Efficacy of new post-emergence herbicides for rice

Abstract

Weeds are the major problem in rice production. Manual removal of weeds is laborious and due to labour shortage and high cost of labour, farmers are resorting to chemical weed control. Many new molecules of selective herbicides are now coming in the market which are having broad spectrum activity or effectiveness against a group of weeds. The present study was conducted to evaluate the relative efficacy of some of these herbicides in wet land rice culture.

The experiment was conducted at Alappad Kole in farmer’s field during October 2011 to February 2012. The treatments included application of graminicides alone ( metamifop, fenoxaprop p-ethyl, cyhalofop-butyl); graminicides with follow up application of herbicides to kill non grass weeds (cyhalofop-butyl fb Almix, metamifop fb carfentrazone ethyl, metamifop fb Almix, fenoxaprop p-ethyl fb Almix, fenoxaprop p-ethyl fb ethoxysulfuron) and herbicides with broad spectrum activity (bispyribac sodium, penoxsulam and azimsulfuron). Hand weeded and unweeded controls were also included for effective comparison. All herbicides in the study were post emergent in action and were sprayed at 20 DAS. Observations on weeds as well as crop were taken at 30 DAS, 60 DAS and at harvest. Visual phytotoxicity scoring on rice as well as on weeds was done at three and seven days after spraying.

As revealed by the weed spectrum in unweeded control, grasses were the dominant weeds than sedges and broad leaved weeds. Among grasses, Echinochloa stagnina and Echinochloa crusgalli among sedges Fimbristylis miliacea, Cyperus iria and Cyperus difformis and in broad leaved weeds Lindernia crustacea and Ludwigia perennis were the dominant weed species.

Sedges and broad leaved weeds were present in treatments applied with graminicides alone at 30 DAS. At 60 DAS and at harvest, higher weed population was observed compared to 30 DAS. Bispyribac sodium, cyhalofop-butyl+Almix and fenoxaprop p-ethyl+Almix continued to perform better in terms of weed control as evidenced by low weed dry matter production both at 60 DAS and at harvest. N, P and K uptake by weeds at different stages of crop followed the same trend as that of weed dry matter production.

Among various herbicides or herbicide combinations only carfentrazone ethyl applied treatment showed phytotoxicity to rice. The variations in plant height among treatments were significant only at 30 DAS and the tallest plants with 54 cm height were observed in unweeded control. However, there was a slight reduction in the height of rice in plots applied with Almix, fenoxaprop p-ethyl and bispyribac sodium compared to hand weeded control, though visual phytotoxicity symptoms were not expressed.

The highest number of panicles/m2, filled grains per panicle as well as grain yield was registered in hand weeded control. Cyhalofop-butyl+Almix and fenoxaprop p-ethyl+Almix were on par with respect to grain yield and recorded comparable yield with that of hand weeding. Cyhalofop-butyl+Almix and fenoxaprop p-ethyl+Almix also registered low weed index of 5.2 (for both) and high weed control efficiency of 88 and 90 percent, respectively.

From this study it can be concluded that, cyhalofop-butyl followed by Almix or fenoxaprop p-ethyl followed by Almix or bispyribac sodium alone can be recommended for effective post emergence weed control and higher yield in wet seeded rice. If grasses are the predominant weeds, cyhalofop-butyl or fenoxaprop p-ethyl alone without follow up spray of Almix can also be recommended.