ABSTRACT
Plant-animal interactions in grazing systems D. F. Chapman, DairyNZ Lincoln, New Zealand; and W. M. Griffiths, Hamilton, New Zealand
1 Introduction
2 Control of dry matter intake: pasture offered and herbage nutritive value
3 Control of DMI: managing sward canopy structure
4 Sward canopy structure: systems factors
5 Future trends
6 Conclusions
7 Where to look for further information
8 Acknowledgements
9 References
Plants and animals interact continuously in pasture-based livestock production systems. Every bite taken from the sward by animals to meet their energy needs for production and maintenance deprives the plants of their energy source (photosynthetically active leaf area) for growth and maintenance. Plants respond to this through a suite of physiological responses (Richards 1993; Matches 1992) that restore their previous energy status. As a result, the phenotype of plants subject to grazing is highly variable in space and time (Chapman and Lemaire 1993; Briske 1996). Animals react to this variability by altering their grazing behaviour, for example by preferentially selecting where to take their bites from within the horizontal and vertical planes of the sward. The extent to which they do this is influenced by the mass, botanical composition and physical structure of herbage available to the cohort of grazing animals. The same amount of feed can be grown from a sward through different combinations of tiller density and leaf size (and, sometimes, number of leaves per tiller) mediated by plant ecophysiological processes (Chapman and Lemaire 1993). Likewise, the same quantum of feed eaten can be achieved by different combinations of grazing time, bite rate and bite mass (Penning et al. 1991), mediated by animal metabolic and neurophysiological processes (Weston 1982). These interactions are unavoidable, and highly dynamic. The production, profit and environmental footprint of pasture-based livestock systems are strongly influenced by the degree to which management practices are able to control these interactions.
