The Department of Agriculture of Western Australia is helping wheat growers determine optimum seeding rates based on minimum plant populations required for expected yield.

Principal researcher Wal Anderson (photo) told delegates at this week’s Agribusiness Crop Updates that adjusting the seed rate according to the expectation of wheat grain yield would maximise yield potential.

Expected yield related to the amount of pre-sowing rainfall, expectation of seasonal rainfall, actual sowing date and soil type.

Dr Anderson said the Department’s research indicated that to maximise yield, a minimum of 40-50 plants per square metre were required for every tonne of anticipated yield.

He said using a seeding rate suitable for only an average season could limit yield in a good season.

Using the seed rate for an average season, however, was unlikely to be detrimental to yield or grain quality in a poor season.

“The Department has collected data from over 30 cropping experiments conducted during 1996-2001 at locations ranging from Yuna to Lake King,” Dr Anderson said.

“The trials involved up to five target plant populations and a range of varieties to assess the minimum plant populations needed to maximise wheat grain yield.”

Dr Anderson said season, soil type, sowing time and sometimes variety affected the optimum plant population and corresponding minimum seed rate required to maximise yield.

He said the more pre-sowing rainfall, more rainfall in the growing season and earlier sowing time all increased the minimum plant population required to maximise yield.

Where rainfall in the growing season was less than 205mm, the optimum plant population increased from 35 to 124 plants per square metre (20-65 kg/ha of seed) with the increase in rainfall and earlier sowing.

Yields at the optimum population in this set increased from 1.39 t/ha - 2.85 t/ha. 

Where rainfall in the growing season was greater than 205mm, the trend to increased optimum population with increased yield was also evident. Yields in this group ranged from about 2.5 - 3.5t/ha.

In general crops on clay loam soils required higher seeding rates but fewer plants to maximise grain yield compared with sandy soils.  This was due to the lower establishment percentages that are common on heavier soil types.

“Better estimates of the minimum seed rate (plant population) for wheat can be made if pre-sowing rainfall, rainfall zone, sowing date, soil type and variety are taken into account,” Dr Anderson said.

“Optimum seed rates (and associated populations) should be viewed as the minimum requirement to avoid limitations to grain yield caused by too few plants under weed-free conditions.

“Higher rates should be used to provide competition in weedy situations.”

The research and the Crop Updates seminars are supported by the Department of Agriculture and the Grains Research and Development Corporation.