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Effects of zero tillage practices on soybean yield and soil cover

D.M. Wheatley, D.A. MacLeod and R.S. Jessop

Department of Agronomy and Soil Science, University of New England Armidale NSW 2351

Summary. Dryland soybeans are considered as an alternative land-use to summer cereal crops and fallows. Two field experiments compared the grain yields and levels of soil protection from zero tillage soybeans compared to soybeans grown by conventional tillage. Zero tillage soybeans maintained high levels of soil cover (>80%) and increased grain yield when sown as an opportunity crop compared to a cultivated system. When sown after a fallow, zero tillage produced equivalent yields to conventional tillage and maintained a higher level of soil cover (>70%).


In the summer rainfall cropping regions of southern Queensland and northern New South Wales cropping consists of continuous crop-fallow rotations. This contrasts with the southern Australian cropping region where extensive use is made of pasture leys after several years of cropping. Without the benefit of a legume pasture ley, total soil nitrogen levels have declined, with a 19-63% decline in nitrogen fertility measured on six southern Queensland soils with cropping histories of between 20 and 70 years (1). Legume crops are being considered as a means of reducing this decline in soil nitrogen; they also have the potential to reduce the build up in cereal diseases when used in crop rotation (2). Disease control is also achieved in legume crops by the ability to control alternative cereal disease hosts, including grass weeds, by selective herbicides such as haloxyfop and fluazifop.

However, legume stubbles provide poor levels of soil protection due to rapid rate of breakdown of stubble and reduced production of crop residues compared to cereals. Retention of crop residues especially from cereal crops by zero and reduced tillage has been shown to reduce soil erosion in the following crop by maintaining high levels of soil cover (4,8). This increase in soil protection has been reported to reduce annual soil loss from 30-60 t/ha/year with stubble removal and conventional tillage to under 2 t/ha/year with stubble retention and zero tillage (3). Soybean research in the USA has found that soil erosion was reduced by 54-64% by zero tillage, and narrow row spacing decreased soil erosion by 37% (9).

Dryland soybean crops can be grown after a proceeding fallow or as an opportunity crop instead of a fallow when soil moisture is available. This paper reports on two soybean experiments one grown as an opportunity crop the other after a winter fallow. Grain yields are compared for soybeans sown by zero tillage or minimal tillage practices compared to conventional tillage practices at wide and narrow row spacing. The soil erosion protection provided by the growing soybean crop and previous crop residues was also compared for the different tillage treatments and row spacings at three stages during the crop.


Opportunity-crop experiment

The crop of dryland soybeans (Glycine max var. Valder) was sown at Willow Tree, NSW (near Tamworth) on a black earth (Ug 5.14) (6) on 19 January 1990, two weeks after harvesting the wheat crop, with 5 t/ha of stubble remaining. The experimental design was a randomised complete block experiment with four replications of four tillage treatments and two row spacing The tillage treatments were zero tillage (standing stubble), minimal tillage (stubble slashed), stubble incorporated (cultivated twice by chisel plough) and stubble removal (above ground stubble removed, with no cultivation). The soybeans were sown at a wide row spacing of 100 cm and a narrow row spacing of 65 cm, at a sowing rate of 230,000 seeds/ha. Weeds were controlled by glyphosate applied to all treatments prior to sowing and Haloxyfop to control grass weeds four weeks after sowing. Each plot measured 10 m long by 8 m wide.

After-fallow experiment

This experiment was sown at Armidale, NSW, on a chocolate soil (Uf 6.32) (6) on 13 December 1990 with the soybean variety Valder . Six tillage treatments were established over the winter fallow following a crop of millet (Echinochloa utilis) resulting in 4.5 t/ha of stubble at harvest, with weed control using glyphosate. The experimental design was a complete randomised block with four replicates of six tillage treatments in plots 10 m long by 8 m wide. The tillage treatments were zero tillage, zero tillage stubble burnt, mulch (simulated tillage of sweep plough), stubble incorporated late (one passage of chisel plough prior to sowing), stubble incorporated early (three passages of a chisel plough during winter fallow), burned and incorporated (stubble burnt and three passages of chisel plough during winter fallow). Sowing density was 650,000 seeds/ha sown by a Stanhay seed spacing drill (precision belt seeder) with a 0.5 m row spacing and 250 kg/ha of single superphosphate applied.


Emergence counts were taken by counting soybean establishment along two random 1 m lengths of row. Final grain harvest was measured by sampling three random 1 m rows and hand thrashing. The percentage of soil cover by crop and stubble residues was measured at 15 days after sowing in the after-fallow experiment and 35 days after sowing for the opportunity crop and for both experiments at full soybean canopy and after crop harvest. Measurements were by a photographic technique with photographs taken using a 35 mm lens at a height of 1.5 m, then projected on to a grid of random points to determine soil cover (10).

Results and discussion

Emergence and grain yields

Tillage type had no significant effect on soybean emergence when sown as a opportunity crop into wheat stubble with an average plant establishment of 170,000 plants/ha. This was not the case in the after-fallow experiment in which stubble removal by burning or by incorporation significantly increased (P<0.05) soybean emergence (580,000 plants/ha) compared to zero tillage with stubble retention (29,000 plants/ha). The reduction in soybean emergence caused by the zero tillage treatment could be attributed to the sowing equipment used in this experiment which tended to have poor stubble handling ability. This especially affected the tillage treatments with high levels of crop residues (zero tillage and mulch). The variation in plant establishment between tillage treatments is within the range found to have no affect on soybean grain yields (5). In the opportunity crop experiment, in which sowing equipment designed for zero tillage was used, no difference in emergence was found.

In the opportunity crop experiment yield from zero tillage was 60% greater than cultivated treatments and 100% greater than treatments with mulch or stubble removed (Fig. 1). There was no significant effect of tillage on grain production when soybeans were grown following a fallow, with the average grain production across all tillage treatments being 3.2 t/ha.

Figure 1. Tillage and row spacing effect on opportunity-cropped soybean grain production.

Increased grain pi (Auction obtained from zero tillage in the opportunity experiment may result from the potential for standing wheat stubble to reduce transpiration loss of water by shading and reduced wind speed. The absence of this effect in the after fallow experiment may be due in part to the flattening of the millet stubble after the winter fallow, which would remove any shading or wind break effects. The after fallow experiment soybean yield was 300% greater than the opportunity cropped experiment. This could be due to the extra 180 mm of rain received in 1990/1991 crop combined with the lower summer temperatures experienced at Armidale compared to Willow Tree. The opportunity crop experiment was also sown one month later than the after fallow experiment. This delay in sowing has been shown to decrease soybean grain production by 20-40% (7). The lack of response to zero tillage in the after fallow experiment could arise from the lower water stress experienced by the crop compared to that at Willow Tree. The narrow row spacing of 0.65 m in the opportunity crop increased grain production by 18% compared to the 1 m row spacing (Fig. 1). This increase in soybean grain production by narrow row spacing agrees with previous work (5) in which January sown soybean crops had increased grain production when sown in narrow row spacing.

Opportunity cropped soybeans sown by zero tillage maintained soil cover significantly higher (P<0.05) throughout the growing season compared to stubble incorporation and stubble removal treatments (Fig. 2). At all times soil cover was maintained by zero tillage at over 80% compared to the other tillage treatments with soil cover reduced to under 60% at harvest. Soil cover was not significantly affected by narrow or wide row spacing. The 80% soil cover at harvest provided by zero tillage soybeans is comparable to the level of soil cover provided by wheat stubble at this time of year. Soil cover in the soybean after fallow experiment was reduced to below 30% at emergence for any tillage treatment that removed crop residues by either tillage or burning. (Fig. 3). It is considered that for soil erosion protection a soil cover level of greater than 30% is needed especially over summer when there is a high risk of erosive rain (>25 mm/ h). Soil cover was increased to approximately 90% at full canopy development for all tillage treatments. At harvest zero tillage soybeans had a significantly (P<0.05) higher soil cover than any tillage treatment that removed stubble at 92% compared to the average for stubble incorporated or burnt treatments was 78% (Fig. 3). The soybeans grown in the after fallow experiment produced greater soil cover at harvest than the opportunity experiment after crop establishment due to reduced dry-matter production by the latter.

Figure 2. Effect of tillage on soil cover of soybeans and remaining wheat stubble of opportunity-cropped soybeans.

Figure 3. Effect of tillage on soil cover of soybeans and remaining millet stubble grown after fallow.


The establishment of soybeans by zero tillage maintained crop yields when grown alter fallow and increased yields when grown as an opportunity crop. Compared to conventional tillage practices, zero tillage soybeans maintained high levels of soil cover during the growing season of over 70% for both cropping strategies. Zero tillage maintained high levels of soil cover by conserving a high level of previous crop residues. Narrow row spacing has the potential to increase grain production of soybeans sown in January. Further work is in progress to study comparative difference in losses of soil and water from the different tillage treatments.


This project was funded by the National Soil Conservation Programme of Australia.


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