Brazil has recorded its first case of multiple herbicide resistance in weedy rice (Oryza sativa), marking a new challenge for rice growers and resistance management programs across the country.

The alert was issued by the Herbicide Resistance Action Committee – Brazil (HRAC-BR) after confirmation that populations of weedy rice from Viamão, Rio Grande do Sul, are resistant to both quizalofop-p-ethyl, an ACCase inhibitor (Group 1), and imazetapir, an ALS inhibitor (Group 2).

The case was validated by researchers from the Federal University of Rio Grande do Sul (UFRGS) using internationally recognized dose-response methodologies and has now been incorporated into the International Herbicide-Resistant Weed Database (WeedScience.org).

According to HRAC-BR, the finding represents an evolution from previously reported cases in Brazil, in which weedy rice populations had previously developed resistance to ALS inhibitors alone. The emergence of resistance to two distinct mechanisms of action significantly reduces available post-emergence control options and highlights the increasing "selection pressure" resulting from repeated herbicide use.

Weedy rice is considered one of the most damaging weeds in irrigated rice production, due to its competingwith the crop while belonging to the same species, making selective control particularly difficult. The new resistance profile raises concerns about the long-term sustainability of current weed management programs in southern Brazil, the country's main rice-producing region.

In response, HRAC-BR is urging growers, agronomists and the rice industry to strengthen Integrated Weed Management (IWM) strategies, rather than relying primarily upon chemical control.

Among the recommendations are the use of certified rice seed to prevent the introduction of contaminated seed lots, thorough cleaning of planting and harvesting machinery to avoid spreading resistant populations, elimination of surviving plants before seed production, rotation of herbicide modes of action, adoption of appropriate application technology, and early-season weed management combined with adequate soil cover, instead of leaving fields fallow.

The committee also emphasizes the importance of closely monitoring fields where herbicide control failures occur so that suspected resistant plants can be identified and managed before resistant populations expand further.

According to HRAC-BR, preserving the effectiveness of existing herbicide technologies will increasingly depend upon integrated agronomic practices that combine chemical, cultural and mechanical control measures rather than continued dependence on a limited number of herbicide modes of action.The new report adds Brazil to the growing list of countries facing increasingly complex herbicide resistance scenarios and reinforces the global need for diversified weed management strategies as resistant biotypes continue to emerge. (from agropages)