the area of animal nutrition have been characterized by an increasing interest in the potential impact of plants, herbs and spices on performance, gut health and the immune function of animals. In Europe, the prohibited use of antibiotic growth promoters since the beginning of the year 2006 necessitated to consider alternatives that might help to support the health status of farm animals. Botanical products, also known as phytogenic feed additives, have inconsistent effects on animal performance mainly due to differences in their botanical origin, processing and composition. Herbs, spices, essential oils and oleoresins contain a huge variety of chemical substances. Most of the properties effective additives are considered to be related to essential oils which may stimulate blood circulation, lower load of pathogenic bacteria, and enhance production of digestive secretions as well as immune status in birds. More recently, focus was laid on physiological and health effects of phytogenic feed additives. Animal health is considered as important prerequisite for the production of healthy food from animal origin. Problems occur especially in young animals, for instance in piglets around weaning. The gastrointestinal tract is the primary organ coming that is exposed to botanical ingredients. The significance of a “balanced” intestinal microecology for the health and well-being of animals and humans has been recognized since long time and has also some impact in the immune function of the host. The intestinal microbiota is a complex system that is important for the stability of the digestive processes, the prevention of the entry and colonization of pathogens and the development of the adaptive immune system. Evidence for the effectiveness of botanicals for stabilizing the microbiota is given by clinical and physiological studies. Several field observations in pigs indicated that botanicals may affect the occurrence, severity and duration of post weaning diarrhoea in large scale production units. The mechanisms by which the active compounds interact with the intestinal microflora are not well characterized. However, in vitro studies demonstrated effectiveness of the essential oils against pathogenic Escherichia coli and other bacteria. Effectiveness of plants and extracts may differ considerably depending on the test system. In vitro studies tend to indicate more frequently positive effects of herbs or spices against pathogenic bacteria compared to in vivo trials. Effects on the metabolic activity of the gastrointestinal microbiota have also been reported for chicken, however, more in vivo studies are needed to confirm the result. Only limited evidence is available considering the potential impact of plant extracts or botanicals with the intestinal or general immune system. The difficulty in such trials is the choice of parameters and the model. Some study protocols chose vaccination trials, others used healthy animals or pathogen challenge trials. Few studies are available on other plants or plant constituents on the immune function. Other species than ruminants, pigs and broilers have been rarely considered. Plants and plant extracts have the potential to influence immune function in farm animals. The current situation is characterized by a lack of well controlled studies fulfilling the requirements that would allow a comprehensive assessment of the effects on the immune system. A major flaw of many studies is the study design, especially the use of blends that do not allow a conclusion on the efficacy of individual plants. Further research work in that field is urgently warranted and offers great opportunities for transdisciplinary projects.