Interaction of Plant Species Diversity on Grazing Behavior and Performance of Livestock Grazing Temperate Region Pastures

The importance of plant species diversity on performance of livestock grazing temperate region pastures is summarized in this review. As livestock producers seek less capital-intensive production systems, emphasis is redirected toward low-input pasture systems that rely on complex species mixtures to produce forage. Increased plant species diversity has been linked to improvements in ecosystem function. While it is recognized that grazing management can have a profound impact on sward composition, which in turn can affect grazing behavior and animal performance, the effects of increased plant species diversity on animal productivity (and vice versa) have not been well explored. This review addresses mechanisms by which grazing livestock alter cool-season plant species diversity, mechanisms of diet choice, effects of plant species diversity on animal performance, and implications to sustainable domestic livestock grazing systems.We review evidence for these effects at one trophic level, that of grazing livestock in agroecosystems. While grazing behavior research conducted during the last several decades has led to advances in the understanding of plant– animal interactions, improved knowledge of these interactions is crucial for predicting animal performance. Also important is the evaluation of the impact of grazing livestockonmixed sward dynamics. This knowledge will lead to new opportunities to develop environmentally and economically sustainable grazing systems. FARMERS CONTINUALLY FACE new challenges in pasture management, such as evolving agrienvironmental schemes to protect natural resources, and therefore need new management techniques to remain sustainable. Increased plant species diversity has been linked to improvements in ecosystem function, including increased primary (plant) productivity, greater stability in response to disturbance, improved nutrient cycling, and greater resistance to weed invasion (Hector et al., 2005; Spehn et al., 2005). These improvements in function could be of great benefit to agriculture (Tilman et al., 1999; Minns et al., 2001). Preliminary research suggests that manipulating plant diversity can improve primary production in grazed systems (Sanderson et al., 2005) and can reduce weed pressure (Tracy and Sanderson, 2004). It is not clear, however, whether the benefits suggested by these limited-scale studies will applymore broadly tomanaged forage and grazing lands. Definitions of diversity vary widely, so it is important to establish a common ground when discussing the importance of diversity to ecosystem functioning. Diversity encompasses two concepts, the idea of richness—the number of species present—and evenness—the relative abundance of species present (Magurran, 2004). Species richness is often used as a surrogate for diversity in studies of the diversity–function relationship, but is not a complete measure of the diversity of a community. Two sites with the same number of species can vary widely in their levels of evenness, and thus diversities. For example, one site may be dominated by a single species and the other species are all rare, while the second site has similar abundancesof all speciespresent. Speciesabundances areprobably also important to ecosystem function, although this information is often neglected. As used here, diversity refers to taxonomic diversity, the number and abundance of species present; but in other contexts, diversity may describe genetic, functional, or structural variability. Increasing input costs and volatile prices received for agricultural products have resulted in some dairy and livestock operations opting for low-input pasture systems that rely on complex species mixtures to produce forage (Rotz and Cropper, 1998; Sanderson et al., 2001). While research with clipped plots has shown advantages of increased plant species diversity on forage production (Tracy and Sanderson, 2004; Deak et al., 2004), the effects of increased plant species diversity on secondary (animal) productivity have not yet been well explored. Factors such as species richness, sward patchiness, and individual species distribution and their effects on animal grazing behavior and resulting performance remain a subject of debate. Much of the research to date has been conducted using either monocultures or simple two-species mixtures of one legume and one grass species. The effects of complex mixtures (i.e., mixtures of several grasses, legumes, and forbs) on animal performance (and vice versa) are still relatively unknown. Improved knowledge of plant–animal interactions is crucial for predicting diet selection, intake, and performance of grazing animals on complex mixtures, as well as evaluating the impact of grazing livestock on the dynamics of mixed swards. In this paper, we summarize the current knowledge on plant species diversity and performance of livestock grazing temperate region pastures. We discuss mechanisms by which grazing animals alter plant species diversity, mechanisms of diet choice, effects of plant species diversity on animal performance, and implications to sustainable domestic livestock grazing systems. IMPORTANCE OF PLANT SPECIES DIVERSITY IN PASTURES: A PLANT-BASED VIEW The role of plant species diversity in pastures, mainly fromaplant and soil viewpoint,was reviewedbySanderson K.J. Soder, M.A. Sanderson, and S.C. Goslee, USDA-ARS, Pasture Systems and Watershed Management Research Unit, University Park, PA 16802-3702; A.J. Rook, Institute of Grassland and Environmental Research, North Wyke, Okehampton, Devon, EX20 2SB UK. Received 30 Jan. 2006. *Corresponding author (Kathy.Soder@