Crop sequencing - what are the key issues to making profitable and sustainable crop sequencing decisions?

Roger Lawes,

CSIRO Sustainable Agriculture Flagship

Keywords: break crops, profitability, weed control, disease control

Take home messages

• Break crops can improve cereal yields by 0.6 t/ha.
• While they can be costly to grow, the increase in profitability of future crops can more than offset these short term costs.
• Break crops should be considered, providing they help control weeds and disease.  If break crops do not help weed management, then other options, like fallow, should be considered.

Background

Break crops form an important part of the cropping system.  Break crops can provide alternative options for weed control, reduce the likelihood of soil borne diseases, and if legumes are grown, provide an additional source of nitrogen (N) to the subsequent cereal crop.  In general break crops increase the yield of the following cereal crop by an average of 0.6 t/ha in Western Australian systems (Seymour et al., 2012). The size of the yield benefit following a break crop is influenced by the ability to control grass weeds as these can host crop diseases like take-all and reduce the effectiveness of the break crop (McLeod et al., 1993).

While break crops have historically been an important part of the cropping system, recent farm and paddock level surveys suggest farmers are growing fewer break crops (~ 10-12% of farm area) than is economically optimal (~20-30% farm area) (Robertson et al., 2010).  It is conceivable that the prevailing management strategy revolves around growing a cereal crop until there is a problem and then spelling the paddock from cropping operations to control weed and disease issues.

Therefore, when should you consider growing a break crop?  The answer is defined by what price, disease load, weed load and break crop yield do you need to justify not growing a cereal.  We explore these questions using the LUSO (Land Use Sequence Optimiser), which is a bio-economic model designed to evaluate crop sequencing issues.

Materials and methods

The LUSO model was parameterized from data generated in a FACEY group crop sequence trial.  Lupins, canola, wheat and Cadiz serradella pasture were sown in 2011 where the growing season rainfall was 257 mm. Wheat (cv Mace) was sown over every plot in 2012, creating crop sequences of lupins-wheat, canola-wheat, wheat-wheat and Cadiz-wheat.  Since yields from the crop sequencing trial were abnormally low for wheat (0.4) and high for lupins (2.3) (Table 1), an alternative parameterization of the model was considered with the following yields (wheat, 2.5 and lupins,1.5, Table 1).

Table 1. Yields and prices used with the LUSO model.

*Wheat was frosted, so the district average of 2.5 t/ha was used in its place
** Cadiz yields refer to biomass produced and the price refers to the feed value of the forage given lamb prices.

Model scenarios were conducted on a 5 year cropping sequence and sequences considered were:

• Wheat, wheat, wheat, wheat, wheat.
• Wheat, wheat, canola, wheat, wheat.
• Wheat, wheat, lupins, wheat, wheat.
• Wheat, wheat, Cadiz, wheat, wheat.

The management choice was to evaluate the economic cost or benefit of growing a break crop instead of growing wheat in a continuous cropping scenario.  These crop sequences allow us to consider what the future cost or benefit to wheat production and profit is when you decide to include a break crop in the sequence after two years of wheat. A break crop would be deemed successful if it contributes to higher wheat yields and future profits that offset the cost of growing the break crop or pasture.

Results or economic analysis scenarios

In the continuous wheat scenario, annual profit declined steadily from $48/ha to just $12/ ha over the 5 year period because wheat yields were reduced by the increase in disease and weed pressures (Figure 1). 

Figure 1. Annual profit, disease impact on yield and weed impact on yield for wheat grown in sequence for 5 years.

The continuous wheat sequence generated an overall profit of $46 when the value of the sequence was discounted for weed seedbank. When lupins were grown in the 3^rd year in the place of wheat, this increased to $63. In a separate sequence, when canola was grown in the 3^rd year in place of wheat the profit increased to $78.  Sprayed pasture generated a loss return of $66, while grazed pasture generated a return of $140 when it was grown in the 3^rd year of the sequence.  The final economic returns are discounted for the size of the weed seed bank remaining at the end of the sequence.

In this situation, when break crops were grown in the 3^rd year, they generated losses of $60 for canola, $111 for lupins, $134 for grazed pasture and $344 for sprayed pasture in that year.  However, because weeds were controlled, disease was managed and in the case of lupins and pasture, N was added to the system profits increased considerably in the 4^th and 5^th years of the sequence (Figure 2).  In the case of canola, lupins and grazed pasture, the increase in wheat yields over two years combined with possible savings in fertiliser N generated a positive economic return that justified growing a break crop in place of wheat.

 Figure 2. A comparison of  annual profit from a 5 year cropping sequence, where wheat, canola, lupins, grazed pasture or sprayed pasture are grown in the 3^rd year of the crop sequence.

From these simulations, break crops can improve cereal yields and long term profits, by suppressing weeds and disease. However break crops will only be profitable if weeds are controlled and diseases are suppressed. In some instance, where ryegrass cannot be controlled in lupins, the benefit of the break crop to the cereal crop may be compromised and in these situations alternatives such as fallow may need to be considered.  In this simulation, it was assumed that weed populations could be managed effectively in wheat crops and in the break crops.  Weed populations that cannot be controlled may require different crop sequences.   In closing, break crops should be viewed from the perspective of their economic return and the benefit they confer to the subsequent cereal crops. Growers should ask the question; what will be the future profit that this break crop will generate?

References

Macleod WJ, MacNish GC, Horn CW (1993) Manipulation of Ley Pastures with Herbicides to Control Take-All. Aust. J. Agric. Res., 1993, 44, 1235-44.

Robertson, M., Lawes, R., Bathgate, A., Byrne, F., White, P., Sands, R., 2010. Determinants of the proportion of break crops on Western Australian broadacre farms. Crop & Pasture Science 61, 203-213.

Seymour  M, Kirkegaard J A, Peoples M B, White PF, French R J (2012) Break-crop benefits to wheat in Western Australia- insights from over three decades of research.  63. 1-16.

Acknowledgements

I thank Sarah Hyde and Graham Manton from the Facey group for conducting the trial the yield data originated from.

Contact details

Roger Lawes

CSIRO Sustainable Agriculture Flagship, Wembley,

roger.lawes@csiro.au