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Water balance, growth and yield in Mallee fallow and continuously cropped rotations

M.G. O'Connell1, D.J. Connor2 and G.J. O'Leary1,3

Joint Centre for Crop Improvement, 1Department of Natural Resources and Environment, Agriculture Victoria, 2The University of Melbourne, 3Present address: ICRISAT, India.

Abstract

Over a 4-year (1993-1996) comparison of two crop rotations (long fallow vs continuously cropped) wheat growth, grain yield and water use were unaltered by fallowing in seasons of average rainfall. In a dry season (1994), a preceding crop of Indian mustard reduced root-zone soil water (233 to 184 mm), above ground biomass (2.8 to 1.6 t/ha), water use (160 to 132 mm) and yield (0.7 to 0.1 t/ha) of wheat compared to wheat following fallow.

Key words Soil water, water use, wheat, field peas, Indian mustard, semi-arid, dryland.

Fallowing increases and stabilises wheat yields in the semi-arid Victorian Mallee (4), but can cause land degradation via wind erosion and may contribute to dryland salinity by increasing deep drainage (3). The effects of fallows are complex providing one or more of the following benefits to wheat: preparation of a seed bed, additional root-zone soil water, additional nitrate at sowing, weed control, and disease break (4). Continuous cropping with Indian mustard (Brassica juncea)-wheat rotation was examined as an alternative to fallowing as it offers the advantage of a cereal root disease break without the sacrifice of a harvestable product prior to the wheat. This study compared fallow-wheat and continuously cropped systems in terms of water balance, crop growth and yield.

Materials and methods

A field study (1993-1996) located in the Mallee region of north western Victoria, at Walpeup (35o 07' S., 141o 58' E.; elev. 85 m) on a gradational calcareous earth (Gc 2.22; (2)), compared a long fallow vs continuously cropped rotation. The cropping systems compared were two 3 phase rotations: "fallow" (long fallow-wheat-field peas) and "continuously cropped" (Indian mustard-wheat-field peas). Each phase was represented each year. Three replicates were arranged in a blocked design, plots size was 20 m x 21 m with a 5-10 m buffer area. Wheat (cv. Meering), field peas (cv. Dundale) and Indian mustard (F2 selection: early maturing, zero erucic acid, yellow seed) were sown at 60, 120 and 6 kg/ha, respectively, with 75 kg/ha of double super (0:17:0:4). Weeds, pest insects and mice were controlled with appropriate pesticides at manufacture-recommended rates. Fallow weed control was implemented in August.

Above ground biomass was measured from five replicate 1 m drill rows (0.2 m2 each) oven dried (75 oC) samples. Grain was harvested from the entire plot. Root-zone (0-1 m; (1)) soil water was measured with a neutron moisture meter (NMM) in two PVC access tubes per plot. The NMM calibrations have been established at a neighbouring site (5). Surface layer water content was measured gravimetrically. Water use was calculated as the change in soil water storage within the root-zone plus rainfall. Analysis of variance of crop and soil water data was conducted using GENSTAT. Differences in treatment means of P<0.05 were considered to be significant.

Results and discussion

Above ground biomass, grain yield, soil water content within the root-zone and water use varied both within and between growing seasons. In 1993, no soil water or water use differences were observed. Biomass and grain yield of Indian mustard was significantly less (P<0.05) than field peas and wheat throughout the study. At sowing in 1994, fallow-wheat had an additional (P<0.05) 49 mm soil water within the root-zone compared to mustard-wheat (Table 1). However, fallowing did not change the soil water conserved compared to the continuously cropped treatment in 1994/95 or 1995/96. In these years below average rainfall was received over the fallow weed control period. Soil water content declined progressively under all crops during the 1994 drought. As a consequence, fallow-wheat growth (2.8 vs = 1.6 t/ha), yield (0.7 vs = 0.1 t/ha) and water use (post-anthesis and growing season) was significantly (P<0.05) greater than all other crops in 1994. Here, crop water use was increased by 28 mm due to fallowing. By harvest the soil profile was drier (P<0.05) under field peas (150 mm) compared to mustard (175 mm). Mustard had the driest profile at sowing, but wettest at anthesis in 1995, reflected by poor growth and yield following a late sowing.

The wettest profiles (>270 mm) of the entire study were observed during late winter of 1995 under all treatments, after a period of above average rainfall. During 1995, no differences in soil water content or water use between crops were observed (Table 1). Irrespective of cropping system, similar biomass, yield and water use patterns were observed for wheat and field peas, respectively, in 1995 and 1996. However, fallow-wheat biomass (1996), water use prior to anthesis (1995 and 1996) and over the entire growing season (1995) was greater (P<0.05) than continuously cropped field peas and Indian mustard.

Table 1. The influence of cropping system on wheat root-zone soil water, water use, above ground biomass at anthesis and grain yield at Walpeup during 1994-1996.

Cropping system

Sowing soil water
(mm)

Water use
(mm)

Biomass
(t/ha)

Grain yield
(t/ha)

Harvest soil water
(mm)

1994

Fallow-wheat

233

160

2.70

0.68

172

Mustard-wheat

184

132

1.59

0.10

152

LSD P<0.05

23.7

13.9

0.59

0.15

24.1

1995

Fallow-wheat

216

244

7.18

2.36

189

Mustard-wheat

218

238

6.53

1.96

197

LSD P<0.05

21.6

37.3

2.32

0.43

39.7

1996

Fallow-wheat

208

192

7.18

2.29

164

Mustard-wheat

222

202

5.24

1.88

169

LSD P<0.05

31.4

28.3

1.16

0.39

23.7

Conclusion

Wheat growth, yield and water use was unaltered by fallowing in average growing seasons. However, in a drought year (1994), water conservation under fallow allowed adequate biomass and insured a harvestable grain yield of wheat compared to a continuously cropped system. Simulation modelling will provide a more detailed insight into crop performance and water balance in relation to climatic variability.

Acknowledgments

The technical assistance of S.Blandthorn, A.Manley, A.Corbett, S.Wisneske, J.Latta, M.Brown, M.J.Ferguson and M.W.Ferguson is acknowledged. Financial support from Murray-Darling Basin (NRMS) and Victorian Sate Salinity programs and Agriculture Victoria is acknowledged.

References

1. Incerti, M. and O'Leary, G.J. 1990. Aust. J. Exp. Agric. 30, 817-824.

2. Northcote, K.H. 1979. In: A Factual Key for the Recognition of Australian Soils. 4th Edn. (Rellin Technical Publications: Glenside, South Australia.)

3. O'Connell, M.G., O'Leary, G.J. and Incerti, M. 1995. Agric. Wat. Man. 29, 37-52.

4. O'Leary, G.J. and Connor, D.J. 1997. Field Crop. Res. 52, 209-219.

5. O'Leary, G.J. and Incerti, M. 1993. Aust. J. Exp. Agric. 33, 59-69.

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