Effect on weeds
Weed flora
Weed flora of the experimental field was composite in nature comprising of grasses, sedges and broad-leaved weeds (BLW). The major grass weeds were Echinochloa crus-galli (L.), Echinochloa colona (L.) and Cynodon dactylon (L.) and common sedges included Cyperus rotundus (L.) and Cyperus iria (L.). Among the BLW, Eclipta alba (L.) and Ammania baccifera (L.) were the dominant species in direct wet seeded rice ecosystem. Such broad spectrum of weeds in direct wet seeded rice ecosystem was also reported by Parameshwari et al., (2015), Raghavendra et al., (2015), Goswami et al., (2018) and Rathika and Ramesh (2019).
Total weed density and dry weight
Weed management practices significantly influenced the total weed density and dry weight at all the stages of observation (Table 1). The data on total weed density and dry weight revealed that application of PE pyrazosulfuron ethyl 25 g/ha fb EPOE bispyribac sodium 25 g/ha, PE pyrazosulfuron ethyl 25 g/ha fb EPOE metsulfuron methyl + chlorimuron ethyl 4 g/ha and PE pyrazosulfuron ethyl 25 g/ha fb conoweeder weeding on 20 and 40 DAS resulted in greater reduction in density and dry weight of total weeds at 20 DAS and this was comparable with other treatments applied with PE pretilachlor 0.75 kg/ha. This might be due to complete control of first flush of weeds by application of PE pyrazosulfuron ethyl 25 g/ha on 3 DAS and pretilachlor 0.75 kg/ha 8 DAS in these treatments. These results are in agreement with the findings of Payman and Singh (2008) and Kanimozhi and Chinnamuthu (2010).
At 40 DAS, the lowest total weed density and dry weight was recorded in application of PE pyrazosulfuron ethyl 25 g/ha
fb EPOE bispyribac sodium 25 g/ha and it was on par with PE pretilachlor 0.75 kg /ha
fb EPOE bispyribac sodium 25 g/ha. This might be due to combined application of PE and EPOE herbicides, in which first flush of weeds were controlled by the PE herbicides pyrazosulfuron ethyl and pretilachlor. The late emerged weeds were controlled by EPOE herbicide bispyribac sodium. This is in conformity with the findings of
Raghavendra et al., (2015).
At 60 DAS, application of PE pyrazosulfuron ethyl 25 g/ha
fb EPOE bispyribac sodium 25 g/ha resulted in greater reduction in density and dry weight of total weeds because first and second flush of weeds were controlled by pre emergence and early post emergence herbicide application. The next lowest total weed density and dry weight was found in application of PE pretilachlor 0.75 kg/ha
fb EPOE bispyribac sodium 25 g/ha. Effective control of weeds in these treatments might be due to application of two herbicides in sequence could be more useful have resulted in decreased weed seed reserve in the soil. This is in line with the findings of
Ajay Singh et al., (2017) and
Rathika and Ramesh (2019).
EPOE application of bispyribac sodium 25 g/ha along with PE herbicide significantly reduced the density and dry weight of weeds in direct wet seeded rice. It indicated that bispyribac sodium is a selective herbicide, it is effective for the control of grasses, sedges and broad-leaved weeds in rice and is effective as a soil or foliar treatment
(Schmidt et al., 1999). It is a member of the pyridiminyloxy benzoic chemical family and inhibits the enzyme aceto hydroxyl acid synthase also known as aceto lactate synthase (ALS) in susceptible plants. This ultimately reduces transport of photosynthate from source leaves to roots, resulting in root growth inhibition which leading to death of weeds
(Ferrero et al., 2002) and
(Sheeja and Elizabeth, 2017).
Weed control efficiency
The highest WCE as registered in application of PE pyrazosulfuron ethyl 25 g/ha
fb EPOE bispyribac sodium 25 g/ha and PE pretilachlor 0.75 kg/ha
fb EPOE bispyribac sodium at 25 g/ha. This might be due to lesser competition of weeds achieved by effective control of first and second flush of weeds which resulted in reduction in weed density and dry weight. PE herbicide along with EPOE herbicide effectively reduced the weed biomass. This is in agreement with the findings of Bhagat
Singh et al., (2009) and
Rathika and Ramesh (2019) who reported that weeds in DSR could be kept at lower level by herbicides (Table 2).
Weed index
The minimum weed competition treatment
i.
e. plot with application of PE pyrazosulfuron ethyl 25 g a.i. ha
-1 fb EPOE bispyribac sodium 25 g/ha was taken as base to work out weed index as maximum grain yield was obtained by this treatment. PE application of pretilachlor 0.75 kg/ha
fb EPOE bispyribac sodium 25 g/ha resulted in minimum weed index of 6.2 per cent. This was followed by HW twice on 20 and 45 DAS and PE pretilachlor 0.75 kg/ha
fb HW on 40 DAS. However, other treatments recorded WI ranging from 18.1 to 36.0 per cent. The unweeded control registered higher WI of 53.3 per cent. This corroborated with the findings of
Ajay Singh et al., (2017).
Effect of weed management practices on rice
Yield parameters
The yield parameters
viz., productive tillers/m
2, total grains/panicle and filled grain percentage were altered significantly by adoption of weed management practices. Application of PE pyrazosulfuron ethyl 25 g /ha
fb EPOE bispyribac sodium at 25 g/ha registered more number of productive tillers/m
2, total grains/panicle and increased filled grain percentage and it was followed by PE pretilachlor 0.75 kg/ha
fb EPOE bispyribac sodium 25 g/ha. The effective control of weeds in these treatments resulted in lesser competition by weeds for nutrients, space and light ultimately resulting in increased number of productive tiller/m
2. PE herbicide application controlled weeds at early stage and supplemental EPOE herbicide controlled weed growth at later stage which resulted in higher WCE and yield attributes. This is in agreement with the findings of
Manisankar et al., (2019). Unweeded control resulted in reduced yield parameters due to severe crop weed competition coupled with reduced uptake of nutrients by crop and increased uptake of nutrients by weeds. Panicle length and 1000 grain weight was not significantly influenced by different weed management practices. This corroborates with the findings of
Rathika and Ramesh (2019) (Table 3).
Grain and straw yield
Adoption of different weed management practices were significantly influenced the grain and straw yields. The highest grain (4665 kg/ha) and straw (6531 kg/ha) yields were recorded with PE pyrazosulfuron ethyl 25 g/ha
fb EPOE bispyribac sodium 25 g/ha and it was followed by PE pretilachlor 0.75 kg/ha
fb EPOE bispyribac sodium 25 g/ha. This might be due to cumulative effect of increased levels of yield attributes was due to lesser crop weed competition, better light transmission for photosynthesis, reduced nutrient removal by weeds and increased nutrient uptake by crop. These results were in conformity with the findings of
Nath et al., (2014), Ajay Singh et al., (2017) and
Manisankar et al., (2019).