MSc

Most of the herbicides used in wet seeded rice are selective and weed specific, and thus control only a small portion of the diverse weed flora. Several new pre-mix herbicide products having broad spectrum activity are now available in the markets which could exclude the labour of tank mixing as well as the possibility of non-compatibility. However, information on the effectiveness of these new herbicides in the wetlands of Kerala is lacking. The present study entitled “Bio-efficacy of pre-mix herbicide combinations for broad spectrum weed control in wet seeded rice” was carried out to test the effectiveness of three new pre-mix herbicides in the Kole area of Thrisuur in Kerala.
The experiment was conducted from October 2019 to January 2020 in a farmer’s field in Kole area of Alappad, Thrissur district with the objective to study the efficacy of different pre-mix herbicide combinations for broad spectrum weed control in wet seeded rice. The rice variety used was Manuratna. The experiment was laid out in RBD with 10 treatments and 3 replications. Three pre-mix herbicide combinations at two different times of application viz., pendimethalin + penoxsulam at 5 and 10 DAS (T1 and T2), cyhalofop-butyl + penoxsulam at 12 and 18 DAS (T3 and T4), and florpyrauxifen-benzyl + cyhalofop-butyl at 12 and 18 DAS (T5 and T6) were evaluated along with the sequential application of cyhalofop-butyl fb (chlorimuron ethyl 10% + metsulfuron methyl 10%) at 18 and 19 DAS (T7), bispribac sodium at 18 DAS (T8), hand weeded control (at 20 and 40 DAS) (T9), and unweeded control (T10).
Major weed species observed in the experimental field were grasses consisting of weedy rice (Oryza sativa f. spontanea), Echinochloa stagnina and Leptochloa chinensis, and sedges consisting of Cyperus spp and Fimbristylis miliacea. Broad leaf weeds were comparatively very low, such that at 60 DAS, grasses, sedges and broad leaf weeds constituted 92 per cent, 7 per cent and 1 per cent of the total weed population.
The lowest weed dry matter production was recorded in hand weeded control (T9) at both 30 and 60 DAS. Among the herbicide treatments, pendimethalin + penoxsulam applied at 5 and 10 DAS (T1 and T2) and florpyrauxifen-benzyl +
cyhalofop-butyl applied at 12 DAS and 18 DAS (T5 and T6) were on par with hand weeded control at 30 DAS with regard to weed dry matter production. Weed control efficiency at 30 DAS was also highest in these herbicide treatments. The nutrient removal by weeds at different growth stages followed the same trend as that of weed dry matter production.
Among the various herbicide combinations and herbicides, only pendimethalin + penoxsulam applied at 5 and 10 DAS (T1 and T2) caused slight injury on rice at three days after spraying; however, the crop completely recovered within seven days. At 30 DAS, there was no significant difference in tiller count per sq. m between the treatments. By 60 DAS the highest tiller count per sq. m was observed in hand weeded control (T9), which was found to be on par with pendimethalin + penoxsulam sprayed at 10 DAS (T2).
The highest number of productive tillers per sq. m, number of grains per panicle, percentage of filled grains per panicle, and grain and straw yield were recorded in hand weeded control (T9). Among the herbicide treatments, the highest grain yield was recorded in florpyrauxifen-benzyl + cyhalofop-butyl applied at 12 DAS (T5), due to the high weed control efficiency of 82.96 per cent and the lowest yield reduction to the tune of 2.61 per cent. This treatment was found to be statistically on par with hand weeded control (T9), and pendimethalin + penoxsulam sprayed at 5 DAS (T1) with respect to grain and straw yield.
On herbicide application, the microbial biomass carbon and the dehydrogenase activity of the soil were affected at earlier stages of crop growth, but at harvest both were found to be restored and the balance was almost attained. Thus, none of the herbicides showed pronounced adverse effect on the soil microbial properties.
The herbicidal combination florpyrauxifen-benzyl + cyhalofop-butyl applied at 12 DAS (T5) fetched highest gross returns, net returns, and B:C ratio, followed by pendimethalin + penoxsulam applied at 5 DAS (T1). These treatments also recorded the lowest weed indices. All the herbicide combinations from T1 to T6 performed significantly better than the sequential application of cyhalofop-butyl fb (chlorimuron ethyl 10 % + metsulfuron methyl 10 %) (T7), bispribac sodium (T8), and unweeded
control (T10). Although hand weeded control (T9) recorded highest gross returns, net returns and B:C ratio were lowest due to high cost of cultivation.
Among the herbicidal combinations sprayed at different times of application, all the treatments at earlier time of application i.e., pendimethalin + penoxsulam at 5 DAS (T1), cyhalofop-butyl + penoxsulam and florpyrauxifen-benzyl + cyhalofop-butyl both at 12 DAS (T3 and T5), were observed to perform better in terms of both growth and yield parameters than the later applied sprays.
All the herbicide combination treatments performed well in terms of yield attributes and crop yields, and thus fetched higher net returns and B:C ratio. Pre-mix herbicide combinations were as good as hand weeding twice in rice with respect to grain and straw yield, and gave higher monetary returns. Florpyrauxifen-benzyl + cyhalofop-butyl when sprayed at 12 DAS (T5) recorded the lowest weed dry matter production, highest WCE and highest crop yield, which in turn gave highest economic benefit, and thus can be regarded as the best pre-mix herbicide combination. All the herbicide combinations tested can be recommended to effectively control mixed weed flora in wet seeded rice in the Kole lands of Kerala.