DO SPRING COVER CROPS IN THE LONG-FALLOW BETWEEN SKIP SORGHUM AND WHEAT ROB WATER AND REDUCE WHEAT YIELDS?

 

Why do the work

During the 14-month fallow that arises when moving from summer to winter crops, stubble breakdown can denude the soil surface reducing water infiltration and leaving it vulnerable to erosion in a traditional zero-till farming system. Cover crops of millet have been proposed as a solution, but this then raises the question, how often is there sufficient water in the system to grow a cover crop without reducing the soil water reserves to the point of prejudicing the following wheat crop?

 

What was done

An on-farm research approach was used to compare the traditional zero-till/chemical weed control long fallow (TF), with a millet fallow (MF) in which a short-term millet crop was sown solely to produce cover. A total of 31 commercial paddocks were studied over three years with each paddock simulated using APSIM (Keating et al. 2003) to investigate the outcomes over a longer timeframe and to determine how often a millet fallow could be successfully included within the farming system.

 

Results

The results showed that if millet was sown early and removed early (40 days after sowing) there was a neutral or positive impact on wheat yield compared to a traditional fallow. Not removing the millet on time and allowing it to progress to maturity reduced wheat yields more than 90% of the time. However, examination of the quantities of cover produced after 40 days millet growth produced insufficient sufficient cover to protect the soil (estimated at, at least 30% Freebairn and Wockner, 1986). Growing millet to provide cover is a balancing act, sufficient cover to prevent erosion must be grown without using more water than can be replaced over the summer season. To assist this balancing act management rules were developed. These where:

 

 

Figure 1

 

Probability of exceedence figures with 100 years of simulated data showing the strategic management of a millet fallow (▬ ▬), in almost all years, improved wheat yield, reduced runoff and soil loss, but increased deep drainage when compared to a conventional long fallow (▬▬), for (a) the following wheat yield, (b) cumulative runoff between the 1st of March year 1 (after sorghum harvest) and wheat harvest of year 2, (c) cumulative drainage below the wheat root zone between the 1st of March year 1 and wheat harvest of year 2, and (d) cumulative soil loss between the 1st of March year 1 and wheat harvest of year 2.

 

Conclusions

The results show that it is possible to include a millet cover crop within a northern grains sequence without detrimental effects on the yields of following crops. In fact millet had a positive effect on wheat yields, provided the millet was sown early in spring and terminated once 50 % cover had been achieved. Visually determining cover is considered a better method of identifying crop termination time compared to phonological stage, because it accounts for seasonal variation.

 

The main motivation for farmers in the northern grains region to sow cover crops is to maintain infiltration rates and reduce soil erosion. However, are these benefits worth the $50/ha cost of establishing the cover crop? This research shows that it is, differences in wheat yield when comparing the MF millet and CF traditional no-till fallows is significant and could be used to offset the costs of millet establishment. Whilst the cost of soil erosion is not insignificant, its impact varies with season; so is hard to substantiate within a single season gross margin however, erosion costs in the northern grains region have been estimated to be about 8% of production per decade (Loch and Silburn, 1997). 

 

Farmer’s enthusiasm for cover crops has driven this research project. This suggests that they do value erosion prevention at $50/ha irrespective of any yield improvement in following wheat crops.

 

Similar scenarios have been run for regions from Gunnedah to Emerald and the cover cropping rules hold for these regions too.

 

Acknowledgments

This work was completed as part of the Eastern Farming Systems project and was funded by GRDC, CSIRO and QDPI&F. We also would like to thank MCA Goondiwindi and their clients for supporting this work. 

 

References

Freebairn, D.M., Wockner, G.H. (1986) A study of soil erosion on Vertisols of the eastern Darling Downs, Queensland .I. Effects of surface conditions on soil movement within Contour Bay catchments. Australian Journal of Soil Research 24,135 - 158.

 

Freebairn, D.M., Wockner, G.H. (1986) A study of soil erosion on Vertisols of the eastern Darling Downs, Queensland .II. The effect of soil, rainfall, and flow conditions on suspended sediment losses. Australian Journal of Soil Research 24,159 - 172.

 

Keating BA, Carberry PS, Hammer GL, Probert ME, Robertson MJ, Holzworth D, Huth NI, Hargreaves JNG, Meinke H, Hochman Z, McLean G, Verburg K, Snow V, Dimes JP, Silburn M, Wang E, Brown S, Bristow KL, Asseng S, Chapman S, McCown RL, Freebairn DM, Smith CJ (2003). An overview of APSIM, a model designed for farming systems simulation. European Journal of Agronomy 18, 267 - 288.

 

Loch RJ and Silburn DM (1997). Soil Erosion, In Sustainable crop production in the sub-tropics : An Australian perspective (Eds. A.L. Clarke and P.B. Wylie Queensland Department of Primary Industries ISBN 072425985 6.

 

Contact details

Jeremy Whish

CSIRO Sustainable Ecosystems

203 Tor St Toowoomba 4350

Email: Jeremy.Whish@csiro.au