Responsible use of exotic tropical pasture cultivars â an ecological ...

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Responsible use of exotic tropical pasture cultivars 335 is difficult to come to a general conclusion. However, when considering the use of exotic plants, the following considerations should be made to minimise undesirable effects: • What is the conservation value of the area? Riparian zones and nearby areas are attractive for development but are also key components of landscapes for conservation. Grassy woodlands, particularly those on fertile soils and with gentle topography, are particularly suited to agricultural development, which threatens their survival. They are poorly represented in reserve systems and attractive to sowing due to their responsiveness. Unconstrained development will lead to the loss of this vegetation type, as has occurred in southern Australia. • Exotic species should not be introduced to new areas where they have not previously been used; these areas should be maintained free of exotics. • What is the vulnerability of the existing vegetation to change? At what scale should this decision be made? • Regional, catchment and whole-property perspectives should be taken to planning pasture sowings, not just the individual paddock, as effects may extend off-site well beyond the actual sown area. At the landscape scale, there is a need to identify threshold levels of pasture development beyond which ecosystem processes, including the maintenance of biodiversity, are likely to be adversely affected. • Exotic species are not the solution to all problems — they should be used only where there is an economic gain, and their use should be avoided in other areas. Opportunities for developing improved “environmentally friendly” cultivars As mentioned earlier, the problem is the invasion and colonisation of unsown (and often ungrazed) areas by exotic species (typical of ruderal species) and their subsequent dominance of the vegetation (typical of competitive species). Both colonising ability and the high yields associated with dominance are valuable characteristics in a pasture plant. It may be necessary to sacrifice these to develop “environmentally friendly” cultivars. This could involve selecting for: • grazing dependence — plants with medium palatability (high palatability could lead to loss due to selective grazing and low palatability to dominance as other species are lost) and lower productivity (to reduce the dominance associated with high yields). This can be contrasted with palatable climbing legumes (e.g. Macroptilium atropurpureum), which have poor persistence in grazed pastures but which can dominate ungrazed vegetation. • ability to co-exist — in many situations, diet quality will be higher from a mixture of species in a pasture as it is unlikely one species will be superior at all times of the year and in all conditions. This will involve less emphasis on selecting for high yields as this trait can lead to dominance and suppression of other species. • seed characteristics — a common feature of ruderal plants is strong seed dormancy mechanisms and often large soil seed banks. The use of sterile plants (e.g. Leucaena hybrids) or plants with no seed dormancy (as in crop plants) would greatly reduce weediness. Long-term persistence in such species would depend on the survival of established perennial plants rather than their periodic replacement. Opportunities for management Since a number of exotic plants are causing environmental damage, it will be necessary to manage areas where this is a concern. Management can minimise spread by reducing seed production (e.g. with grazing, mowing, fire) and transport of seed (water, wind, animals, humans). These measures will not prevent invasion but they can reduce the rate of invasion. Management should also aim to prevent establishment and growth of exotic species by minimising disturbance (however, all ecosystems need some disturbance for re-establishment) and encouraging competition from established vegetation. This will be difficult to do except for small areas, but has a role in critical environments. As mentioned in other sections, farm and regional planning of pasture sowings to minimise off-site impacts, and the development of “user-friendly” cultivars, are needed to complement management designed to minimise invasion and establishment.
336 J.G. McIvor and S. McIntyre Opportunities for developing cultivars of native grasses and legumes Australia has a wide diversity of native herbaceous plants but, in contrast with the large efforts expended on exotic pasture species, there have been only limited efforts to develop cultivars from native species (Waters et al. 1997). A number of grasses (e.g. Danthonia, Themeda, Astrebla, Microlaena, Elymus) have been studied in southern Australia and the native legume Glycine latifolia is on pre-release in Queensland. Very high expectations have been held for exotic plants to alleviate production constraints where native plants have been considered inadequate. In some cases, these expectations have been too high. As noted earlier, no plant strategies have evolved for sites with high levels of both stress (droughted, infertile soils) and disturbance (high grazing pressure). The exotic “magic bullet” is not ecologically obtainable for these sites and a change in management to lower the stress and/or disturbance levels is the only solution. Development of cultivars in the future will involve new selection criteria and it will not be possible to focus on single traits to the exclusion of other considerations. Trade-offs will be necessary and in this consideration, native species have a number of advantages: • They have the necessary climatic and edaphic adaptation, but suitable sowing methods and management regimes for their persistence need to be determined. • There are no recognised adverse environmental impacts of native herbaceous species. • Differences in growth and quality between species are frequently less than the differences due to environmental variation, and when native and exotic species are compared under similar conditions, differences are often small. • There are native species with moderate productivity and grazing tolerance (e.g. Heteropogon contortus, Bothriochloa and Dichanthium species). These could form the basis of low-input systems which may not be as productive as high-input systems, but may have lower risks of failure. However, just as developing cultivars from exotic species is a long process, developing cultivars from native species will also take a long time and will require a substantial effort over many years. Conclusions Those actions which constitute responsible use will depend on the value systems of the judge and different judges will come to different conclusions. However, in future, the potential environmental impacts will need to be considered as well as agronomic performance and economic benefits. The continuing use of sown pastures will be a balancing act and it is essential that environmental issues are considered. There are opportunities to develop new cultivars, both exotic and native, and also to develop management systems for them. This will involve tradeoffs and sacrifices and there will always be some risk, but this can be minimised so that their benefits can be captured and unwanted environmental effects prevented. References CAMPBELL, B.D. (1990) Pasture cultivars in ecological perspective. Proceedings of the New Zealand Grassland Association, 51, 139–142. CLEMENTS, R.J. (1996) Pastures for prosperity. 3. The future for new tropical pasture species. Tropical Grasslands, 30, 31–46. DAVIES, J.G. and EYLES, A.G. (1965) Expansion of Australian pastoral production. Journal of the Australian Institute of Agricultural Science, 31, 77–93. EBERSOHN, J.P. and LEE, G.R. (1972) The impact of sown pastures on cattle numbers in Queensland. Australian Veterinary Journal, 48, 217–223. GRIME, J.P. (1977) Evidence for the existence of three primary strategies in plants and its relevance to ecological and evolutionary theory. American Naturalist, 111, 1169–1194. GRIME, J.P. (1979) Plant Strategies and Vegetation Processes. (John Wiley and Sons: New York). GRIME, J.P. (1988) The C-S-R model of primary plant strategies — origins, implications and tests. In: Gottlieb, L.D. and Jain, S.K. (eds) Plant Evolutionary Biology. pp. 371–393. (Chapman Hall: London). HACKER, J.B. (1997) Priorities and activities of the Australian Tropical Forages Genetic Resource Centre. Tropical Grasslands, 31, 243–250. HUMPHRIES, S.E., GROVES, R.H. and MITCHELL, D.S. (1991) Plant invasions: The incidence of environmental weeds in Australia. Kowari, 2, 1–134. LONSDALE, W.M. (1994) Inviting trouble: Introduced pasture species in northern Australia. Australian Journal of Ecology, 19, 345–354. MCIVOR, J.G. and GARDENER, C.J. (1995) Pasture management in semi-arid tropical woodlands: effects on herbage yields and botanical composition. Australian Journal of Experimental Agriculture, 35, 705–715. WALKER, B. and WESTON, E.J. (1990) Pasture development in Queensland — A success story. Tropical Grasslands, 24, 257–268. WATERS, C.M., LOCH, D.S. and JOHNSTON, P.W. (1997) The role of native grasses and legumes for land revegetation in central and eastern Australia with particular reference to low rainfall areas. Tropical Grasslands, 31, 304–310. WESTON, E.J., HARBISON, J., LESLIE, J.K., ROSENTHAL, K.M. and MAYER, R.J. (1981) Assessment of the agricultural and pastoral potential of Queensland. Technical Report No. 27, Agriculture Branch, Queensland Department of Primary Industries, Brisbane.
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