MSc

Herbicidal weed control is very common in rice cultivation. Application of pre- emergence and post emergence herbicides or two post emergence herbicides as follow up sprays is usually recommended to take care of diverse weed flora in rice.
As this increases the cost incurred in spraying, farmers prefer single spray of a
mixture of herbicides or a broad spectrum herbicide. Cyhalofop-butyl is a common cost effective post emergence selective herbicide that controls grass weeds, especially
the rice associated weed Echinochloa spp. and Leptochloa chinensis. The present
study was carried out to study the efficacy and economics of tank mix application of cyhalofop-butyl with selected herbicides (pre emergence, post emergence and early post emergence) and to study the response of weed flora to tank mix application.
A field experiment was conducted in Alappad kole lands from September
2015 to January 2016, using the rice variety Uma (MO-16). There were a total of 16
treatments replicated thrice. Almix® [chlorimuron-ethyl (10%) + metsulfuron-methyl
(10%)], ethoxysulfuron, carfentrazone-ethyl, pyrazosulfuron-ethyl, pretilachlor,
pendimethalin were tank mixed with cyhalofop-butyl and were also applied as follow
up sprays, two days after cyhalofop-butyl application. For better comparison, sole application of cyhalofop-butyl, as well as a broad spectrum post emergence herbicide, bispyribac sodium, were also included apart from hand weeded and
unweeded controls.
Pre emergence herbicides were sprayed at six days after sowing (DAS), and
early post emergence herbicide at 10 DAS. All tank mix herbicide applications were done at 18 DAS and follow up post emergence herbicide applications at 20 DAS (i.e.,
two days after the application of cyhalofop-butyl). Hand weeding was carried out at
20 DAS and 40 DAS.
The data on weed spectrum revealed that broad leaf weeds and grasses were dominant and at 30 DAS they constituted 47 per cent and 46 per cent of the population respectively, whereas sedges constituted only 7 per cent. Echinochloa stagnina was the dominant grass and Monochoria vaginalis, the dominant broad leaf
weed.
Among tank mix applications of herbicides, cyhalofop-butyl +
pyrazosulfuron-ethyl recorded the least weed dry matter production while among various sequential application of herbicides, the lowest weed dry matter accumulation
was noted in cyhalofop-butyl followed by (f.b.) Almix® and both were statistically
comparable.
Among various herbicides and herbicide mixtures, both tank mix and sequential applications of carfentrazone-ethyl caused severe phytotoxicity in rice. However, the crop recovered by seven days after spraying and plant growth parameters were not affected with all the treatments registering comparable plant
height at all stages. At both 30 DAS and 60 DAS, hand weeded treatment registered
the highest tiller count. However, this was on par with bispyribac sodium, pyrazosulfuron-ethyl + cyhalofop-butyl and cyhalofop-butyl f.b. Almix®. Highest and statistically superior grain yield was registered in hand weeded treatment. Application of cyhalofop-butyl + pyrazosulfuron-ethyl and bispyribac
sodium were the next best treatments with respect to grain yield and were superior to
all other treatments. Except for pyrazosulfuron-ethyl all other herbicides resulted in lower grain yields when tank mixed with cyhalofop-butyl with weed index in the range of 13to 18 per cent and weed control efficiency in the range of 61 to 76 per
cent (at 30 DAS). Application of follow up sprays of herbicides for control of broad
leaf weeds after cyhalofop-butyl resulted in grain and straw yields comparable to that of single application of bispyribac sodium, which recorded a higher B:C ratio of 2.4.
From this study it can be concluded that tank mix application of cyhalofopbutyl
with pyrazosulfuron-ethyl at 18 DAS can be recommended for effective control
of mixed weed flora in wet seeded rice as this treatment resulted in the highest B:C ratio (2.5) as well as net returns. It is not advisable to go for tank mixing of cyhalofop-butyl with Almix® as it will lead to complete loss of activity of cyhalofopbutyl. Tank mixing of pre emergence herbicides with cyhalofop-butyl was found to be less effective than their sequential application.