Why is sowthistle on the increase?

NEW research is having its first long hard look at sowthistle, a serious and growing threat to graingrowers in the north.

The scientist doing the looking is weeds research agronomist Michael Widderick, at the Leslie Institute at Toowoomba. Mr Widderick's University of New England project is supported by growers and the Federal Government through the GRDC.

His project looks at the reasons for the emerging dominance of this weed under changing tillage and weed control systems; it may also spot other potentially troublesome weeds with similar ecology, and will train a weed scientist for work in the northern cropping region. Mr Widderick says common sowthistle (Sanehus aleraeeus), pictured above, has become one of the most common weeds throughout the northern grains region of Australia and is difficult to control in both crop and fallow situations.

Increase under zero-till systems?

Northern graingrowers and agronomists confirmed this in a recent survey and that it has increased over the past five to 10 years. Published surveys from the northern wheatbelt of NSW indicate an increase of 30 to 67 per cent during the 1980s.

The weed is difficult to control under fallow systems, relies heavily on herbicides for its control, and seems to be more common under zero tillage than in minimum and conventional tillage systems. Seed can germinate over a wide range of temperatures and moisture levels, although germination is greatest at higher water availabilities.

Mr Widderick believes one reason sowthistle is doing so well may be its adaptation for germination and growth under reduced tillage systems. Field measurements showed that emergence was greatest under zero tillage and least under conventional tillage (300 and 70 seedlings respectively). Greater levels of seed remained in the top 2 cm of soil in zero-tillage treatments compared with conventional tillage (50 seeds and 6 seeds respectively).

Another reason may be sowthistle's tolerance to the common herbicides used in crop and fallow andlor an ability to acquire resistance to herbicides such as chlorsulfuron, a widely used herbicide in wheat.

The project aims to develop effective management strategies that will lessen the weed's impact on northern crops. A field experiment this year is investigating its impact on crop productivity and the usefulness of crop competition as a management tool.

Program 3 Contact: Dr Brian Sindal 02 6773 3747. Mr Michaal Widdark:k 07 4639 8856

Region North, South, West