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Ryegrass is adapted to a wide climatic range

Once considered a winter weed of southern cropping zones of Australia, annual ryegrass is being increasingly observed in more northerly districts and in summer crops in Victoria and South Australia.

With investment from the Grains Research and Development Corporation, researchers at the Queensland Alliance for Agriculture and Food Innovation, University of Queensland conducted a series of ecological studies to better understand the environmental parameters required for germination of this widespread and costly weed.

Bhagirath Chauhan, professor at the University of Queensland’s Centre for Crop Science, Michael Thompson and Gulshan Mahajan studied the effects of temperature, salinity, drought, light and burial depth on germination of seed collected from annual ryegrass (Lolium rigidum) populations from northern and southern NSW and Victoria. One of the three populations was known to be resistant to glyphosate.

“The results of this study suggest that annual ryegrass could easily establish in areas outside the southern cropping zones of Australia, and that glyphosate resistance had effect on germination rates,” said Dr Chauhan. “The three populations achieved germination rates of over 90 per cent under day night temperature regimes ranging from 20/10 to 35/25 degrees C, and around 80 per cent at day/night temperatures of 15/5 degrees C. This suggests that this weed has the potential to germinate in field conditions in Queensland and New South Wales throughout the year.”

It is worth noting that this study investigated only germination of ryegrass seed and not other attributes of the weed, such as seed production, under varying environmental conditions.

With no apparent bounds on the geographic range and seasonality of annual ryegrass, the presence of individual plants or small patches should prompt urgent attention from growers, keeping in mind that the weed incursion could already be resistant to a suite of herbicides.

ryegrass plants near cotton crop

Annual ryegrass established and setting seed adjacent to a cotton crop.

Transient saline conditions affect up to 67 per cent of Australia’s dryland cropping soils and high salt loads on the surface can inhibit ryegrass germination somewhat, however, if the seed does not decay due to long exposure, some seed is likely to germinate when there is sufficient rainfall to dilute the salts. Similarly, annual ryegrass seeds can maintain dormancy during drought conditions and ‘revive’ when moisture is available.

“Soil moisture is considered the main catalyst for germination of ryegrass seed,” said Dr Chauhan. “Typically, ryegrass seed is dormant over summer months in southern growing regions when moisture is limiting, and germination occurs in late autumn. If ryegrass were to establish in areas where sufficient moisture is available for most of the year, then multiple germinations could occur through the year, making management very difficult.”

Although exposure to light plays a role in breaking dormancy of ryegrass seed, even seed kept in complete darkness achieved germination rates above 70 per cent. The high rates (90 per cent) of germination in alternating light and dark conditions correlates with the highest germination rate also occurring when seed was placed at the soil surface rather than buried.

“Germination from a burial depth up to 4 cm was over 80 per cent,” said Dr Chauhan. “Germination rates dropped significantly, but was still around 40 per cent, for seed buried at 8 cm, indicating that shallow seedbed cultivation would not sufficiently bury ryegrass seed to prevent germination.”

Annual ryegrass is endemic throughout Western Australia, South Australia and Victoria where it is considered the #1 cropping weed and has an enviable track record for evading herbicides. In areas where this weed is ‘emerging’ it is safe to say that herbicides will not provide the best control. Annual ryegrass often establishes in patches and can be eradicated if early and decisive action is taken to prevent seed set using the WeedSmart Big 6 integrated weed management program.

In addition to following good farm hygiene and monitoring watercourses, boundaries and access roads, the single most effective tool growers can use against annual ryegrass is strong crop competition. Further research is required to determine the optimal sowing time and planting configurations for crops in the northern growing region to minimise the impact of this weed.

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