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EFFECT OF TILLAGE ON RHIZOSPHERE MICROBIAL POPULATIONS, EARLY GROWTH AND GRAIN YIELD OF WHEAT THROUGHOUT SOUTH EASTERN NSW

S. Simpfendorfer1, J.A. Kirkegaard2, P.T.W. Wong1 and D.P. Heenan1

1NSW Agriculture, Wagga Wagga Agricultural Institute; 2CSIRO Division of Plant Industry, Canberra

Abstract

The effect of tillage on early wheat growth was examined at 21 farm sites, to survey the reported problem of reduced seedling vigour in direct drilled (DD) crops and the possible involvement of deleterious rhizosphere microorganisms. The experiments were conducted by cultivating strips in paddocks that were otherwise direct drilled. The average early vegetative growth of direct drilled crops was reduced at 13 of the sites relative to the cultivated strips. Reduced early vegetative growth generally carried through to decreased anthesis biomass but there was no significant difference between the final grain yield of wheat grown in direct drilled (2.79 t/ha) and cultivated (2.77 t/ha) soil when averaged across the sites due to a dry spring. The probable reason for the yield compensation by direct drilled crops was soil water conservation due to reduced vegetative growth, leading to greater availability during grain filling.

Key words: Direct drill, wheat, early growth, deleterious soil microorganisms.

Direct drilling (DD) and stubble retention have been widely promoted throughout the grains industry as a key component of conservation cropping systems designed to sustain soil fertility and crop production. Despite the demonstrated benefits of this system in reducing soil erosion, increasing soil organic matter, and improving soil structure, only 30% of farmers routinely practice direct drilling due to concerns over crop yields (6). A recent review of long-term tillage trials in Australia by Kirkegaard (4) highlighted that reduced seedling growth is the most consistent factor associated with reduced yield of DD wheat. Studies have also found that this reduced early growth could be overcome by soil fumigation which indicates that soil microorganisms are responsible for reducing the growth (1, 5, 7). Field experiments were conducted at 21 farm sites throughout south-eastern NSW in 1997 to determine the effect of tillage on the early growth of wheat seedlings, rhizosphere microbial populations and final grain yields.

Materials and methods

Three conventionally cultivated (C) strips (3 workings to ~10 cm in autumn) were established within 21 paddocks across south-eastern NSW and the whole area then DD sown with wheat. Wheat plants growing in both the DD and C plots were assessed throughout the growing season for vigour (dry shoot weight), and rhizosphere populations of total bacteria and fungi, pseudomonads, bacilli and actinomycetes. Wheat growing under these two tillage systems were also assessed for differences in establishment; disease throughout the season; tiller and head counts; grain size and final grain yields.

Figure 1

Results

Early vegetative growth in DD crops was reduced at 13 out of the 21 field sites, compared to the cultivated treatment (Fig. 1). At nine of these sites the average population of pseudomonads was higher in the rhizosphere of DD seedlings. Furthermore, three of the four sites which had increased early vigour in DD strips had associated decreases in rhizosphere populations of pseudomonads compared to the C treatment. There was no correlation between populations of fungi, total bacteria, bacilli or actinomycetes and the occurrence of reduced early vigour in DD plots. However, reduced early vigour in DD crops resulted in decreased final grain yields at only four of the 13 sites. Reduced early growth in DD plots predominantly carried through to a decreased anthesis biomass. However, there was generally a negative correlation between anthesis biomass and grain yield across the 21 farm sites.

Discussion

Reduced early vigour of DD wheat was found to be a widespread problem throughout southern NSW. Decreased early growth was evident at 13 out of 21 (62%) farm sites even in a dry season. Poor early seedling growth of DD wheat is believed to be most severe in moist soil at low temperatures (below 15&degree;C, J. Kirkegaard, pers. comm.). Wheat seedlings are also known to be more sensitive to growth inhibitory pseudomonads under these conditions (2). The start of the 1997 season was relatively dry with soil temperatures around 15&degree;C at the sowing depth. Such conditions were potentially less conducive to the activity of deleterious soil microorganisms and the reduced early growth of DD wheat. The observation that plots with a lower anthesis biomass generally had a larger average grain size and higher final yield can also be attributed to the climatic conditions as there was little rainfall following anthesis at most sites (3). Thus in a season with higher spring rainfall, poor early vigour in DD wheat would be expected to impact more heavily on grain yields. There also appears to be a correlation between increased populations of pseudomonads in the rhizosphere of DD seedlings and poor early vigour. Further studies are continu- ing to determine the effect of these bacteria and other deleterious soil microorganisms on the early vigour of DD wheat in contrasting seasons.

Acknowledgments

We gratefully thank the Grains Research and Development Corporation for funding and Graeme Heath for technical assistance. We also thank farmers involved in this project for use of paddocks and their co-operation and support throughout the season.

References

1. Chan, K.Y., Mead, J.A., Roberts, W.P. and Wong, P.T.W. 1989. Aust. J. Agric. Res. 42, 221-228.

2. Elliott, L.F. and Lynch, J.M. 1985. Plant and Soil 84, 57-65.

3. Fischer, R.A. and Kohn, G.D. 1966. Aust. J. Agric. Res. 17, 281-295.

4. Kirkegaard, J.A. 1995. Aust. J. Exp. Agric. 35, 835-848.

5. Kirkegaard, J.A., Munns, R., James, R.A., Gardner, P.A. and Angus, J.F. 1995. Aust. J. Agric. Res. 46, 75-88.

6. Steed, G.R., Ellington, A. and Pratley, J.E. 1993. In "Conservation Tillage in Temperate Agroecosystems-Development and Adaptation to Soil and Climatic Constraints." Edited by M.R. Carter. Lewis Publishers, Boco Raton. pp. 231-251.

7. Simpfendorfer, S., Kirkegaard, J.A. and Heenan, D.P. 1998. Proc. 9th Aust. Agron. Conf. Wagga Wagga. pp. 533-534.

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