Herbicide Resistance in the Northern Grains Region of Australia
The Tropical and Sub-tropical Weeds Research Unit at The University of Queensland has conducted ongoing research, financially supported by
GRDC, into herbicide resistance in the northern grain region of Australia since 1993. Since then, resistance has been confirmed in nine major weed species for the three of the most commonly used herbicide groups in wheat and sorghum, Fops (group A), sulfonylurea (group B) and triazine (group C).
Between 1993 and 1997 five broadleaf weed and two grass weed species were discovered to be resistant to herbicides from three chemical groups. Turnip weed
(Rapistrum rugosum), climbing buckwheat (Fallopia convolvlus), African turnip weed
(Sisymbrium thellungii), common sowthistle (Sonchus oleraceus), Indian hedge mustards
(Sisymbrium orientale) were resistant to the recommended rates of
chlorsulfuron. Liverseed grass (Urochloa panicoides) was resistant to the recommended rate of atrazine and wild oat
(Avena sterilis ssp. ludoviciana) was resistant to the recommended rate of
clodinafop-propagyl. This was the first case where resistance was reported for turnip weed, climbing buckwheat, African turnip weed or liverseed grass anywhere in the world. Resistance was found to have developed after 3 – 10 years use with
chlorsulfuron, five years of “fop” use or 2 – 15 years of atrazine use. There was no correlation between the frequency of use and the degree of resistance. Cross-resistance to herbicides from the same group was common but there was no evidence of multiple herbicide resistance.
Since then further wild oats and paradoxa grass populations have been investigated and a further 17 cases of wild oats were identified as being strongly resistant to “fop” herbicides (Group A) with two of these populations also resistant to “dim” herbicides. Dose response trials also found Paradoxa grass to be strongly resistant to several “fop” herbicides but showed little or no resistance to “dim” herbicides.
The decline in Wild oats fenoxaprop-ethyl resistance within the seedbank over time was also determined. It was seen that within 2 years the number of resistant seeds had declined from
200/m2 to 2/m2. This information suggests that changing from winter to summer crops over a two to three year period, and completely preventing any replenishment of the seed bank can manage resistant wild oats populations.
The current phase of research is detecting and mapping the true extent of herbicide resistance in the northern grains region. Weed seed collected from winter cereal crops between September and November 2001 is currently being screened using the Rothamsted Rapid Resistance Test
(RRRT) and a molecular technique, the Cleavase Fragment Length Polymorphism
(CFLP) assay is being developed to assist with this screening. The RRRT and CFLP will be used to detect resistance to group B and C herbicide chemistries and the CFLP assay will be further developed for the detection of group A and D herbicides.
For further information please contact Chris O’Donnell