A comparison of agroecosystems : organic and conventional broadacre farming in south east Australia

Abstract

An agroecosystem comparison of broadacre organic and conventional farming under Australian conditions was undertaken from 1991 to 1993. The research was based on a pair of adjacent farms at Ardlethan in southern New South Wales. The farms were located in the agroecological region of Australia called the semi arid slopes and plains. The research had four objectives: 1. to obtain detailed information on the functioning of the two farming systems; 2. to compare and contrast the functioning of each system; 3. to develop hypotheses on the functioning of the organic system that could be tested in further studies; 4. to address the hypothesis that organic systems are likely to be more sustainable than conventional systems. The research approach incorporated the concepts of agricultural systems and agroecosystem analysis. It also focussed on the areas of difference between natural ecosystems and agroecosystems. The farm boundary was used as the appropriate system boundary. Mean annual rainfall at Ardlethan is 487 mm but is variable. Rainfall in 1991 was low with a dry spring but in 1992 and 1993 it was much higher. These years provided a good contrast between wetter and drier years to assess the performance of each system under differing climatic conditions. The organic farm was 1074 ha in total area and had 764 ha of arable land. The conventional farm had 374 ha of arable land. The main crop on each farm was wheat with smaller areas of oats and rye on the organic farm and lupins and barley on the conventional farm. On the organic farm the rotation was two years crop followed by six years pasture, on the conventional farm it was three years crop followed by three years pasture. Both properties grazed sheep and the organic farm also had cattle. The soil type on both farms was a red earth. Soil from the conventional farm had higher concentrations of total and extractable P, exchangeable Mn and total Fe. Soil from the organic farm had higher pH and concentrations of exchangeable Na, Ca and K. There was no statistically significant difference in total N, Na, Mn, K, Mg and organic carbon levels. Particular attention was paid to wheat, as the main crop on each farm, especially the first crop of wheat grown after pasture. In 1993 a further pair of organic and conventional crops was also monitored. There were many similarities between the management systems used for these crops with similar cultivation techniques and sowing dates. The major difference was the use of fertilizers containing water soluble P and herbicides on the conventional farms. Crop growth and yield were always higher in the conventional crops. Differences were least in 1991 which was a drier season. In 1992 and 1993 which were wetter growing seasons the differences were considerable. Weed competition was consistently higher in the organic crops. Analysis of plant tissue revealed the conventional crops contained higher concentrations of P at tillering, anthesis and in the grain and straw. The organic grain had higher levels of Cu, Zn and Cl. Independent experiments conducted in 1991 and 1992 demonstrated that P was the limiting factor for crop growth on the organic farm. Higher levels of V AM infection were found in the wheat roots on the organic farm but the VAM were not able to match fertilizers containing water soluble P in plant nutrition. In 1992 pasture production was monitored using pasture cages. There was little difference in pasture productivity although the pastures on the conventional farm contained higher concentrations of P. The conventional pasture contained more clover. Grazing pressure was estimated at 4.2 DSE/ha on the organic farm and 4.8 DSE/ha on the conventional farm. Other crops grown were also monitored for yields. Crop yields were lower on the organic farm. Analysis of these crops for nutrient concentrations revealed similar differences to those observed in wheat. Nutrient balances indicated that there were generally only small net gains or losses of nutrients. The major difference was the greater losses of K from the conventional farm due largely to stubble burning. Both farms had similar requirements in terms of embodied energy for machinery~ The only differences due to a difference between organic and conventional farming were increased grain storage capacity on the organic farm and a boomspray on the conventional farm. The conventional farm used more energy in the form of fertilizers and pesticides but this was rewarded by increased yields. The increased crop yields meant that energy efficiency ratios were similar for each cropped ha on the organic and conventional farms. The conventional farm had a superior energy efficiency ratio for pasture. When the energy use was calculated through a 24 year period the organic farm was using substantially less energy but the conventional farm had a superior energy efficiency ratio for the 24 year period. In 1992 the amount of labour input into each farm was monitored. Total labour inputs were similar but more labour was used for livestock on the conventional farm. On the organic farm more labour was used in marketing and office work and labour was also required for flour milling. Five key processes were identified in the functioning of the studied systems. These were: each farmers choice of management system, rainfall, phosphorus, soil pH and energy flow. The sustainability of each system was assessed by comparing them to the five principles of sustainable agriculture developed for Australian agriculture. This comparison revealed that neither system was superior in regard to all the principles. There are many problems which agriculture needs to address to achieve sustainability. In regard to many of these neither system offered definite advantages. For some of these problems the choice of organic or conventional management is irrelevant and changes will need to occur at a societal, rather than a farm, level. Show more