First, in this chapter, those traits that characterize individual pasture genotypes and affect competitiveness through the capture and utilization of light, water and nutrients are reviewed. This is followed by the broader question: 'How far does genotype regulate or influence competitiveness and competitive interrelationships in pasture mixtures?' Finally, the chapter addressed if the relative competitiveness of associated genotypes depends on how well they are adapted to the environment.

The purpose of this chapter is to discuss, with particular reference to south-eastern Australia, the competitive interactions in pastures and how these are modified by management practices. This will include discussion of the general principles of competition within and between sown species and between those sown species and volunteers. Understanding the interconnected nature of the components of pasture plant communities within the conceptual framework of competition provides an ecological context within which to consider the effects of management strategies. The ultimate goal is to contribute to the achievement of truly sustainable pasture-based production systems that will provide an income for livestock producers without degrading the resource base. Emphasis will be on the general principles involved rather than extensive detail on the responses of individual species or pasture types and on competition after establishment.

This chapter examines the composition of pasture seed mixtures that were largely derived from the practical experience of farmers establishing new pastures and renewing old ones. Then principles of plant competition that were derived from this practical experience and supporting experimentation are considered. Finally, special attention is paid to mixing cultivars of the same species in pasture seed mixtures. These considerations are used to provide guidance as to whether the mixing of herbage species and cultivars from the wide choice now available can provide further improvements of pasture yield, quality and stability. How processes of plant competition and succession in pastures can provide these outcomes will also be considered.

The hypothesis to be explored in the present chapter is that plant species diversity conveys stability - in time and space - to pasture performance. Due to environmental heterogeneity in both space and time, it is hypothesized that pasture stability will increase as plant species diversity increases up to an optimum that is greater than would be predicted from short-term, small-plot clipping trials. It is theorized that the simple mixtures commonly recommended today predispose swards to yield fluctuation, weed encroachment, a shorter lifespan and reduced profitability. The needs of commercial grazers would be better served by landscape-based evaluation protocols for both species and mixtures.

This chapter discusses some definitions, concepts and questions frequently encountered in studying competition and succession in pastures. The quantification of competition effects is described.