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Assessing climatic risk to grain sorghum production in water-limited subtropical environments

R.C. Muchow1, G.L. Hammer2, and R.L. Vanderlip3

1CSIRO Division of Tropical Crops and Pastures, 306 Carmody Road, St. Lucia QLD 4067
2QDPI/CSIRO Agricultural Production Systems Research Unit, PO Box 102, Toowoomba QLD 4350
3Department of Agronomy, Kansas State University, Manhattan KS 66506 USA

Rainfed crop production in the subtropics is a risky enterprise due to high rainfall variability. The frequency of occurrence of planting opportunities is low. Grain sorghum can be planted from spring through to autumn. Farmers face a decision of whether or not to plant when a planting opportunity occurs. This decision is influenced by the soil water present at planting and the likely yield, and the yield expectation from a later planting. Before deciding on a later planting, the probability of obtaining another planting opportunity and the likely change in soil water at the later planting need to be considered. In this study we assessed the risks associated with this planting decision by quantifying:

  • the effect of planting date and soil water at planting on sorghum yield expectation; and
  • the likely change in soil water with delay in planting.

Methods

A sorghum crop growth simulation model (1) was coupled to long-term sequences of climatic data to generate probabilistic estimates of yield for a range of planting conditions and probabilistic estimates of change in soil water with delay in planting. We used Dalby as the location in subtropical Australia to demonstrate the approach. Continuous daily temperature and rainfall data for 101 years were available for this location. Three levels of available water capacity, either full or half full at planting, were simulated for planting dates from August to March.

Results and discussion

Median yield was relatively stable for planting times between October and February. Earlier spring plantings achieved higher yield in the more favourable years, but showed greater yield variability. Higher yield levels were found with increased level of soil water at planting. Greater soil available water capacity was associated with higher yield levels only if the level of stored water at planting was greater. Grain yields were higher at all levels of cumulative probability where the soil profile was full rather than half full at planting.

Delay in planting was associated with a good chance of increased stored soil water in the next 30 days for sowings later than November. However, information on likely occurrence of planting opportunities in the next 30 days, and the significant chance of losing stored soil water, indicate that the farmers have little option but to plant whenever a planting opportunity occurs. However, this option would be tempered by farmers yield goals, their attitude to risk, and other influences not considered in our analysis, such as disease and pest pressures and requirements to fit a crop rotational regime.

Reference

2. Hammer, G.L. and Muchow, R.C. 1991. In: Climatic Risk in Crop Production: Models and Management for the Semiarid Tropics and Subtropics. (Eds R.C. Muchow and J.A. Bellamy) (CAB International: Wallingford, United Kingdom). pp. 205-232

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