Source: The Crop Doctor, GRDC

A new annual medic, Angel SU*, will provide cereal-livestock producers of southern Australia with an annual pasture legume tolerant of sulfonylurea (SU) herbicide residues.

Believed to be a world first, Angel has been bred from Herald strand medic by researchers Dr John Heap (SARDI) and Dr Chris Preston (CRC for Weed Management Systems).

South Australian Research and Development Institute (SARDI) Pasture Group research officers Jake Howie and Craig Bell discussed trial results of Angel at the Grains Research and Development Research Update in Adelaide earlier this year.

SU herbicides such as chlorsulfuron and triasulfuron, used extensively in the cereal-livestock zones of temperate conditions around southern Australia , are generally considered to be effective, cheap and safe to apply with useful levels of residual activity.

However, these residues can persist beyond the cropping year into the following year, particularly in low rainfall areas with alkaline soils.

Legumes are extremely susceptible to even very low residues of SU herbicides which can cause severe stunting, reduced dry matter production, seed yields, persistence and nitrogen fixation.

Interactions under stressful conditions are also likely to result in increased susceptibility to root disease, and nutrient and moisture stress.

Trials have been conducted to compare the relative effect of SU herbicide residues applied the previous year, upon Angel and Herald. These have all confirmed Angel's ability to tolerate SU residues from a range of application rates.

One trial conducted in the Mallee Sustainable Farming core site at Waikerie, SA in May, 2003 saw Herald and Angel sown into wheat stubble containing herbicide residues.

The study showed that application of triasulfuron reduced shoot dry weight, seed yield and regeneration of Herald by > 50% compared with Angel .

Further research to test Angel's tolerance to SU residues from a summer application were conducted at Wirrulla on Upper Eyre Peninsula , SA and Walpeup in the Victorian Mallee.

The results showed Angel's production was unaffected whereas Herald was reduced by 80 per cent.

SU herbicides are commonly used to control weeds such as Lincoln weed, caltrop and skeleton weed over summer before going into pasture.

According to Mr Bell and Mr Howie, the ability of Angel to tolerate short term SU residues will enable it to both survive and to take advantage of the residual moisture and nutrient benefits of weed control.

Research has found that Angel can be effectively controlled to an acceptable level by a range of commonly used chemicals. These control options include both non-SU and post emergent SU applications (excluding triasulfuron).

Angel was commercialised last year with Seedmark, with large scale seed production to occur in 2006 before being commercially available in 2007.

* Varieties protected under the Plant Breeders Rights Act 1994.

The Crop Doctor is GRDC Managing Director, Peter Reading