Eighteen years of Clearfield™ rice in Brazil: what have we learned?

Abstract

Clearfield™ (CL) rice (Oryza sativa L.) is a weedy rice (Oryza spp.; synonym = red rice) control tool that has been used in Brazil since 2003. This system includes the use of an imidazolinone (IMI)-tolerant cultivar and the application of IMI herbicides. In this review article, Brazilian weed scientists evaluate the challenges and lessons learned over 18 yr of CL use. CL system benefits include selective weedy rice control, better crop establishment during the most advantageous period of the year, and more efficient fertilizer use. In Rio Grande do Sul state, the CL system, in conjunction with other improvements, has contributed to rice grain yield gains from 5,500 kg ha−1 before 2002 to around 8,400 kg ha−1 currently. In contrast, the main problem that has arisen over this period is the rapid evolution of IMI-resistant weedy rice, caused by gene flow from CL rice cultivars. The off-label use (rate and continuous use) of IMI herbicides has contributed to the evolution of resistance in Echinochloa spp. and other weeds. IMI herbicide carryover has also affected susceptible crops grown after CL rice. Crop rotation with soybean [Glycine max (L.) Merr.] is increasing, ensuring system sustainability. The importance of minimum tillage has also become apparent. Such cultivation includes applying nonselective herbicides before sowing or just before crop emergence (at the spiking stage to eliminate as much weedy rice as possible and other weeds at an early growth stage). It also includes the use of certified seeds free of weedy rice, following label instructions for IMI herbicides, applying the herbicide PRE followed by POST, and complementary weedy rice management practices, such as roguing of surviving weedy rice plants.

Insights into the genetic spatial structure of Nicaraguan weedy rice and control of its seed spread

“…In addition, although planting herbicide‐tolerant varieties along with the use of chemical control is convenient and effective, 45 our study documented outcrossing events randomly occurring in each district, indicating that there is pollen‐mediated gene flow between WR and cultivated rice. Similar cases have also been reported in the USA and Brazil 46,47 . If herbicide‐tolerant rice production systems are not properly managed, populations with introgression of herbicide‐tolerannce traits will emerge and survive under high selective pressure.…”

“…Similar cases have also been reported in the USA and Brazil. 46,47 If herbicide-tolerant rice production systems are not properly managed, populations with introgression of herbicide-tolerannce traits will emerge and survive under high selective pressure. In Malaysia, because of the long residual effects of herbicides, farmers have limited options for planting traditional rice without herbicide tolerance.…”

Surveys of weed management on flooded rice yields in southern Brazil

“…The yield rise of around 2,000 kg ha −1 from the 2009 to 2010 growing season to the 2019 to 2020 growing season could be related to soybean rotation, in spite of a decreased rice harvest area during that time (Avila et al 2021a). Soybean crop rotation started with 11,000 ha in the 2009 to 2010 growing season, compared with approximately 1 million ha of cultivated rice (IRGA 2020).…”

“…Soybean crop rotation started with 11,000 ha in the 2009 to 2010 growing season, compared with approximately 1 million ha of cultivated rice (IRGA 2020). Despite the importance of using soybean in rotation in flooded rice fields as a resistance management tool, currently only ~29% of these fields in RS have adopted this practice (Fruet et al 2020), mainly due to the difficulty of managing dryland crops in lowland environments (Avila et al 2021a). Farmers who adopted crop rotation for resistance management reported yields around 20% higher than farmers who reported increasing the herbicide dose (Figure 4).…”

“…In this sense, efforts must be made to elucidate and raise farmer's awareness of the real risks involving the increase of resistant weeds, so that good management practice strategies can be adopted to mitigate the evolution of resistance. Thus, more than 15 yr after the first CL cultivars were launched in Brazil, it is relevant to assess the impact of problems related to weed resistance (Avila et al 2021a). The objective of the present study was to evaluate the association of weed management practices adopted by flooded rice farmers in the RS with yield.…”