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Keep Trials Simple When Assessing Variable Rates

30 April 2012


With heightened interest among growers in using variable-rate application (VRA) to reduce input costs, it is recommended they first conduct on-farm trials to generate paddock data.

The Grains Research and Development Corporation (GRDC) funded ‘The Agronomy Jigsaw’ project has identified simple steps that growers can take to maximise the effectiveness of these trials.

A simple nitrogen trial repeated at two locations in the paddock to cover different soil types.

These measures and other information about VRA are outlined in a GRDC Variable-Rate Application Fact Sheet mailed to growers in the March-April edition of Ground Cover and available at

Variable-rate application involves adjusting input rates to match changing local requirements, instead of applying a blanket rate over an entire paddock.

The Agronomy Jigsaw project involves the Department of Agriculture and Food (DAFWA), the South East Premium Wheat Growers Association (SEPWA) and Precision Agronomics Australia (PAA).

It aims to improve farm production efficiency by providing knowledge about how to better use inputs and rainfall.

SEPWA project officer Nigel Metz said the project had included the use of on-farm strip trials, which had led to insights about how they could be better conducted to generate meaningful information and minimise error.

“While data collected from farmer trials using yield mapping software will never be as accurate as small plot trials conducted by researchers, they are a simple and efficient way of gathering information about variation within paddocks,” he said.

“One way of being confident about yield differences observed in strip trials is to keep them simple: stick to one or two treatment differences and make the size difference between treatment rates large.

“For example, use your normal rate of nitrogen, a treatment of double the usual rate and a treatment using no nitrogen.

“You need those large differences in treatments to be confident that the yield differences you observe in your strip trial are due to the effect of the treatment, and not simple variation in data from your yield map.”

Mr Metz said growers could also improve their on-farm trials by harvesting the trial area using a single harvester and harvesting in the same direction, not up-and-back.

“Harvesting up and back can cause yield data errors of up to 20 per cent,” he said.

Growers should ensure that individual strip trials were conducted in paddock areas with the same soil type.

“If there is room, repeat the trial in different parts of the paddock where there are different soil types,” Mr Metz said.

He said that in line with the GRDC Variable-Rate Application Fact Sheet, growers should:

  • Question whether variation exists when considering variable-rate application, as the level and area of variability needs to be big enough to justify adoption;
  • Use on-farm trials to diagnose the constraint or factor which limits crop yield when managing different zones within a paddock;
  • Prioritise lifting the yield potential of constrained soils with ameliorants before adjusting nutrient inputs;
  • Keep on-farm trials simple and consider equipment capability in the trial design.

The fact sheet includes grower case studies, including that of Esperance grower Phil Longmire, who has implemented a management system that modifies fertiliser and seed rate applied on headlands in his paddocks.

In one year this approach saved him nearly 24 tonnes of fertiliser, valued at $16,700, as well as savings in seed, which have not yet been calculated.

As well as being available via the GRDC website, the fact sheet is available free (plus postage and handling) through GRDC’s Ground Cover Direct – freecall 1800 110044 or email [email protected]

Growers can access additional information about variable-rate application and precision agriculture through the GRDC’s ‘Precision Agriculture Links’ (PALinks) at

Among the several resources available through PALinks are YouTube videos about PA technologies, produced under The Agronomy Jigsaw project.

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