The Pest Management Network of the Argentine Association of No-Till Producers reports a new case of weed resistance, this time involving the Argentine rice sector. Researchers confirmed that more than 60% of the evaluated biotypes of weedy rice (Oryza sativa) from the provinces of Santa Fe, Chaco, Entre Ríos, and Corrientes are resistant to the herbicide imazapyr, a member of the imidazolinone family.

What is unique about this case is that the resistance arose from hybridization between weedy rice and cultivated rice. Through spontaneous crossbreeding between the two populations, weedy rice directly incorporated the resistance gene present in cultivated rice.

The work was led by Alejandro Presotto of the National University of the South in Argentina, in conjunction with specialists from INTA (National Institute of Technology), the University of Massachusetts Amherst, and the University of British Columbia.

This is the first report in the country of resistance in this weed known as weedy rice or red rice (Oryza sativa). Similar cases have already been reported in other producing countries such as Brazil, Colombia, Greece, and the United States.

Genomic analysis revealed that the weed not only acquired the gene for resistance to the herbicide imazapyr, but also other traits that reinforce its survival.

Among these are early shelling, which facilitates seed dispersal before harvest; dormancy and the reddish color of the pericarp, which ensure persistence in the soil; as well as morphological and cycle traits that increase its adaptability. Even the synchronization of flowering with the crop favors the continuity of this genetic exchange process.

The combination of resistance and adaptive traits makes weedy rice a formidable competitor within the production system. Its rapid spread is aggravated by the movement of uncertified seeds and dispersal via agricultural machinery.

Therefore, the new Alert issued by the REM underscores the need to strengthen management strategies. The use of certified seeds, thorough cleaning of harvesters and equipment, crop rotation and herbicide modes of action, and the inclusion of service crops are key tools for containing the spread.