Simulation of soil water storage and sowing day probabilities with fallow and no-fallow in southern New South Wales: I. Model and long term mean effects.

In southern New South Wales, wheat is sown in the autumn (April–June), usually after a period of fallow to conserve soil water. In order to predict the effect of fallow and of new tillage systems, in particular no-fallow followed by direct drilling, on total available soil water (ASW) at sowing and on moisture-based sowing day probabilities, a simulation model combined with historical daily weather data was used. The model comprised a fallow subroutine with no plants, a no-fallow component involving weed germination, growth and water use, both followed by a wheat crop phase for continuous simulation. The model was calibrated and satisfactorily validated using data from several tillage experiments conducted in the region over the period 1979–1984. It was then used to compare long fallow (LF, 1 September start), short fallow (SF, post-wheat-maturity start, November–December), late short fallow (SFl, 15 February start) and grazed no-fallow (NoFg) over 41 years at Yass (684 mm rainfall), Wagga Wagga (566 mm) and Griffith (420 mm). On average, ASW on 1 May was 88 mm (LF), 61 mm (SF), 52 mm (SFl) and 33 mm (NoFg), with greater fallowing gains the wetter the site. Sowing probability in May averaged 77%/day for all fallow treatments and 54%/day for grazed no-fallow. In a number of years, especially following no-fallow, there were few sowing days in May. The average green herbage (weeds) consumption by sheep (at 10 animals/ha) on NoFg from harvest to 1 May was only 33 g/m2. Results were insensitive to the assumed starting weed leaf area index (range 0·001–0·025) after weed-germinating rain, and to stocking rate (range 0–40 per hectare), but were moderately sensitive to the upper and lower limits of seed-bed water content for sowing. Model assumptions are discussed and it was concluded that quantification of the likely cost of not fallowing in terms of reduced ASW and reduced number of sowing days was possible.

[1]  R. Papendick,et al.  Tillage Effects on Fallow Water Storage in the Eastern Washington Dryland Region1 , 1974 .

[2]  R. D. Berndt,et al.  A simulation-based evaluation of three cropping systems on cracking-clay soils in a summer-rainfall environment , 1976 .

[3]  W. Baier.  An agroclimatic probability study of the economics of fallow-seeded and continuous spring wheat in southern saskatchewan☆ , 1971 .

[4]  H. van Keulen,et al.  Simulation of water use and herbage growth in arid regions. , 1981 .

[5]  H. A Nix,et al.  An index of crop water stress related to wheat and grain sorghum yields , 1969 .

[6]  M. Freer,et al.  An evaluation of cereal stubbles for sheep production , 1976 .

[7]  Joe T. Ritchie,et al.  Model for predicting evaporation from a row crop with incomplete cover , 1972 .

[8]  J. J. Bond,et al.  Soil water evaporation: first stage drying as influenced by surface residue and evaporation potential. , 1970 .

[9]  G. Johns.  Measurement and simulation of evaporation from a red earth. II. Simulation using different evaporation functions , 1982 .

[10]  J. Pratley,et al.  Responses of soil and crop water relations to tillage. , 1987 .

[11]  R. Fischer,et al.  Simulation of soil water storage and sowing day probabilities with fallow and no-fallow in southern New South Wales: II. Stochasticity and management tactics , 1990 .

[12]  R. Fischer,et al.  Tillage practices and the growth and yield of wheat in southern New South Wales: Lockhart, in a 450 mm rainfall region , 1986 .

[13]  P. Biscoe,et al.  Radiation absorption, growth and yield of cereals , 1978, The Journal of Agricultural Science.

[14]  G. Ford,et al.  Effect of fallowing practices on the growth and yield of wheat in south-eastern Australia , 1985 .

[15]  R. Fischer,et al.  Tillage effects on soils - Physical and hydraulic responses to direct drilling at Lockhart, NSW , 1986 .