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Challenging Sowing Rates for Wheat to Achieve Target Plant Densities

A. Johnson1, M. Evans1 and K. Wansink2

1Natural Resources and Environment, Epsom, Victoria.
Natural Resources and Environment, Geelong, Victoria.


TOPCROP Victoria investigated sowing rates for wheat to achieve target plant densities using large-scale paddock demonstrations during the 2000 season. TOPCROP farmer groups established 30 sites across Victoria comparing 75%, 100%, 150% and 200% of the district practice sowing rate, using Silverstar and a farmer selected wheat variety. Initial findings have indicated that poor seeder calibration and a lack of understanding of the influence of grain size has lead to target plant densities not being reached. This highlights the need for sowing recommendations to be based on target plant densities rather than sowing rates.

Key Words

TOPCROP, extension, target plant densities, sowing rates and participatory research.


During the 1999 season, there was much interest generated by the Victorian grains industry in the inherent grain size differences between wheat varieties and the implications for sowing rates, in particular that of Frame and Silverstar wheat varieties. At harvest, the majority of Silverstar across the state was found to have high screenings. Indications from trial work undertaken by the Birchip Cropping Group and Natural Resources and Environment (NRE) imply that increasing sowing rates might help in the reduction of screenings in Silverstar (1,2). This raises a question. Is traditional district sowing rates still appropriate to achieve target plant densities, given modern varieties and management systems? To address this question, TOPCROP Victoria conducted a State Focus in the year 2000, which investigated the effects of target plant densities on yield and quality of wheat.

The State Focus is a focused paddock demonstration extension program across the State coordinated by TOPCROP (S. Holden, pers. comm.). Critical factors of influence were sowing rate and seed size on plant densities with subsequent implications for yield and grain quality at harvest. Across the state TOPCROP farmer groups established 30 State Focus sites.

The sites were sown, managed and monitored by district TOPCROP farmer groups, using farmer's equipment and land. The wheat variety Silverstar was selected from a common seed source as the standard variety for all sites. The reason being it can be grown throughout Victoria and it was likely to show quality responses at different plant densities. Silverstar was sown at 75%, 100%, 150% and 200% of the district practice sowing rate, which varied between district. The farmer groups included another wheat variety if they wished, however in this paper only the Silverstar variety will be discussed. The nearest neighbour balanced approach, with a control variety at a standard sowing rate every third plot, was used to give an indication of variability at the site and to allow some statistical analysis to be carried out.

Results and Discussion

Recommended target plant densities were met at district practice sowing rates for all or most sites in the Wimmera, Northern and Southern areas. Four out of seven sites in the Mallee did not meet target plant densities at the district practice sowing rates. This may affect final yield at the end of the season. In North Central and North East Victoria, district practice sowing rates resulted in much higher plant densities than the currently recommended target plant densities. It is assumed that recommended plant densities are correct, however, initial results from these sites suggest that the target plant densities for the Mallee, North Central and North East Victoria regions may need to be revised, further work is required into this issue.

Expected plant densities would have been achieved for the tested sowing rates if all seed had successfully established. Under field conditions an establishment of about 80% is considered normal, although in some soil types, establishment can be as low as 60%. Establishment will be less than expected for a variety of reasons, including soil type, seeding depth, waterlogging, dry conditions, herbicides, seed dressings and pests and diseases. This was observed in North Central and North East Victoria, where waterlogging is a common problem often reducing plant establishment, so local experience has shown increased sowing rates are required to compensate. Seeding depth was the probable cause of poor plant densities at the Gymbowen site in the Wimmera, where seed was sown at 10 cm due to machinery difficulties.

Seeder calibration was one of the major causes of expected plant densities not being reached. Seeders were calibrated for most sites, but this did not guarantee accuracy of sowing rate during sowing, particularly as a range of sowing rates were used. Air seeders were more accurate than combines mainly due to ease of calibration and age. Generally, variation from manufacturer’s tables of actual sowing rate was 10%, but was up to 190% for one particular combine. Seeder calibration problems are the likely explanation for high plant densities at the Millewa and Carwarp sites in the Mallee, Nhill South and Kalkee sites in the Wimmera, the St James site in the North East and the Coleraine site in the South West. Low plant densities resulting from poor seeder calibration means a yield penalty and the costs associated with this are likely to be much greater than the cost of the seed needed to sow at a higher rate. Alternatively, a seeder putting out higher rates than expected may result in running out of seed at sowing.

Traditionally farmers determine sowing rates in kilograms per hectare based on experience. However, grain size and weight can vary between varieties and seasons and failure to consider this can result in expected plant densities not being achieved. It was found for one combine that output for Silverstar was very similar to the manufacturer’s tables, but was 20% out for the wheat variety Frame. The higher than expected plant densities at the Kalkee site in the Wimmera was sown with an air seeder having an electronic rate controller. This demonstrates that there is a need to calculate sowing rates in terms of target plants per square metre and to calibrate even the best machines at least once with grain of differing weights and sizes to ensure sowing accuracy.


In 2000, the TOPCROP Victoria State Focus investigated the effects of target plant densities on yield and quality of wheat. In addition to crop environment limitations eg waterlogging, it reinforced three important factors to consider when sowing to reach target plant densities and hence target yield potential: The calculation of grain size each season. The calculation of sowing rates in terms of target plants per square metre rather than kilograms per hectare. The calibration of the seeder with the grain to be sown to ensure sowing accuracy.


The authors gratefully acknowledge the following people:

  • TOPCROP staff, groups and coordinators for their participation and contributions.
  • Russell Eastwood (VIDA Horsham), Ben Jones (VIDA Walpeup) and Harm van Rees (CropFacts) for their technical advice and assistance.


1. van Rees, H., Smallwood, B., McCelland, K., Jackman, J.1999. The Mallee and Wimmera Crop and Pasture Production Manual 1999-2000 BCG Trial Results, 7, (Birchip Cropping Group: Birchip), p126-129.

2. Vallance, N. 2000. Proceedings of the Victorian Cropping Systems Update, 2000. Horsham, p 29-30.

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