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Out of season crops – What are the benefits for no-till farming systems?

ME McNee1,2, Phil Ward3, DR Kemp1 and WA Badgery4

1 School of Agricultural and Wine Sciences, Charles Sturt University, Orange, NSW 2800 Email mmcnee@csu.edu.au
2
Central West Conservation Farming Association, Wellington, NSW 2800
3
CSIRO Plant Industry, Private Bag No 5, Wembley WA 6913
4
NSW Department of Primary Industries, Orange Agricultural Institute, Forest Rd, Orange, NSW 2800, Australia.

Abstract

The impact of growing White French millet (Panicum miliaceum) in the short fallow over summer prior to winter wheat was assessed in Wellington NSW. International evidence suggests that use of cover crops can help suppress weeds, improve soil health and the ability of soils to capture rainfall for use by following crops, prevent erosion and add diversity to cropping rotations. However, there is concern that under Australian conditions replacing fallow phases with cover crops might deplete soil water and compromise the yield of commercial crops. A preliminary appraisal of soil moisture data in the 2007/2008 summer cropping season demonstrates that soil water use in a millet cover crop was significantly greater than in the stubble-retained fallow. However in the top 20 cm there was little difference between treatments, suggesting that wheat germination and establishment might not be significantly affected by a cover crop in the event of good planting rain.

Key Words

cover crops, cropping intensity, diversity, moisture, soil health, weed control

Introduction

The Australian conservation farming movement views the maintenance of groundcover as a natural progression from the adoption of reduced tillage practices. With rainfall use efficiency becoming more and more critical in dryland agriculture and awareness of the potential impacts of declining soil quality and climate change increasing, progressive land managers are investigating how to make better use of summer rainfall that might otherwise be lost through evaporation and runoff. This trend is seen in highly variable rainfall zones around the world. Poor rainfall-use efficiency and declining soil quality under fallow has led to a reduction in the frequency of fallows in the Great Plains of North America. Research is now focused on developing opportunistic cropping systems that can adjust the intensity and diversity of crops used based on soil moisture availability (Liebig, Tanaka et al. 2007). Australian land managers require more opportunistic and flexible cropping systems to manage agroecosystems effectively.

In central and southern NSW, there are limited opportunities to intensify cropping rotations to add diversity. Land managers grow winter cereals, canola and some pulse crops. In no-tillage farming systems land is generally fallowed in the hottest months and wheat stubble from the previous crop is retained at the soil surface. However, historical rainfall records indicate that summer crops could be grown in some regions depending on the season. Even with the potential benefits of no-tillage stubble retention it is still unclear in how many years land managers might expect to establish a cover crop and under what climatic conditions this is possible.

It remains unclear what effect growing summer crops and their management would have on the yield of following winter crops. A likely consequence is that increased plant growth over summer will lead to higher transpiration rates (French and Schultz 1984; Singh, Bird et al. 2003; Ward, Hall et al. 2007) that could reduce the water available for subsequent crops. However, it isn’t clear whether there are actually any significant benefits of storing summer soil moisture by fallowing in the Wellington district. It is generally assumed that after fallowing, timely rainfall is still required to establish a winter crop. In dry years there may not be any stored moisture anyway, and in average years to above average years it may not matter. Sief and Pederson (1978) report that from 37 trials in the district, 86% of the variability in yield in their trials could be accounted for by variability in spring rainfall (3 weeks before anthesis to 2 weeks after anthesis) and that this is the limiting feature in the environment (Seif and Pederson 1978).

Where stored soil moisture is not a major production driver and timely rainfall is more critical, it may be possible to grow cover crops without seriously compromising yields in following crops. Cover crops could then provide some land managers with an opportunity to add diversity and flexibility to their cropping rotation and perhaps realise long term soil quality benefits or more direct benefits such as a reduction in herbicide as a result of increased crop competition.

Methods

Experimental site

The experiment commenced in December 2007 at the Wellington Agricultural Experiment Station (S3230, E14858) on a Red Dermosol in Central West NSW. An average of 268mm of rainfall falls from December to May at the Wellington Agricultural Research Station. From May to November, the winter cropping season, 349mm of rainfall is received on average. The Wellington district of generally receives more summer rainfall than the wider Central-West region which lies within the 350 and 600mm rainfall isohyets (Evans and Scott 2007). Winter rainfall in the Wellington district is more reliable and effective for crop production than summer rainfall, but in some years adequate rainfall for crop growth is available when land managers would normally summer fallow.

Experimental design and treatments

The experiment began in 2008 on land that had been cropped to wheat the previous year. Wheat stubble was retained after harvest. The experimental treatments were arranged in a randomized compete block design with four replications. There were four treatments in each replication, stubble fallow (control), short duration cover crop, long duration cover crop and a late sown cover crop. Except for the control treatment, each treatment was sub-divided into cowpeas and White French millet forming subplots. Therefore, there were 16 major plots (4 Reps x 4 Treatments) of which 12 were split between cowpeas and millet cover crops. This paper only reports on the short duration millet and stubble fallow plots in the experiment (4 short duration millet plots and 4 stubble fallow plots) where electronic soil moisture capacitance probes were installed. The plot size for the millet cover crops was 5m x 80m and the plot size for the stubble fallow was 10m x 80m. The short duration millet was planted on the 22nd of January after the whole experimental area was sprayed with 1.5 L/ha glyphosate on the 10th of January. The seeding rate was 7kg/ha on 25cm row spacing. There was no fertiliser applied. In total, the millet and stubble fallow plots were sprayed three times before wheat (cv Ventura) was planted on the 17th of June. The second spray on the 18th of March was used to kill the millet cover crop and control weeds in the stubble fallow. A mixture of glyphosate and 24-D at a rate of 1.5L/ha was used. After this spray all plant growth ceased in the millet cover crop plots and it was left untouched at the soil surface to fall to the ground. Both the milet cover crop and fallow treatments were sprayed again on the 1st of May with 2 litres of glyphosate to control the emergence of winter grasses prior to wheat planting. No pre-emergent herbicide was used. The wheat was planted at a seeding rate 50kg/ha with 80kg/ha of DAP fertiliser.

Soil moisture measurements

In each plot, an electronic capacitance soil moisture probe (Butler, Dalton et al. 2007) was installed to a depth of 1.5 m on the 1st of February just after emergence of the millet, and after 77 mm of rainfall in January. Sensors were positioned down each probe at depths of 10 cm, 30 cm, 50 cm, 100 cm and 150 cm, and soil water content was measured at hourly intervals. The disturbance to the seedbed around the probe during installation reduced plant populations in the cover crop plots, so the probes were re-installed on the 22nd of February in new locations where the plants were established and the soil had dried sufficiently. Consequently, there was a gap in the recorded data for the cover crop replicates between emergence of the cover crop and re-installation of the probes. Significance tests were conducted using GENSTAT t-tests for different depth increments down the soil profile between the 20th and 31st of March.

Results

The cover crop depleted more soil moisture than did the weeds in the stubble fallow. The main significant rainfall event (38 mm between March 25 and 27) recharged the soil water store under the stubble to zero, but a 80mm deficit remained under the millet (Figure 1). By June the soil water deficit under the stubble was ~100mm and ~150+mm under the millet.

Figure 1. Total change in soil moisture (mm) to 1.75m depth between February and July for Fallow and Millet. Y2 axis is daily rainfall (mm).

Figure 2. Average daily soil moisture (mm) over time (day) for a. 0-20cm, b. 20-40cm, c. 40-75cm and d. 75-75cm.

The average soil moisture deficit was much larger in the lower soil profile than in the top 40cm. The difference in the means was ~22, ~16, ~57 and ~79 for the respective depths graphed in Figure 2. Infiltration occurred down to 150cm in one of the four plots in the cover cropped treatment. In the remaining plots in the cover crop treatment infiltration was confined to 30 cm, 50 cm and 50 cm, respectively suggesting that the top 50 cm of the soil in the cover cropped plots was sufficiently dry to absorb all the incident water in most cases. In the fallow treatments, infiltration occurred down to the 150 cm sensor in two of the plots, and to 100 cm and 30 cm for the other two plots. The magnitudes of the spikes in soil water were generally smaller for the fallow plots than the cover cropped plots reflecting the fact that the soil moisture prior to the rainfall was comparatively higher. It is unclear if runoff was significantly higher in the fallow treatments compared with the cover cropped treatments at this stage, although the fact that water storage in the cover cropped treatments increased by more than the quantity of rainfall suggests that runoff from the fallow plots to the cover crop plots did occur. Regular rainfall events occurred from Mid-May through June slowly re-filling the top soil profile for planting winter wheat. The rainfall events were much smaller and of lower intensity than the large summer event (Figure 1).

Discussion and Conclusions

White French Millet extracted a substantial volume of water from compared with a stubble fallow. Average rainfall for December, January, February and March (1946-2007) at the Wellington Agricultural Experiment Station is 51mm, 65mm, 60mm and 51mm (Bureau of Meteorology, 2008). The cover crop was planted a month later than planned and soil moisture was depleted through transpiration well into March. If planted in late December and desiccated no later than mid-February this would have provided a greater opportunity to make up the soil moisture deficit following the crop’s living phase. Without transpiration, the 56mm that fell in February would not have been used for crop growth, and would have been added to the 38 mm that fell in March (Figure 1).

An early appraisal of the data suggests that the ability of the cover cropped treatments to capture summer rainfall was superior to the fallow treatments. Consequently, it is foreseeable that rainfall in similar years would re-fill the profile after cover crop desiccation, improving the overall rainfall use efficiency of farming systems. Millet cover crops were found to aid water infiltration in southern Queensland (Price, Castor et al. 2006). The advantages of plant litter are well known and include reduction in evaporation, runoff, improved water infiltration, soil moisture holding capacity, soil temperature, and soil structure (Lodge, King et al. 2006). One or more of these factors, combined with the tendency for fallow soils to retain more soil moisture than cropped soils, could explain the results of these studies. The practice of fallowing usually results in lower infiltration rates and increased runoff compared with cropped soils (Freebairn and Boughton 1981) though the role of stubble retention on the fallows may alleviate this to some degree. Cover crops may also have created preferred pathways of water infiltration, due to cracks opening up as the soil dried. With earlier desiccation the cover crop residue left at the soil surface would be present when evaporation rates are high in February and March. Higher residue loads at the soil surface might be more effective in reducing soil surface evaporation compared with the stubble retained fallow treatments after transpiration of the cover crop has ceased.

Regardless of the potential of improved timing, it still remains to be seen whether soil moisture storage between winter cropping phases is a driver of crop productivity in the region. The next stage of this research will determine the importance of soil moisture storage prior to winter sowing by comparing wheat yields in the fallow and cover cropped treatments. If soil moisture storage prior to sowing turns out to be a driver of yields in this region the sowing time and duration of the cover crop will become critical to improve the re-fill potential and reduce evaporative losses when there is high demand. In the event that there is little or no difference in yield, it may be possible to lengthen the growing season of the cover crop to improve groundcover and realise other associated benefits particularly for soil health and weed control.

References

Butler, G., M. Dalton, et al. (2007). Soil Moisture Technologies in Broad Acre Crops. The No-Till Journal SANTFA 4(Spring Edition): 153-200.

Evans, C. M. and B. J. Scott (2007). Surface soil acidity and fertility in the central-western wheatbelt of New South Wales. Australian Journal of Experimental Agriculture 47(2): 184-197.

Freebairn, D. M. and W. C. Boughton (1981). Surface runoff experiments on the eastern Darling Downs. Australian Journal of Soil Research 19(2): 133-146.

French, R. J. and J. E. Schultz (1984). Water use efficiency of wheat in a Mediterranean-type environment. I. The relation between yield, water use and climate. Australian Journal of Agricultural Research 35(6): 743-764.

Liebig, M. A., D. L. Tanaka, et al. (2007). Dynamic Cropping Systems: Contributions to Improve Agroecosystem Sustainability. Agronomy Journal 99(4): 899-903.

Lodge, G. M., K. L. King, et al. (2006). Effects of pasture treatments on detached pasture litter mass, quality, litter loss, decomposition rates, and residence time in northern New South Wales. Australian Journal of Agricultural Research 57(10): 1073-1085.

Price, L., P. Castor, et al. (2006). Cover cropping in low stubble situations. QDPI&F. Toowoomba, Grains Research and Development Corporation.

Seif, E. and D. G. Pederson (1978). Effect of rainfall on the grain yield of spring wheat, with an application to the analysis of adaptation. Australian Journal of Agricultural Research 29(6): 1107-1115.

Singh, D. K., P. R. Bird, et al. (2003). Maximising the use of soil water by herbaceous species in the high rainfall zone of southern Australia: a review. Australian Journal of Agricultural Research 54(7): 677-691.

Ward, P. R., D. J. M. Hall, et al. (2007). Water use by annual crops. 1. Role of dry matter production. Australian Journal of Agricultural Research 58(12): 1159-1166.

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