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Assessing the potential of wide-row farming systems in the Mallee

Jones B.R.1 and Desbiolles J.M.A.2

1 Agriculture Victoria Walpeup, Victorian Institute for Dryland Agriculture, Vic.
2
Agricultural Machinery Research and Design Centre, The University of South Australia, The Levels Campus, Mawson Lakes, S.A.

ABSTRACT

A wide-row system (WRS) is proposed to make better use of water in the Mallee, by increasing crop intensity to 100% and cropping at wide row spacing. WRS will use water in-season, rather than conserve it for the next year (long fallow), and manage the inter-row, into which the next crop is sown, to provide a disease break, or use the extra water to grow break-crops. Simulation modelling and edge-row measurements were used to assess the potential of WRS to increase crop water supply in the inter-row (CWS), before agronomic trials, and the whole-farm effect was demonstrated with yield data from coincident long-fallow trials. On the Walpeup sandy loam soil type, WRS increased simulated CWS by at least 10 mm for each yearly rainfall decile above 2.4. The average CWS was 22 mm in a decile 2 year and 70 mm in a decile 8 year. Edge-row measurements supported the model predictions, and WRS increased whole farm yield at least 15% above long fallow – cereal rotations.

KEY WORDS

Wide row spacing, OLEARY model, fallow performance, soil evaporation, edge row.

INTRODUCTION

In the Victorian Mallee, farming systems are dominated by cereal-fallow and cereal-pasture-fallow rotations, with an average cropping intensity of 37% (2). Although these systems provide the crop with mineralised nutrients and a disease break, they make poor use of rainfall in the fallow year (average 22mm stored (1)). Controlled traffic and guidance technology could be used to re-arrange these systems to make better use of water. Crop could be sown over the whole farm each year at wide row-spacings, and rotations made into the inter-row instead of between paddocks. This could significantly increase whole-farm yield. There would be a big increase in cropped area (from 37% to 100%, nearly 3-fold), with yield reduced only slightly by wide row spacing (because of compensation by the plant). At sufficiently wide row spacing (eg. 50 cm), the surface soil of the inter-row might be managed independently to keep it weed- and root-free, thus providing a disease break. Alternatively, extra water from the inter-row may allow disease-break crops to be grown profitably.

This paper reports two attempts to assess the potential of wide-row systems (WRS), before commencing agronomic trials. These were, to simulate the water available under the inter-row, by treating the inter-row as a fallow and using a fallow simulation model, and to estimate the extra water available to edge-rows of experimental plots, by measuring the difference in yield between edge- and inner-rows. The simulation exercise was used to describe variation in the system across a range of years, and allowed two key questions to be answered: How much extra water can be supplied to crops from the inter-row, given that evaporation from the soil surface is high? Will WRS be effective on heavier clay flats where infiltration rates are low and storage capacity in the top-soil higher? The edge-row measurements were used to ‘validate’ the simulation exercise. In essence, WRS tries to capture the benefits of edge-rows across whole farms. The extra crop water supply from WRS can be compared to that of a long-fallow, but the wide-row crop water can come from rain falling in the season of the crop, not just storage before sowing. Because the aim is not to store water for the following crop, as in long fallow, the frequency of groundwater recharge is likely to be lower.

Real yield data from cereal-long fallow rotations at Walpeup were available for years that overlapped the simulation (1,7). To demonstrate the likely effect of WRS on a real farm, whole-farm yields for both systems were calculated, by taking into account changes in crop area, together with the difference between long-fallow water conservation and inter-row crop water supply simulated by the model.

The OLEARY crop simulation model (3,4) was developed in the Mallee, simulates differences in total soil water between environments well (R2 = 0.95, RMSE = 20 mm (5)), and exhibits similar in-season behaviour to that of field grown crops (3). Significant deviations between simulated and measured soil water evident in validations (3,5) are cause for concern, and may be caused by the high sensitivity of the model to the depth of the surface soil layer (data not shown). Stage one (constant rate) drying, according to Ritchie (8), only occurs in the surface soil layer, which dries quickly. The deeper the layer, the more water lost, and vice versa. However, the OLEARY model was considered adequate to answer questions in the Victorian Mallee environment, given sufficiently conservative assumptions and later being verified with independent field data.

The assumptions were that the crop did not shade or reduce wind speed above the inter-row, and that the soil profile began each year at wilting point (no stored moisture from the previous year). Rather than simulate crop use of water in the inter-row, water was accumulated in the soil. This would have increased the rate of stage two soil-drying (proportional to the relative available water between 0 and 50 cm). The non-conservative assumption was that, from sowing, crop water use from the inter-row would be sufficient to prevent further drainage below the maximum rooting depth. The rooting depth may be shallower in soils with subsoil limitations, and deeper in sandy soils. Crop water supply from the inter-row will be lower where rooting depth is shallower than assumed (110 cm).

MATERIALS AND METHODS

OLEARY model

The model (4) was modified to record plant-available soil moisture below 25 cm on November 1 (assumed date of physiological maturity). The simulated drainage from sowing to November 1 was added to this figure to give an estimate of the potential crop water supply (CWS), to plants either side of the inter-row, between sowing and harvest. Daily weather data for Walpeup (1965 to 1992, maximum and minimum temperatures, rainfall, pan evaporation and cloud cover) were used for simulations, together with long-term average wind run and day:night wind ratio as a substitute for daily data. The data set had an average yearly rainfall of 340mm, 3 mm below the long-term average.

Hydrological parameters for Walpeup sandy loam (5), were used for all simulations except the heavy clay loam. For this, lower storage limits (LSL) for a clay loam at Birchip (R. Armstrong, pers. comm.) were used, together with estimated bulk densities and remaining parameters interpolated between Dooen clay and Walpeup sandy loam (5).

The model was run with combinations of 0 and 2.5 t/ha stubble cover, and 0 or five growing-season inter-row cultivations. The no stubble, cultivation treatment was repeated with the heavy clay loam soil data.

Edge-row measurements

Four metres of paired edge- and inner-rows were hand-harvested from each of the 11 treatment x 3 replicates at the Waikerie no-till seeding system trial site in 1999. There were no significant treatment effects so the trial means were used in the analysis. The trial was on sand, with evidence of a transition to clay loam below 60-100cm depth. The yearly rainfall was 274 mm (decile 5), with 152 mm April-October rainfall. The edge-rows of adjacent plots, spaced 80-90cm apart, were considered to be at ‘wide’ row spacing compared to the inner-rows (25 cm). To estimate the extra CWS to the edge-row, compared to inner-rows, the difference in yield/m row was multiplied by 1.5 g grain per litre of water (= 15 kg/ha/mm transpiration efficiency, calculated on whole-plot yields with 60mm evaporation assumed). To calculate the mm-equivalent rainfall of the extra water in the space between plots (=litre/m2 cf. litre/m calculated above), the litres of water/m row value was divided by ((plot spacing– inner-row spacing ) / 2 = (0.85 – 0.25)/2 = 0.3 m). This calculation assumed that the edge-row had access to the same amount of water in its 0.25 m as did an inner-row. Extra water must have come from the inter-row beyond.

SIMULATIONS

Growing season rainfall at Walpeup varied between 33 mm in 1982 and 338 mm in 1973 (data not shown) and corresponded well to yearly rainfall (between 101 mm in 1982 and 684 mm in 1973).

Crop water supply from the inter-row – Sandy loam

The simulated crop water supply from the inter-row (CWS - water available between sowing and physiological maturity, below 25 cm in the inter-row), related well to yearly rainfall decile (YRD) for the no-stubble scenarios on sandy-loam soil. Cultivation had no effect on total evaporation without stubble, because even without cultivation, water in the surface soil layer was almost always evaporated before the next rain. CWS in the no-stubble scenarios (Figure 1) was well described (R2=0.79) by the model CWS (mm) = 7.9 x YRD + 6.3. The lower frontier for CWS had the equation CWS (mm) = 10.0 x YRD – 24.4. The upper frontier increased to a plateau at 71mm at YRD above 5. Retaining stubble increased the minimum and maximum CWS, lower being linear through the origin with a slope of 10 mm/YRD to plateau at 71 mm at YRD 7, and upper increasing to a plateau above 80 mm from YRD 3 (data not shown). The effect was slightly less if the stubble was cultivated.

Crop water supply from the inter-row – Clay loam

No stubble, conventional tillage on the synthetic heavy clay loam reduced simulated CWS at lower YRD (Figure 2). The intercept for the lower frontier decreased 18 mm. The upper frontier, rather than reaching a plateau, was approximately linear through the origin, with a slope of 13 mm/YRD.

Figure 1. Simulated CWS vs. YRD, on sandy loam under no-stubble, five cultivations. Heavy line is the model CWS (mm) = 7.9 x YRD + 6.3, R2 = 0.79.

Figure 2. Simulated CWS vs. YRD, on ‘heavy’ clay loam under no-stubble, five cultivations.

RESULTS

Edge-row measurements

The additional grain yield of the edge row (Table 1) corresponded to an additional 17 l of water per metre of row, equivalent to 56 mm CWS from the extra 0.3 m inter-plot space. This compared to 125 mm average water use (WUE 15 kg/ha/mm for an average 1800 kg/ha yield) in the inside rows of the plots.

Table 1. Extra Yield in the Edge Row of Direct-Drill Trial Plots at Waikerie, 1999.

Treatment

Grain/m row

Extra Water for Edge-Row

 

Inside

Edge

Per m Row

from Inter-Row

 

(g)

(g)

(litre/m row)

(mm)

Mean (30 Plots)

46.60.9

71.31.5

16.90.9

56.03.0

Means presented standard error of the mean.

DISCUSSION

Simulation with the OLEARY model suggested that wide rows on lighter soil types would provide extra CWS from the inter-row in most years. If the lower frontier of Figure 1 is taken to represent the absolute minimum, there will be increased CWS in all years with rainfall above YRD 2. The average CWS is at least 6mm, increasing a further 8 mm for each YRD. The plateau of the upper CWS frontier is equal to soil water storage below 25 cm, limiting the potential CWS in years with greater than YRD 4. While the CWS at low YRD is generally low, when there is high summer-autumn rainfall, the CWS can be quite high. Stubble retention increases the probability and magnitude of the CWS (as it does for fallow in conventional systems). The minimum CWS from the system is likely to be less in heavier soils, but the potential maximum is higher. The edge-row measurements verify that extra water is available from the inter-row, and the estimation of CWS according to edge-row measurements is close to what the model predicts (8 x YRD 5 + 6 = 46 mm) for YRD 5 at Walpeup. The CWS at Waikerie is likely to be higher because the soil is lighter in texture than the Walpeup sandy loam.

Compared to a cereal - long fallow (LF) rotation, wide-row systems (WRS) have the potential to produce on average 400 t more grain on a 2000 ha farm, an increase of 15%, in the years for which data was available (Table 2). This calculation is conservative because LF WUE, used to calculate ‘h’, is calculated including evaporation, whereas CWS implicitly accounts for evaporation. The marginal WUE for CWS should thus be higher, and CWS later in crop growth is also likely to improve grain quality. If the 37% intensity figure for the Mallee is used in the calculation (eg. LFCA = 740 ha), the increase in whole-farm production is much greater (ca. 50%).

Table 2. 2000 Hectare Farm Production Under Long Fallow (LF) and Wide-Row Systems (WRS).

Year (Rain)

Long Fallow

 

CWS

Short F.

Whole Farm Production (t)

 

Yield1

WUE1

 

LF1

WRS

Yield2

Long

 

WRS

WRS

WRS

 

Difference

 

(t/ha)

(kg/ha/mm)

 

(mm)

(mm)

(t/ha)

Fallow

 

Row Area

Inter-Row

Total

 

Calculation3

a

b

c

d

e = a -(c x b / 1000)

f=

LFCA x a

g =

WRRA x e

h = WRIRA x d x b / 1000

i = g + h

j = i - f

1985(267)

1.9

9.2

22

19

1.7

1900

1910

152

2062

162

1986(374)

3.3

10.1

23

73

3.0

3270

3417

642

4060

790

1987(318)

1.8

6.9

26

43

1.6

1760

1778

260

2038

278

1988(322)

2.2

9.1

32

37

1.9

2200

2147

297

2444

244

1989(310)

2.7

15.1

5

34

2.6

2680

2930

455

3385

705

1989(310)

2.2

7.9

25

34

2.0

2240

2299

237

2536

296

1990(288)

3.7

11.8

19

30

3.5

3680

3889

305

4193

513

1991(293)

3.2

16.5

45

54

2.5

3210

2778

784

3561

351

1992(517)

2.3

8.9

59

70

1.7

2270

1963

545

2508

238

                     

Mean(333)

2.6

10.6

28

44

2.3

2579

2568

408

2976

398

1. Data from Incerti et al. (1), 1985-1989, O’Leary and Connor (6,7), 1989-1992.

2. Short fallow yield, so that WRS row area yield can be calculated without the benefit of LF CWS.

3. LFCA (Long fallow crop area) = 1000 ha, WRRA (WRS row area) = 2000 x (45/80) = 1125 ha, assumes 30 cm strip of crop is equivalent to 3 rows x 15 cm spacing, WRIRA (WRS inter-row area) = 2000 – WRRA = 875 ha.

CONCLUSIONS

Wide-row systems are likely to increase the crop water supply (CWS) in climates and soil types similar to those of Walpeup. Minimising evaporation is likely to increase both the size and reliability of CWS. CWS in wide-row systems is likely to be less with heavier soil types, and minimisation of evaporation on these soils is likely to be more important. Edge-row measurements confirm that, if nutrition is adequate and the disease-break benefits of the fallow can be retained in the inter-row, whole-farm yield increases can be obtained from wide-row systems compared to traditional farming systems in the Mallee environment.

ACKNOWLEDGMENTS

The authors wish to thank the GRDC for funding projects DAV437 ‘Novel farming systems to increase productivity and reduce risk in the Mallee’ and USA25 ‘Seeding system trials for the drier Mallee conditions’, Dr John Angus for many helpful discussions and suggestions for the manuscript, and Dr Roger Armstrong and Mr James Nuttall for providing ‘real’ data for the synthetic soil.

REFERENCES

1. Incerti, M., Sale, P.W.G. and O’Leary, G.J. 1993. Aust. J. Exp. Agric. 33, 885-894

2. Martin, P. 1998. Australian Grains Industry 1998 (ABARE: Canberra). pp. 77.

3. O’Leary, G.J., Connor, D.J. and White, D.H. 1985. Ag. Systems. 17, 1-26.

4. O’Leary, G.J. and Connor, D.J. 1996a. Ag. Systems. 52, 1-29.

5. O’Leary, G.J. and Connor, D.J. 1996b. Ag. Systems. 52, 31-55.

6. O’Leary, G.J. and Connor, D.J. 1997a. Field Crops Res. 52, 209-219.

7. O’Leary, G.J. and Connor, D.J. 1997b. Field Crops Res. 54, 39-50.

8. Ritchie, J.T. 1972. Water Resources Res. 8, 1204-1213.

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