Scott, J.M. Dr.: Ph. (02) 6773 2436; Fax. (02) 6773 3238; firstname.lastname@example.org
Boschma, S. Dr.: Ph. (02) 6763 1202; Fax. (02) 6763 1222; email@example.com
Research organisation: University of New England, Armidale NSW 2351
Sponsors: LWRRDC, Land and Water Resources Research and Development Corporation; MRC, Meat Research Corporation
1. To examine the effects of grazing intensity and droughts of varying intensity and seasonal timing on the persistence/mortality of a wide range of introduced and native temperate perennial grasses;
2. To define the stress thresholds which threaten the persistence of this range of perennial grasses in a way which will be useful both to producers and to those developing models incorporating persistence and botanical composition changes, such as GrassGro;
3. To provide the necessary factual information on seasonal susceptibilities to drought and grazing stresses to the major perennial grasses which will provide a basis for economic assessments to be made of the risks due to grazing during drought;
4. To link with the 18 grazing management sites established under the Meat Research Corporation (MRC) Key Program and located in the high rainfall zones of New South Wales, Victoria, Tasmania and South Australia. An understanding of grazing and drought stresses on perennial grass production and persistence will be a vital component in interpretation of the results from the grazing management sites.
The experiment was conducted in the field under a rain-out shelter at CSIRO "Chiswick", Armidale NSW. Six perennial grass species were transplanted into 1m2 plots as spaced plants. The perennial grass species included four introduced species: Phalaris aquatica (phalaris cv. Sirosa), Festuca arundinacea (tall fescue cv. Demeter), Dactylis glomerata (cocksfoot cv. Porto), and Lolium perenne (perennial ryegrass cv. Victorian), and two Australian native species: Microlaena stipoides cv. Shannon and Danthonia richardsonii cv. Taranna.
The species were subjected to two defoliation intensities: 'severe' and 'moderate'. The severe defoliation plots were maintained between 600 and 900 kg DM/ha while the moderate defoliation plots were maintained between 1,500 and 2,000 kg DM/ha.
Three moisture treatments: two droughts and a non-stress 'control' were applied. The droughts, determined from Armidale historical rainfall records were: 10-percentile (seasonal rainfall which occurs 1 year in 10) and 40-percentile rainfall (seasonal rainfall which occurs 4 years in 10). The non-stress moisture treatment received rainfall equivalent to 80% of evaporation. The treatments were imposed for six months in two seasonal combinations. The two seasons were: spring-summer (1 September - 28 February) and summer-autumn (1 December - 31 May).
Plant traits measured included dry matter yield, nutritive quality, basal cover, plant greenness, carbohydrate reserves, soil moisture and rooting distribution. Multiple regression was used to determine which traits were most important in determining persistence.
The period up to September 1994 was taken up in establishing the pasture swards and in constructing the twin rain-out shelters to impose the drought conditions. The droughts applied proved to be effective in causing more plant deaths compared to the non-stress moisture treatment. The non-stress moisture treatment resulted in very few plant deaths regardless of defoliation intensity or season. The results from the 1994-95 season form the basis of the conclusions as during that year the droughts imposed were without incident and indeed the monitored soil moisture status confirmed this.
One of the most significant findings of this experiment is that a moderate drought resulted in greater mortality of perennial grasses than a more severe drought. The rainfall during a moderate drought appears to have been sufficient for the plants to continue to grow but at the expense of carbohydrate stores. In contrast, a severe drought appears to have resulted in greater plant dormancy, thereby enabling plants to save their reserves. This suggests that it is important for graziers to graze their perennial pastures carefully during any dry times and not just during severe drought. All droughts begin as 'moderate' droughts; graziers cannot know whether a drought will be moderate or severe until after the fact.
In general, more plants were lost in the spring-summer drought than the summer-autumn drought. This highlights the importance of lenient grazing during spring and early summer, especially during moderately dry periods. Spring is also the time which is most crucial for the accumulation of water soluble carbohydrate (WSC) reserves.
The effect of severe defoliation was species dependent, having a greater effect on some species than on others. Cocksfoot was greatly affected whereas Danthonia was virtually unaffected by defoliation intensity. Defoliation intensity had a greater effect on plant water soluble carbohydrate concentrations than drought intensity, especially during the most vulnerable time of spring-summer.
One of the few findings that applied to all species is that defoliation intensity caused a consistent reduction in the regrowth potential of plants following drought. This suggests that moderate defoliation is one way of ensuring greater regrowth potential from all of the species assessed.
Persistence through drought is strongly linked to changes in basal area, changes in water soluble carbohydrate levels and changes in plant quality at certain times of year. These parameters vary considerably between species and consequently management recommendations need to be formulated with care for each individual species.
The question which has been commonly asked is "Which is the best species?" A simple answer is impossible as the goals of pasture sustainability include many sub-goals of persistence, plant and animal productivity.
More work is needed to model the effects of the various treatments on mortality over a range of soil types and climatic environments before the recommendations can be extended more widely. Suggestions for improvements and further work are also made.
The results from the 1995-96 season have not been reported as this season was interrupted by a 1 year in 50 rainfall event which caused a perched water table to move laterally within the soil under the rain-out shelters thus rendering the drought treatments irrelevant in that year.
Period: starting date 1993-07; completion date 1997-09
Keywords: persistence, perennial grasses, drought
Boschma, S.P. (1997). Performance and persistence of six perennial grasses under different intensities of drought and defoliation. PhD thesis. University of New England.
Scott, J. M., Rapp, G. G. and Boschma, S. P. (1994). Managing the persistence of perennial grasses through drought. Proceedings of the 9th Annual Conference of the Grassland Society of NSW. pp. 112. Poster presentation.
Boschma, S. P., Scott, J. M. and Rapp, G. G. (1995). Yield of perennial grasses under different drought and grazing intensities during spring and summer. Proceedings of the 10th Annual Conference of the Grassland Society of NSW. pp. 90. Poster presentation.
Boschma, S. P., Hill, M. J., Scott J. M. and Rapp, G. G. (1996). Effect of different intensities of drought and defoliation upon the mortality of perennial grasses. Proceedings of the 8th Australian Agronomy Conference, 1996. pp. 624. Poster presentation.
King, J. R., Boschma, S. P., Scott J. M. and Hill, M. J. (1996). Etiolated regrowth as a measure of potential forage grass recovery following drought stress in New South Wales, Australia. The Canadian Society of Agronomy, June 1996. pp. 811. Poster presentation.
Boschma, S. P., Hill, M. J., Scott, J. M. and Lutton, J. J. (1997). Carbohydrate reserves of perennial grasses: Effect of drought and defoliation intensity. XVII International Grassland Congress 1997, Canada. pp. 22-41-22-42. Poster presentation.
King, J. R., Scott, J. M. and Boschma, S. P. (1997). Forage persistence under extremes of cold and drought. XVII International Grassland Congress 1997, Canada. In press. Plenary paper presented by J. King.
Scott, J. M., Boschma, S. P. and Rapp, G. G. (1997). How can graziers manage perennial grasses to survive through drought? Proceedings of the 12th Annual Conference of the Grassland Society, NSW. pp. 140-142. Poster presentation.
Scott, J.M. and Boschma, S.P. (in press). Improved management of perennial grass pastures during drought: implications for pasture sustainability. Australian Journal of Experimental Agriculture.
Scott, J.M., Rapp, G.G., Bourke, G. and Boschma, S.P. (in preparation). Drought research using a Rain-out shelter.
Boschma, S.P., Hill, M.J., Scott, J.M. and Rapp, G.G. (in preparation). Perennial grasses under different intensities of drought and defoliation. I. Production and quality.
Boschma, S.P., Scott, J.M., Hill, M.J. and Lutton, J. (in preparation). Perennial grasses under different intensities of drought and defoliation. II. Carbohydrate reserves.
Boschma, S.P., Scott, J.M., Hill, M.J. and Rapp, G.G. (in preparation). Perennial grasses under different intensities of drought and defoliation. III. Soil water and root distribution characteristics.
Boschma, S.P., Hill, M.J., Scott, J.M. and Rapp, G.G. (in preparation). Perennial grasses under different intensities of drought and defoliation. IV. Plant mortality and plant traits important in persistence.