It is widely accepted that many plant secondary metabolites ( PSMs) act as plant defense against herbivores。Therefore most studies of the effects of PSMs on animals traditionally have focused on their adverse effects。There are many examples of PSMs causing food avoidance or reducing food intake，but there is as yet relatively little evidence for positive selection of them by herbivores。However，recent public concerns about utilization of PSMs to enhance animal performance and health issues，have stimulated research interest in the role of phytochemicals in diets (Iason 2004，Acamovic and Brooker 2005)。This research attempted to examine the role of functional plants ( here defined as plants containing different PSMs) in sheep foraging from the perspective of potential positive effect of PSMs on herbivores。
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