Trials by the Department of Agriculture, Western Australia have shown summer weeds can have a serious impact on wheat grain yield and protein and at the same time seriously reduce soil nitrogen.

The trials, funded by CRC Australian Weed Management, saw wheat grain yield down by as much as 40 per cent in two out of three years, soil nitrogen levels down by as much as 66 per cent and grain protein levels substantially reduced.

“Given the weather we have had this year the timing of the trial results is very apt,” said Department of Agriculture Senior Researcher Dr Abul Hashem, who along with Dr Shahab Pathan, carried out the trials.

“Summer weed growth is a real issue after recent summer rains and the messages from these trials are clear. It is of the highest importance for growers to make every effort to control summer weeds,” said Dr Hashem.

The trial was carried out between 2003 and 2005 on sandy loam duplex soil at Merredin Research Station and measured soil nitrogen and soil water up to a depth of 90cm in January and May (before sowing wheat crop), growth, grain yield and quality of winter wheat over three years. 

Key findings were:

• A combination of summer rain and uncontrolled weeds reduced soil nitrogen within the rooting zone of crop plants by up to 66 per cent - winter crops would have 66 per cent less nitrogen in the soil to start with in a paddock where summer weeds were not controlled.

• Summer weeds did not use any more soil water than lost otherwise from a weed-free plot. This is probably because the evaporative demand on the site during summer period was 3-4 times higher than the amount of total summer rain received.

• Wheat emergence was reduced by 22% in 2003 only when caltrop was the dominant summer weed species. This means caltrop may have a strong allelopathic effect on wheat emergence.

• Summer weeds reduced wheat grain yield by up to 40 per cent in two out of three years.

• Wheat grain protein was substantially reduced by a combination of summer rain and summer weeds in all the three years. 

• Soil nitrogen fixed by summer weeds will be released slowly as their biomass decomposes although this released nitrogen will not be available to wheat plants during vegetative stages when demand is high.

“Summer weeds can also serve as green bridges for insects and diseases. They can also block the seeding machine leading to poor crop establishment. Early treatment is essential,” said Dr Hashem.

Grazing is a very cheap management option for summer weeds. The palatability of summer weeds species such as melons, radish, capeweed and doublegee may be increased by spraying 2,4-D amine @ 250 g ae/ha.

Most summer weeds can be effectively controlled by spraying a mixture of glyphosate @ 250-500 g ae/ha + 2,4-D ester @ 200-400 g ai/ha when the wind speed is between 3-15 km/hour.

Where there is a restriction on 2,4-D spraying or growers are not sure how summer weed herbicides are going to affect their winter cropping plan, they should seek advice on appropriate herbicides to control summer weeds.