Yield Prophet is an internet based computer interface to the crop model APSIM (Agricultural Production Systems sIMulator). It simulates crop growth based on paddock-specific inputs of soil type, pre-sowing soil water and nitrogen, rainfall, fertiliser application and climate data. Over the past three years, this model has predicted grain yields reasonably accurately.

This year, Yield Prophet is being used at three sites on upper Eyre Peninsula – a grey calcareous sandy loam at Lock and a red loam and a heavy red flat at Minnipa. Yield Prophet uses actual rainfall figures to date and then predicts the rest of the season based on historic data. Consequently, as each season progresses, the accuracy of the model improves.

Since the start of April, Lock and Minnipa have received 135 and 128 mm of rainfall respectively. Probablility curves of wheat yield outcomes were generated using Yield Prophet. At Lock, the current wheat yield prediction is between 0.8 and 1.2 t/ha with a 50% chance of exceeding 1.0 t/ha. At Minnipa, the prediction is between 0.8 and 1.4 t/ha with a 50% chance of exceeding 1.0 t/ha on the loam soil and 0.4 to 0.9 t/ha with a 50% chance of exceeding 0.6 t/ha on the heavy red flat. In a year with a severe lack of end of season rainfall, these yield predictions do seem quite optimistic but time will tell.

Yield Prophet is a useful tool but it is essential to have accurate soil characterisation. Many Eyre Peninsula soils have been characterised and these are continually being added to the Yield Prophet database where they are available for farmers and consultants to use.

At this time of year, it is also possible to estimate crop yields in the paddock, based on head numbers, grains per head and estimated seed weights. The first step is to work out the number of heads in a metre of crop row. Take several counts and average them. Multiply the number of heads per metre of row by 100 and then divide by the row spacing (cm) to work out the number of heads per square metre. For example, 50 heads/m × 100 ÷ 20(cm) = 250 heads/m2. The next step is to calculate the number of grains per head or pod by counting several heads or pods and averaging them.

To calculate grain yield in tonnes per hectare, multiply the number of heads per square metre by the number of grains per head. Multiply this by the estimated seed weight and then by 100. Seed weights can vary markedly, but as a rule of thumb, use 0.030 g for wheat, 0.035 g for barley and 0.20 g for peas. For example, 250 heads/m2 × 20 grains/head × 0.037 (g) ÷ 100 = 1.85 t/ha.

AUTHOR: Jon Hancock, Research Officer - Low Rainfall Farming Systems SARDI, Minnipa
CONTACT: For further information contact Jon Hancock on 08 8680 6212 or visit the Yield Prophet website – www.yieldprophet.com.au