Benzofenap - a new herbicide for weed control in water-seeded rice

Benzofenap (MY-71) is a pyrazole herbicide first synthesized in 1981 by Mitsubishi Petrochemical Company. It was first registered in Japan in 1987 for pre- and early postemergent control of annual grass, sedge, and broadleaf weeds in transplanted rice. Rhone Poulenc Rural Australia Pty. Ltd. began evaluating benzofenap for water-seeded rice in New South Wales (NSW) in 1994. The commercial introduction of benzofenap (as Taipan® herbicide) in Australia occured in the 1998-99 season. Benzofenap exhibits low water solubility and volatility. It is primarily absorbed through roots, resulting in carotenoid biosynthesis inhibition. Toxicity to mammals, birds, and aquatic organisms is very low and no residues have been detected in grain and straw at harvest. Forty-seven replicated field trials were conducted over four seasons to demonstrate the efficacy of benzofenap against seedlings of the following weed species in water-seeded rice: Sagittaria montevidensis, Alisma plantago aquatica, A. lanceolatum, Damasonium minus, and Cyperus difformis. Optimum results were obtained by applying benzofenap at 600 g ai ha¯¹ to standing water within 10 d of flooding. The application of benzofenap to dry soil before inundation resulted in poorer weed control than when applied to floodwater. The addition of molinate at 2,400-3,600 g ai ha¯¹ to benzofenap ensured the effective control of grass weeds (e.g., Echinochloa crus-galli). Crop safety of benzofenap plus molinate combinations was acceptable and equivalent to or better than alternate commercial standards. Monitoring of benzofenap performance under commercial conditions has demonstrated that prolonged floodwater retention after application is critical to attaining effective weed control. Where benzofenap-treated floodwater has been held static for minimum of 5 d, excellent aquatic weed control was attained, resulting in high rice grain yields. MCPA sodium has been recommended for application following benzofenap in NSW as a second mode-of-action herbicide to minimize selection for benzofenap-resistant weed biotypes.